So anyway, the first photo shows the roof of my Taurus in beautiful, scenic Fauquier County, Virginia.

Ugly Mag Mount in Beautiful Countryside

Farms, countryside, cattle, horses, even foxes with the view spoiled by the unsightly, ugly cable coming from the magnetic mount antenna on the auto roof top.

I never considered punching a hole in the roof of my Ford Taurus. I don’t have anything against this, but mag antennas have, quite simply, always been around to serve my mobile antenna needs. After years of weathering, it’s time to consider a through hole mount on the old Ford Taurus.

CBs, K-40 and Big Momma

What, on Earth, am I talking about? I am not embarrassed to say I worked in a popular CB Shop in Fairfax, VA during the college years – 1980s. This was a superb experience. I installed CBs and Antennas in vehicles from compact cars to Firebirds to pickups to Tractor Trailers to RVs. I even designed a CB Antenna for someone’s hot air balloon.

The easiest antenna to install was the K-40. However, another popular antenna of the day was the Antenna Specialist “Big Momma” CB aerial. This antenna was once found on every VA State Police car. The key difference between the K-40 and Big Momma twist off mount was the addition of car ground connection whereas the K-40 only connected the coax center conductor. I never quite understood, then, why a base loaded 5 foot whip would need a ground connection. Little did I know I saw my first NMO mount antenna.

New Motorola (NMO) Mounts

Back then I thought the NMO mount was an over designed mount. Who needs the ground connection to the whip? If it is a simple quarter wave or loaded short antenna, you don’t. However, if you use a transformer with one end tied to ground, the NMO is the perfect mount to use.

Indeed, my further research found that NMO is pretty much the defacto mount used in professional applications for antennas of all kinds. One standard threaded connection provides for an unlimited choice of antennas that one can replace as requirements change. Nice.

NMO Through-hole Mount Choices

Current Antenna with various NMO Through Hole Replacements.

Research on the Internet reveals quite a few selections to ponder when considering through hole NMO antenna mounts. Choices include:

• 3/8″ or 3/4″ hole
• Open or shielded cable connection point
• Name brand of cheap knockoff
• Hole Saw or Metal Punch

3/8″ vs. 3/4″ Hole
It is quite clear the traditional installation of NMO uses a 3/4 inch hole. This is precisely what I thought I was going to do. However, I stumbled onto one web site offering a 3/8 inch option and I had to know more.

It seems the 3/8 inch option is primarily used for mounts that must attach to very think metal… 1/8″ or thicker. If I can get a way with a 3/8″ hole in my Taurus I was willing to try it. Let’s compare the advantages of each.

Advantages of 3/4″ NMO through-hole mount include:

• Mount can be drilled and installed from the outside (Cable routing extra of course)
• Seems to be the “way” to do it
• 3/4 inch specialty drills available to drill hole, but not go any deeper

Advantages of 3/8″ NMO through-hole mount include:

• Smaller hole
• Potentially sturdier with smaller hole

Open or Shielded Connection Point
While searching for NMO mounts to consider, I stumbled onto a web site showing an NMO mount with a completely metal enclosed connection area. This is to ensure little RF energy escapes into the human occupied area. Apparently some countries require this. The US does not seem to have such a requirement, but I purchased one of these for review… see below.

Name Brand or ‘other’
One thing that became obvious after reading the forums about NMO mounts is the suggestion to use brand name parts from Laird, Motorola, Larson, etc. The NMO mount is standardized, but the suggestion is quality tolerances may be loose in off-brand parts.

One problem I had while shopping was knowing just what brand parts were being sold. After receipt of my examples, shown below, Laird and Antenex were the primary manufacturers. However, only two of the four vendors said so on their web site and in the sales receipts.

Hole Saw or Metal Punch
No one in the metal industry uses hole saws for small holes like this, however, it appears just about every installer of mobile antennas does. When installing through-hole CB antennas back in the day, the bossman insisted on the use of his metal punch. It did make superb holes requiring very little de-burring.

One clear problem with punches is the need for tool access to both sides of the metal. I can see where busy installers desire top only access and the hole saw is about the only way to get it.

Fortunately Laird and others make a hole saw specifically for use with NMO installation. The good ones have a pilot drill bit followed by a “wood hole saw” like cylinder and, most important, a shoulder that stops the thing from going too far into your vehicle. I did not see such a tool for 3/8″ installation, but you need bottom access for this anyway, so a punch or carefully used drill may be just fine.

Pictures hard to find
One particularly annoying thing with most of the NMO Vendor web sites is the lack of specific pictures for the particular model you are trying to buy. The descriptions were fine, but seeing the thing helps. So for the readers of this web site, here are some pictures of the actual items purchased.

Vendor A – Off brand NMO Mount for 3/8″ or 3/4″ Hole

Unknown manufacturer NMO Mount for 3/8 or 3/4 inch Hole

I’m not really too hip to that bent metal back support as it feels a bit cheap. The main barrel is for the 3/8″ hole, but the metal back support has too small knockout bumps which help center it in a 3/4″ hole.

Vendor B – Antenex (Laird) MABTO NMO Mount for 3/8″ or 3/4″ Hole

Laird Brand NMO Mount for 3/8 inch or 3/4 inch Hole

This unit has a longer barrel for thicker base-plates than the MABO (below). The sales receipt says precisely “Antenex 3/4″ or 3/8″ Hole NMO Antenna Thick Mount MABTO” as did the web site when I purchased it. The Laird bag it came in says this is for a 3/8″ hole. However, the fine print on the bag describes the purpose of the white washer is a centering washer when you use this mount in a 3/4 inch hole.

Vendor C – Laird MBO NMO Mount for 3/4″ Hole and MABO for 3/8″ Mount

Laird Brand NMO Mount for 3/4 inch Hole

This Laird item may be the “classic” NMO mount. It is for the traditional 3/4″ hole. The top component and backing component are actually just one piece. The backing component has a nice centering cylinder milled right into which looks like it will nestle right into the hole and keep it in place. This unified unit feels and seems very solid.

Laird Brand NMO Mount for 3/8 inch Hole

This is also from Laird with the only difference between it and the MABO is the backing piece shape. This one has no 3/4 inch centering ridge milled in it.

Both the MBO and MABO Laird mounts impress me.

Vendor D – Unknown Manufacturer NMO Mount for 3/4″ Hole with FME Connector

Possible Motorola Brand NMO Mount for 3/4 inch Hole with FME Connector

This product contains a 90 degree coaxial connector. The RF energy is obviously well contained. There are only two pieces and the bottom portion is one solid block of metal. I don’t know a lot about the FME connector.

Conclusions

Laird (Antenex) easily won my admiration for quality and function. Vendor mis-information and lack of product specifics forces the buyer to do their own research into product selection. I hope the pictures of actual products above help you select the best NME through-hole for your application.

My final choice is likely between the MBO and MABO. Upon close examination the only real difference between the two is the milled 3/4″ circular “shelf” on the MBO inner foot and the size of the threaded connection between the top and bottom pieces. Many reports from the field suggest the 3/4″ MBO style is “much stronger” than the 3/8″ MABO style, but the folks never seem to have any clue as to why. The outer dimensions of the inside foot for each are identical and, thus, transfer stresses to the metal identically. The cylinder of metal between the two is the same diameter. It’s plainly obvious the 3/8″ unit transfers mechanical stress over a larger surface area of vehicle metal with the smaller hole. I will prototype both onto a scrap piece of metal, show it to some machinists and MEs to get their take on this.

More details to come.

73
John, kx4o

That’s not the point though. If we were to cut off the connector we would surely still see a spark from center conductor to shield. If the cable were near station ground I would expect a spark jump. That large a charge has to go somewhere.

The point is a wire, any wire, with wind blowing on it will build a static charge. We should keep this in mind as we design our antenna systems. Including a DC path to ground is prudent.

I cannot imagine a more teachable YouTube moment. Thanks to the poster for making this video available.

Here is a link to a very old video which explains quite nicely what and how static charge occurs…

http://video.google.com/videoplay?docid=2306375174608358801&hl=en#

73

1. Crimp on connectors have been the norm in industry since at least the 1970s. I’m just ashamed it took me so long to figure this out.

I cannot comment on the mechanical realities of the antenna flying off the roof during rapid changes in speed, but can say I have never seen one do so. I encourage anyone with data to propose their article to this web site

So…

Let’s talk about the electrical conductivity of a standard mount vs. a magnetic mount antenna. Here is a quote from the newsgroups concerning antennas for scanners…

The average mag-mount antenna gets its “ground” from the magnetic attraction between the mount and the metal of the car body. This is nowhere near the quality of a good, solid ground connection to the body. We are relying here on the mount, the paint on the car, and the metal underneath to form a “capacitive ground connection”. (I scared myself with that one !) The smaller in diameter the magnetic mount is, the less effective this is. It is a simple matter of total area covered by the mount.

First while the mag mount antenna does hold itself to the car via magnetic attraction, there is no other direct electrical benefit related to the magnetism. There is no magnetic “coupling.”

The post author correctly identifies the ground connection happens due to the capacitor created between the bottom of the antenna and the metal of the vehicle. The claim this is worse than a direct connection via set screw or other demands verification. Let’s do some math to check this out.

First some assumptions:

• Paint thickness of autos seems to range from 75-200 microns. Add in clear-coat and primer this might grow to 3x or so. I am not a paint guy so I am not at all sure about these figures, but since capacitance is reduced at greater thickness, we will assume 600 microns for our calculations or about 24 mils (1 mil = 1/1000 inch).
• Typical small 2m ham antenna with 3 inch base.
• Braid of coax electrically attached to the foil on the bottom of the antenna. Did you notice that mag mount antennas have electrical foil to make the antenna side of the capacitor?
• We will derive the capacitive reactance of the above details at 50MHz, 144MHz and 440MHz
• Dielectric Constant, K, of clear coat and paint something like 3

Figure 1 shows a simplified model of antenna electrical parameters:

Fig 1 - Currents in and around a magnetic antenna.

• The Radiation Resistance is the thing you want your transmitter to work into.
• The Ohmic loss is the simple resistive loss of the antenna and its inner connections.
• The sum of the Shield Currents exactly equal the current on the inner conductor and flow back to the transmitter on the inside of the outer conductor. If you do not connect the antenna’s ground to the local ground around the antenna, the current will simply flow back on the outside of the outer coax conductor and create havoc for your radio and possibly you.
• This fact applies equally well to ground mounted HF vertical antennas and is why radials are so important.
• Xc is actually part of the resistance from coax shield to ground and adds directly to the overall impedance.
• Keeping every resistance or impedance small with respect to the radiation resistance makes or breaks an antenna’s efficiency.

Calculating Capacitance between Antenna and Metal

The forumla for the capacitance between two conducting plates is:

C = 0.2258 * K * A / d

Where…

• K is the dielectric constant of the material,
• A is the overlapping surface area of the plates in square inches,
• d is the distance between the plates in inches, and
• C is capacitance in pF (pico Farads – 1×10^-12 Farads)

Capacitor Area = pi * r^2 = 3.14159 * 1.5^2 ~ 7 sq. in.

Capacitance(pF) = 0.2258 * 3 * 7 / 0.024

Capacitance(pF) = 198pF

Xc = Capacitive Reactance = 1/(2*pi*f*C)

Results:

• When f = 50MHz => Xc = 16 Ohms
• When f = 144MHz => Xc = 6 Ohms
• When f = 440MHz => Xc < 2 Ohms

This probably represents the worst case where the car’s paint and clear-coat are super thick as assumed above. Let’s redo the calculations for a more probable 8 mils…

Capacitance = 0.2258 * K * A / d

Capacitance(pF) = 0.2258 * 3 * 7 / 0.008

Capacitance(pF) = 593pF

Xc = Capacitive Reactance = 1/(2*pi*f*C)

Results:

• When f = 50MHz => Xc = 5 Ohms
• When f = 144MHz => Xc < 2 Ohms
• When f = 440MHz => Xc < 1 Ohm

The above calculations reveal mag mount antennas do, indeed, provide a very robust AC coupling of the RF shield currents to the metal of the car via capacitive currents.

The impedance presented by the mag-mount/metal interface adds directly to the radiation impedance of the antenna resulting in the final impedance of the antenna approaching 50 ohms… ~36 ohms + Xc. This also directly affects the efficiency of the antenna just like a ground radial system helps a ground mounted vertical.

The post author does understand the area of the mag mount bottom in contact with the car matters with larger being better. He goes on to say…

I am not going to launch into a big tech discussion here, with foot-long words, and math that would give you (and me) a headache. You will just have to take my word for it. And, the really sad part is, the higher the frequency that you monitor, the worse this gets

This is where his understanding breaks from reality. The higher the frequency the better. The math is straight forward; You do not have to take anyone’s word for it. All other things being equal, capacitors conduct better at higher frequencies as the above math shows. There is no need for foot-long words, but full understanding does require doing some math.

What we have shown above is magnetic mount antennas do, indeed, provide a good connection to the metal it is attached to so long as there is a foil bottom attached to the shield of the coax and the area of this bottom is reasonable large. It is true that as the frequency goes down, the mag mount antenna efficiency also goes down. Many CB antennas for 27MHz have much larger bases not only to support the longer whips, but to provide that extra contact area to keep the Xc low.

It can be argued this large capacitor in contact with the metal of the vehicle may well be more effective than trunk lip mounts that make their contact with one or two small set screws dug through the paint to the metal of the trunk lid or gutter. This is, of course, if the trunk lid is made of a conductor… many are not. This is all especially true if high power is applied to the antenna system. The magnetic mount antenna provides the unique advantage of spreading out the ground currents over a large area rather than through the finite points of locking screws.

A properly installed though-hole NMO mount probably beats the mag mount in shield to ground plane connection as its design provides a large direct contact area from shield to hole avoiding the set screw problem.

When shopping for a magnetic mount antenna you might well benefit from a unit that keeps the Capacitive reactance below, say, 10% of the antenna impedance. For a quarter wave antenna of about 36 ohms that means the mag mount impedance should be under 3 ohms. Much depends on the frequency, paint thickness, dielectric constant and area. Armed with the above technique you can comparison shop with better clarity.

With the above method of calculation, you can easily convince yourself that, at least electrically, the magnetic mount antenna works just fine in many situations and provides the added bonus of putting the antenna in the best position without need for a hole in the roof.

What follows is the experiment which provides proof Velocity Factor of Coaxial Cables does not apply to cables used simply as a wire in parallel with another wire.

The Experiment

The two Baluns are an excellent pair to compare. The Coaxial Cable Balun is just two transmission lines tied together one 1/4 wavelength long, the other 3/4 wavelength long. In this case the electric and magnetic fields are entirely within the dielectric material. Thus the velocity factor adjustment should apply. The Pawsey Stub relies entirely on becoming a parallel transmission line with the feedline coax. Its fields are entirely outside the structure of the coax.

I built the two baluns using some RG316 coax. My target frequency for both is 300 MHz. I cut pieces with no corrections applied to see how the frequency changes.

The Coaxial Cable Balun has the following dimensions based on calculations…

• 1/4 wave section = 300m/s / 300 MHz * .25 = 25cm ~ 9.8 inches
• 3/4 wave section = 300m/s / 300 MHz * .75 = 75cm ~ 29.5 inches

I simply cut two pieces of coax to the above lengths and added a third with an SMA connector for the feed line.

Figure 4 shows the Coaxial Cable Balun with a 50 ohm resistance at the balanced feed point.

Fig 4 - Coaxial Cable Balun with 1/4 and 3/4 sections of coax, cut for 300MHz Freespace, in parallel.

With the resistance at the feedpoint, the Balun should show some very obvious frequency dependence on a Return Loss or SWR plot. Because this is the case where Velocity factor does apply, the target frequency of 300 MHz should be lowered by .695 or about 208 MHz. Given that I flayed the coax shield and center, thereby reducing some of the coaxial cable length, the actual frequency should be a little higher.

Figure 5 shows what happens when I connect the Coaxial Cable Balun to the VNA…

Fig 5 - Return Loss of Coaxial Cable Balun from 100 - 500 MHz. Markers M1 and M2 are at 300 and 214 MHz respectively.

Wow the 214 MHz pretty confirms that Velocity Factor is well in play for this style of Balun.

Now let’s try the Pawsey Stub. The only critical dimension is the stub length.

• 1/4 wave section = 300m/s / 300 MHz * .25 = 25cm ~ 9.8 inches

Once again, my trimming slightly shortens the actual electrical length to about 9.2 inches which raises the frequency just a bit. Figure 6 shows my test unit with 50 ohms at the feed point.

Fig 6 - Pawsey Stub Balun with 1/4 of coax, cut for 300MHz Freespace, in alongside the feedline.

What’s the prediction here? If you listen to the web sites which suggest coax cable VF applies, the frequency will be between 210 and 230 MHz. If I am right and this is free space, or very close to free space, then the frequency should be about 300 MHz, or since the connection points on the stub are a little closer together, about 320 MHz. Let’s see the Return Loss plot…

Fig 7 - Return Loss of Split Coax Balun from 100 - 500 MHz. Markers M1 and M2 are at 300 and 322 MHz respectively.

Bulls Eye!!!!

This conclusively proves external stubs, like the Pawsey Stub, should be calculated using free space wavelengths without corrections for Velocity Factor.

The funny thing is, this has been the case in the ARRL Antenna book for many many years. The book never suggests to apply velocity factor to external stub calculations. However, they never say don’t either.

Conclusion
It is a good thing Amateur Radio Operators know about transmission line Velocity Factor specifications. Without this knowledge you would be cutting stubs incorrectly. However, some Amateurs assume that just because a piece of coax is used for a stub, Velocity Factor applies. The key to understanding when it does and when it doesn’t is found by following the electric and magnetic fields. Where are they? Are they inside the coax or outside between pieces of coax? If inside, apply VF. If outside assume freespace.

Measure, but verify…
This experiment relies on some crude cutting of cable lengths and assumed values of Velocity Factor. Potential sources of error include:

• Cable End Trimming – Source for my slight errors above
• Coax Velocity Factor not what the manufacturer says – When you do need to know VF, it might be a bit off especially with lower quality coax
• Freespace not quite freespace – Even the best air gap transmission line has some dielectric material in the fields – VF values of 0.95 are common even with parallel transmission ladder line.

So what does this suggest? If you are going to the trouble to make something as critical as a Balun for your Yagi antenna, electrically measure each cable with your SWR meter or VNA to ensure you are spot on the frequency of choice.

This was an interesting test. I am now so interested in Baluns, I will focus on posts for each type in the near future.

Thanks for reading.

John

VHF and UHF antennas, especially beams, often have balanced feed points. Several methods exist to connect unbalanced coax to the dipole element: gamma, t-match, etc. Some VHF/UHF Balun options use tuned stub or shunt topologies to achieve the unbalanced to balanced goal; Methods include the Pawsey Stub, Split Coax Balun and Coaxial Cable Balun. Each of these requires specific lengths of conductors based on multiples of 1/4 wavelength at the design frequency.

When calculating 1/4 wavelengths, however, it seems the amateur radio community has lost the collective knowledge of when Velocity Factor (VF) of Coaxial Cable Dielectric Material applies. Indeed despite the decades of college texts and ARRL Antenna books showing otherwise, the idea Velocity Factor always applies to coaxial cable length calculations when used as nothing more than a wire stub in a Balun seems to have gone viral on the Internet.

The web site of G0KSC provides innovative new approaches to Yagi-Uda design. The author provides focus on the topic of Baluns on his Creating a Balun web page.

The topic of baluns is vast so let’s just focus on the ones for VHF/UHF antennas. Most often these are tuned assemblies using some combination of coax and/or wire stubs at specific lengths.

G0KSC highlights several Baluns on his web page:

• 1:1 Coaxial Cable Balun I have never seen before using 1/4 and 3/4 wavelengths of coax. This is similar to the 4:1 Balun we have seen for decades in the literature.
• 1:1 Pawsey Stub – a method using a 1/4 wavelength wire off the coax center conductor and tied a 1/4 wavelength back.

One comment made on the web site concerns velocity factor of coax. Certainly the Coaxial Cable Balun has this effect which requires shortening wavelength dependent coax pieces. The author also claims the Pawsey Stub requires the same adjustment for velocity factor. The electric and magnetic fields for the Pawsey Stub are outside the realm of the coaxial cable dielectric suggesting this is not the case.

I decided to build working models of both to see for myself which Balun requires Velocity Factor correction.

First let’s examine the two Balun approaches.

1:1 Coaxial Cable Balun
Figure 1 shows the pieces of this Balun…

Fig 1 - 1:1 RF Balun using 1/4 and 3/4 wavelength coaxial cables.

This topology is deceivingly simple. A description can be found at I0QM’s web site PDF file.

Pawsey Stub
Figure 2 shows the idea behind the Pawsey stub which is well known in Electrical Engineering circles as a variant of the Split Coax Balun.

Fig 2 - Split Coax Balun (aka Pawsey Stub)

While the Gray conductor in Figure 2 only needs to be a wire of similar size to the coaxial cable feedline, it is often made from a scrap piece of the same cable. Each end of the outer shield of the stub is connected to the feed system. A common thought of many is since this is coaxial cable, we need velocity factor adjustments. Since the electric and magnetic fields (of the stub system) are in air, I think velocity factor does not apply.

Figure 3 highlights my reasoning…

Fig 3 - Electric and Magnetic fields in dielectric materials define when velocity factor applies.

Figure 3a shows the construction of coaxial cable we are all familiar with. In normal operation all electric and most of the magnetic fields are contained between the inner and outer conductors. Since the spacing between these conductors is maintained by an insulating material, the fields are completely dependent on the dielectric constant of this material.

Figure 3b shows the same coaxial cable, but this time there are two pieces side by side as used in the Split Coax Balun. This balun, essentially a parallel transmission line, develops all its fields between the outer skin of the cables’ shields, not within. The stub wire, as used in the Pawsey Stub method, could be a copper wire with the same effect.

The studious reader will notice the external fields are not entirely in free air. The cable jacket certainly has a dielectric constant greater than 1 and, as thin as it is, will retard the speed of light a bit. The effect is much less than the situation inside coax, however. So… the VF of free air is not quite 1, but almost is… about 0.90 to 0.98 or so.

I assert the typical 0.6-0.8 values of velocity factor of coaxial cable should not be used for stub length calculations for stubs with fields outside the cable’s interior.

Am I right?

Part 2 of this series highlights the actual test I performed to prove I am correct.

Despite being a popular and productive way of simulating amateur radio antennas, another method of electromagnetic simulation makes use of the Finite-Difference Time Domain (FDTD) method.

Without going into too many details, the method divides up the simulation space into little cubes of volume. Then the next magnetic field values are derived from the current electric fields. Then the next electric field values are derived from the current magnetic fields. Repeat.

FDTD makes use of Maxwell’s equations to calculate the above fields. Being a Time Domain method, what follows shows a movie like display of what is happening in the antennas in super slow motion.

The cool byproduct of this style of simulation is we get to see a movie of magnetic and electric fields as power is applied to an antenna. The fields build up around the antenna and eventually fill the volume with energy. For both styles of hex beam antennas you can see after a few cycles how the parasitic element becomes energized. Once both elements are energized the beaming action takes place and you can see energy propagating forward.

Both simulation movies reveal the magnetic fields in the first 50 seconds and the electric fields in the second 50 seconds. They, of course, happen at the same time to yield Electro-Magnetic fields, but I could not show both in a meaningful way.

Here is a 10 meter Traffie (Classic?) Hexbeam…

Here is a 10 meter G3TXQ Broadband Hexbeam…

Neat eh?

No simulation technique is perfect, but the FDTD method’s ability to show EM in time is very cool.

If you are interested in FDTD, Google has quite a bit of information. There are some free tools out there for the taking.

I already have a post concerning my selection for the hex beam’s center hub. After reviewing the several aluminum plate styles offered by Leo, Hexkit and many others, I chose the molded Hex Beam Aluminum Hub by DX Engineering.

For wire I purchased 200 feet of 14 AWG Flexweave with Clear UV Resistant PVC Insulation. This wire is simply fantastic and has saved my bacon several times including at Field Day 2010 when I had to unravel the hex beam. My only concern is will it stretch over time and change the tuning. Time will tell.

So I had the hub and the wire. All I needed to finish the beam was spreaders and some rope.

I thought about the spreaders far too much. I read every post on the Hexbeam Yahoo forum. Many folks were using things like fishing poles, plumbing PVC and other locally available products for the spreaders. The consensus seemed to be fiberglass was the material of choice for long term enjoyment of the hex beam antenna.

Some folks said they found fiberglass poles locally. I could not. So I examined what Leo was using and saw Max-Gain Systems provides an appropriate kit of intersecting poles for both the Traffie style MW Hex Beam and Steve’s (G3TXQ) Broadband. I am not a cheap ham, but I did have a budget. After some thought, I realized I had already committed to Cadillac components starting with the amazing DX Engineering Hexx Hub. In about three seconds I decided to order the kit of poles from Max-Gain Systems and be done with the selection process.

Below are some photos of the arrival plus subsequent modifications and assembly of the Max-Gain Systems fiberglass kit. I needed some of the black rope also so simply added that to my Max-Gain order.

I cannot complain one bit about the Max-Gain Systems fiberglass poles. They just work.

Not discussed are the two 1 inch x 48 inch fiberglass poles also shipped by Max-Gain. One of these poles is for the vertical center pole of the antenna. The other is simply a spare pole that comes with the fiberglass kit. I wound up using PVC for the center pole and saved the other pole for future possibilities. However, I did make good use of the spare pole; I used some of it to reinforce the PVC from the bottom of the antenna to just above the hub. The 1 inch fiberglass tube fits perfectly inside 1-1/4 inch PVC. This allayed my concerns the PVC pipe would simply crush when clamped into the rotator and the PVC would buckle during rough handling. The fiberglass pole inside the PVC pipe provides a sturdy inner core.

I wish I could have run the fiberglass the whole length of my hexbeam’s center post, but the wire connection bolts prevent this. I am still satisfied as I think most of the stresses in the center post exist between the rotator and the center hub. PVC plus fiberglass core exist in this critical area.

I now have a portable (mostly) contest band broadband hex beam and it really does work pretty darn well.

This G3TXQ Broadband Hexbeam project exceeds my expectations wonderfully well.

One of the stations used an Elecraft K2 with an external power amp plus the 100 watt auto tuner. The Elecraft auto tuner provides two antenna ports. We connected a large dipole to channel 1 and left the other open for another antenna.

At some point during Field Day I decided things were going well enough for me to concentrate on my 20-15-10-06 meter hex beam of the G3TXQ variety.

As you can tell by the first photo below, the hex was hastily shoved into the trailer. However, I used good Flexweave wire for the elements and untangling the mess was straight forward. I decided the put the hex beam on the K2 station since it had the open antenna port and, for some reason, I think the K2 and the hex beam complement each other.

The hex beam was a bit loose in its rigging causing the elements to slump a bit. However, the thing tuned right up on 20, 15 and 10 meters similar to its debut during the Virginia QSO Party. Six meters was way off; I think the 10 meter wires were a bit too close to the six meter wires. No matter, the K2 had no 6 meters and 10 meters seemed to work well enough.

Results

Did it work? Boy it sure did. During my turn at the K2 Phone I found the hex beam provided two to four S units improvement against the dipole on 20 and 15 meters.

Late Saturday night I worked 4,800 mile contact with Hawaii (PAC) from Virginia with ease on 20 meters… on Phone! The Hex Beam and K2 bagged the only PAC contact of the four stations on site.

Several more 20 meter contacts were easily made to the West coast including LAX, SDG, SCV and WWA. I wish Field Day used Sections as multipliers, but I was glad to be able to add fresh contacts to the club’s totals. Tom, owner of the K2, quickly added many CW contacts using the beam.

The hex beam was up only about 15 feet and still worked quite well. Amazing.

We did have a rotator, but did not have it aligned with North. That was troubling, but I was just glad to be able to steer it at all at Field Day. We used a Channel Master TV type rotator. Yes, it works, but clearly is over stressed by the inertia of the hex beam despite its low mass. Anyone contemplating a rotator for their hex beam is well advised to “up” the rotator ratings quite a bit to ensure mechanical robustness.

Of all the compact compromise Yagi-Uda beam designs out there, I have to say the hex beam lives up to its own hype. There’s no magic in the hex beam (no matter what anyone tells you). It can’t outperform full size single band Yagi-Uda designs. However, for the investment, it is a superb value and deserves to be on anyone’s short list of antenna choices.

Choice Matters

The most important thing learned at this Field Day is having two antennas to select between makes a big difference in operating success. For some reason North Florida was alive with contacts and I sometimes used the hex beam and other times the wire dipole on 20 and 15 meters.

Yes, it’s Field Day and we tend to be minimalist. However, I designed this hex beam to be very portable and think it fits well with the Field Day theme with its simple tent peg tripod mount. It nicely complements a wire dipole.

The G3TXQ 20-15-10-06 meters Hex Beam antenna is a keeper for Field Days of the future.

There is nothing new about small loop antennas. They have been discussed in the literature for decades. The ARRL has some very old articles about them in the 1968 March and July editions.

Constantine A. Balanis’ book on Antenna Theory discusses and defines large vs. small loops. This book is an essential reference if you seek the details on how loops work. The loop described below fits into the “small loop” category where the currents along the conductor are, for all practical purposes, constant. This is unlike full size antennas where current reaches a minimum where voltage approaches maximum. Since this loop is electrically short with respect to wavelength, current does not change “much.”

The bands of interest in this year’s experiments were 80 and 40 meters. We had to balance scientific research with a hard deadline of March 2010… in other words engineering.

Here are some highlights and details of our small loop…

• Loop dimensions 4 – 8 feet – Fits into the bed of a full size pickup truck,
• Feed point middle of bottom horizontal conductor,
• Capacitor middle of top horizontal conductor (Note the literature describes a way to avoid having the capacitor in the top, but we stuck with the simpler approach),
• Current of loop will be tens of Amps, perhaps up to 50 A – Fat pipe necessary,
• Voltage across Capacitor will be thousands of volts, perhaps up to 5 kV – Vacuum Variable is an good choice,
• We used shunt loop feed at bottom,
• We used 1 inch plumbing pipe and elbows,
• Silver solder a must to keep conductivity high,
• Vacuum variable capacitor selected to provide tuning capability – some use coaxial cable caps for fixed frequency use.

Here are various pictures…

Did it work?

It sure did. You might notice we never finished a method to tune the vacuum variable remotely so we were stuck with leaving the thing tuned for 7265 kHz. However, the mobile operator made frequent AB comparisons between the loop and his 40 meter ham stick. I was on the other end of the contact several times and could not tell any difference. This does not suggest the loop was better than the ham stick, but at least the loop didn’t stink.

We were pressed for time to get this thing on the air for VAQP 2010 so the mobile operator took the loop as is from our tree testing. During the contest he found the feed loop worked with a much smaller diameter than we originally anticipated. On the fly adjustments resulted in this final configuration here…

Finished HF Mobile Loop ready for contacts. Feedpoint shunt loop is much smaller than originally anticipated.

There are several things left to do including engineering some way to tune the vacuum variable remotely. This will be well worth the effort since our manual tuning revealed this should work on a variety of bands. The capacitor is a surplus 5-500 pF (5 to 3 kV) model form Surplus Sales.

According to the literature, small “magnetic” loops like this have an antenna aperture similar to a full size dipole. If this is true it should be an impressive benefit for otherwise compromised mobile operations.

If you are considering a small loop antenna be sure to understand this is a very small bandwidth antenna. A future post will provide details, but don’t expect the bandwidth to exceed a few kHz from center. You cannot use a traditional tuner with this antenna. The variable capacitor is the only way to move the frequency.

As a first foray into Magnetic Loop antennas, I think we achieved success. Work still needs to be done to see if this really is a practical antenna for mobile (or even fixed) use, but results show promise.

Thanks go out to W2BRI’s web site for lots of great ideas…

• http://www.standpipe.com/w2bri/

If my plans included the 17 and 12 meter bands I would have purchased one of these kits with no questions asked. However, I felt my need for only 20, 15 and 10 meters (6m maybe) were not sufficiently addressed by the kits. Yes, you can get wiring just for these bands, but the cost differential was slight.

So I decided to custom pick parts for my own custom variation of the hex beam. The hub for the six fiberglass poles was a logical place to begin.

The hex beam has been around long enough to have a cottage industry to support it. Ever since the March 2009 QST article more manufacturers have emerged. The base plate suggested in Leo’s QST article and offered by most of the manufacturers use two U Bolts to hold each fiberglass pole. The HexKit.com baseplate is one example…

The Hexkit.com baseplate

The instantly obvious issue is the way these flat plate units hold the round fiberglass pole. The U-Shape of the bolt is good, but squeezing the round pole to the flat plate can easily stress the pole. This is easily circumvented with saddle clamps and other methods that have been around for centuries, but I guess the stress on these poles is sufficiently low to not be a problem. I did notice many hex beam sites reinforcing the poles in the hub area while others have welded metal pipes instead of clamps. Perhaps there is a problem with fiberglass poles against flat plates after all.

Last year when I started thinking about making a hex beam, I seriously considered purchasing just the aluminum plate and purchasing saddle clamps of the correct size from DX Engineering like this example…

DX Engineering Saddleclamp

The mechanical benefit over U-Bolts is obvious. That said, hundreds of amateurs have been carefully using the flat plates with perfect success so it appears the more proper saddle clamp approach is only marginally necessary. However, I like doing things the more traditional way and swore I would use saddle clamps when it came time to build my own hex beam.

Time passed and other distractions caused me to shelve the project for a bit.

Then lo and behold… DX Engineering decided to enter the hex beam fray. Everything about their hex beam offering seemed pretty much run of the mill with the sole exception being their hub and especially the way they connect the poles to the hub. Here is a close view of the pole attachment point…

Where the pole meets the hub.

Wow. This offers the benefit of a saddle clamp, but with the saddle running the whole length of the pole-hub interface. Brilliant.

The hub costs about the same as the offerings from other vendors so there was no real financial advantage of any particular model. The sole exception is folks who make their own plate from raw materials and trade their time for cash savings; Kudos to them. My time is worth a bit more than the hundred or so dollars hex beam hubs cost.

Needless to say specifying the DX Engineering hex beam hub in my design was a no brainer and that’s exactly what I did earlier this week. The web site said they were out of stock, but it arrived in a few days during a snowy mid-atlantic Saturday anyway.

I have purchased DX Engineering products before with the primary feature being excellent documentation. I decided to see if my Lego building seven year old could put this product together. Let’s see what happens…

The seven year old had no problems at all except for that V-Bolt moment.

So the DX Engineering Hex Beam hub is here and the project is officially underway. I patiently await the arrival of wire and rope from The Wireman and fiberglass poles from MaxGain Systems. Hopefully by March I will have my 20-15-10-6 meter contest band hex beam ready to go for the Virginia QSO Party.

So…

I have been eagerly absorbing all there is to know about the hex beam style of HF antennas.

If you are the least bit interested in building or buying your own hex beam a must read web site is the G3TXQ study of the hex beam…

One word that comes to mind is “WOW” what a great collection of tests, simulations and other thoughts about the hex beam.

My favorite feature of the site is the availability of an EZNEC simulation file for the G3TXQ of the five band broadband hex beam.

I eagerly downloaded the EZNEC file and, because I have the + version of the software with a higher segment limit, changed all the wires to a consistent and even segmentation rather than keeping the tapering feature. I now have 1223 segments in the mode. It takes a lot longer to do simulations, but is no big deal.

What I am doing with this beam is for another post. The question I hear often on the hexbeam Yahoo forum is concerning using the five band hex on 6 meters.

The focus of concern comes from the third harmonic of the 17 meter element which is close to the 6 meter band.

Indeed running a SWR plot on the stock five band hex beam between 48 and 56 MHz yields…

SWR of Five Band Hex Beam for 6 Meters

Hmmm, this is not a fantastic SWR, but sure suggests something in the five band broadband hex beam is absorbing the energy. Let’s see where the currents are…

Current Magnitudes in Five Band Hex at 51 MHz

Sure enough, the 17 meter element has the most current suggesting it is providing the majority of the absorption at 51 MHz. If you look closely, only the driven element has lots of current while the reflector seems quiet. If you look closer still you can imaging the 17 meter M element acting a bit like a folded Extended Zepp antenna. An examination of current phase indeed confirms that center current peak is of opposite polarity than the current peaks at the ends.

So this is all very interesting. The stock G3TXQ Broadband Hex Beam sort of works on 6 meters. Even if you can live with the high SWRs, is the pattern useful? The next figure shows the Elevation pattern of the five band hex beam up about 20 feet above the same ground as defined in the original G3TXQ file…

Elevation Pattern of 5 Band Hex Beam at 51MHz

Hmmm, not too shabby. Remember to not get too excited with gain figures of 8 dBi when simulating antennas above real ground; There is often a gain increase due to ground for any antenna including simple dipoles. The point here is this HF antenna seems to offer some gain at 6 meters. Here is the Azimuth plot of the 12 degree elevation…

12 Deg Azimuth of 5 Band Hex at 51 MHz

The front to back and front to side ratio is a bit weak at 5 dB, but this is certainly a directional antenna at 51 MHz.

My ultimate goal is to build a contest version of the broadband hex beam for just 20, 15 and 10 meters. After I deleted the 17 and 12 meter wires from the model, I ran SWR again to see if things change at 51 MHz…

SWR of 5 Band Broadband Hex Beam at 6 Meters

They sure did. The 12 and/or the 17 meter elements were certainly contributing to the dip in SWR in the 6 meter band. It is pretty obvious the 17 meter wires were operating at the third harmonic so I rule the 12 meter wires are not contributing to 6 meter operation.

So… what does the mean?

Two things.

1. If you have the five band 20-10 meter G3TXQ Broadband Hex Beam antenna, you may well have a workable 6 meter antenna too. If your rig has 6 meters on its HF output and a built-in auto-tuner, press TUNE at 51 MHz and see what you get. You may well have a free 6 meter beam ready for use.
2. Adding an additional 6 meter set of wires to the existing 5 band HF G3TXQ Broadband Hex Beam antenna, may be a waste of time since the 17 meter element is competing for some of the energy at 50+MHz.

The second point may surprise many folks who have invested in adding a 6 meter element to their five band hex beam. They say “it works fine.” That’s great, but it certainly is worth knowing why. In a future post we will simulate the five band with the extra 6 meter wires and see what we find.

Given the above facts, I will never add a 6 meter add on to the current G3TXQ Broadband Hex Beam design as it is clearly competing with the 17 meter antenna.

However, remember, I am building a three band hex beam with 20, 15 and 10 meters. This antenna won’t have the conflicting third harmonic issues caused by the 17 meter portion. I will explore the reasonableness of add 6 meters to my three band configuration in a future post.

Conclusion

For those of you with the 5 band design, you may well have a workable 6 meter beam an autotune button away from use right now. Give it a try and let us know the results by adding comments to this post.

There is an obvious conflict between an added 6m wire set and the existing 17m wire set which may create a complicated situation which looks “good enough,” but may be not what you expect. I would love to post your far-field 6 meter measurements of relative gain, front-back and front-side ratios. Please, no comments about how the hex beam is impossible to simulate.

Kudoes

I can’t thank G3TXQ enough for his extensive research into the hex beam antenna and his Broadband alternative. I very much look forward to building my contest version leveraging his excellent work.

I stopped by for a visit to discover a very large yard with ample room for all kinds of wires antennas.

One issue my friend has it noise on his current antenna.

While his home is an excellent blank canvas waiting for lots of amateur radio antenna possibilities, I thought a full-wave loop might be a real good next-antenna to try.

Which full-wave loop to try first: 40 meter, 80 meter, 160 meter, more?

Google Earth comes to our aid with the excellent program.

Here is a view of my friend’s home…

Using the excellent tools in Google Earth I added two notional loop possiblities: a red line loop as large as the yard, and a 160 meter full wave loop in white. Isn’t this a nice big yard?

For the while loop I included the feed line as part of the total loop length to ensure a reasonable loading appears at the feed point in the house.

Using Google’s measuring tools I confirmed the white loop’s length to be around 525 feet or so including both sides of the transmission line. The red loop is about 680 feet total.

Simulations reveal the usual big lobes at higher frequencies and good behavior at low frequencies.

The big question is do we take full advantage of the yard and make the great big loop, or do we stick with a 160 meter full wave loop and live with that?

Anyone have any thoughts?

He went shopping online only to find many vendors are low in stock of many items including the 400ish ohm windowed ladder line. We both agree making our own is a viable option.

So here is a photographic step by step we took in 2005 to construct the spreaders of our ladder line fed dipole antenna we use for Field Days, JOTA and other events requiring a simple antenna.

Since our antenna is used for various events it is not in the weather long. We opted to use wood spaces soaked in paraffin.

As mentioned above, modern weather proofing methods exist to make the paraffin method obsolete. However, our antenna is holding up well after many years of off and on use. I don’t know how wax holds up to real outside year round use.

The one thing I would not skimp on is the drying process in the oven. If your wood spreaders aren’t very dry, they can and will cause some conductance between your transmission line wires. While it may be true the effect is minimal, this drying step is easy enough to just go ahead and complete.

Ladder line is cool stuff. It works very well. Just be sure to have a tuner with a balanced output or, even better, a fully balanced matcher available from MFJ and other manufactures.

They have just two antennas: a two meter omni and a ladder line fed 135 foot dipole. The dipole is inverted V style atop 12 sections of 48 inch aluminum military mast. The top of the antenna is about 54 feet AGL. Coupled with a good balanced tuner, this thing can work quite well on most of the bands.

The first contact from Virginia was with Japan on 20 meters phone. Not too shabby for such a simple antenna.

Tomorrow we will give it the big test. I know it will work fine because it has already proven its worth during many field days.

If the only antenna you have it a ladder line fed dipole, put it up and go for it.

This post is very late. The actual date of the events within it are just before March 2009 in preparation for the Virginia QSO Party.

In the many posts within this site, it is no secret my examination of various vertical antenna solutions with comparison between BigIR and the 43 Foot products a big part of this. Check out all the 43 Foot posts on HHD here…

43 Foot Antenna Selected
I chose the DX Engineering 43 Foot quick taper model during the price war between DXE and Zero Five. Compromises understood and simulations complete, I carefully considered the Big IR, but liked the idea of no moving parts or switches at the antenna location.

Back in Black
A bare aluminum antenna would stand out too much in my neighborhood so I decided to paint the antenna black. Priming aluminum is a topic I researched very carefully to be sure I applied a lasting finish. As shown below, I applied regular flat black paint over the primed aluminum pieces except where the pieces slide into each other; If you are going to paint your antenna, you need to mask the mating four or five inches of the lower end of each tube.

Conductive Compound
I used the recommended aluminum conductive anti-oxidant compound suggested by DXE on all physically and electrically mating aluminum surfaces. This results is a gooey mess the first few times you apply, but you get the hang of the proper amount soon.

Antenna Tilt Base – Cumbersome
The “kit” from DX Engineering included an Antenna Tilt Base which I gladly installed. It works great. However, you need to really watch what you are doing when lowering the antenna. If you do not seat the lower pivot bolt properly, you will bind it and/or the top sliding bolt and may simply cut it off.

You are well advised to pay very close attention to what is happening to your tilt base while moving it. If you cannot handle the antenna yourself, enlist the aid of a friend who takes direction well. The good news is you will get the hang of raising and lowering your antenna as you realize you will need to apply some downward pressure during transition.

Basically, you will need to hold the antenna nowhere near its center of gravity and manipulate it just as if it was not attached to anything; You and your two widely spaced hands are the antenna support, not the tilt base pivot point. This ensures you are able to “encourage” the tilt base bolts into the correct slots and keep them there during lowering. The 43 foot antenna is not too heavy, but, in my opinion is very cumbersome due to its length and floppy nature.

Photo Gallery

Radials
You probably noticed only eight or so radials on this 43 foot antenna system. Yes, I know… I need more and plan on installing more soon. On 20 and 40 meters, the feed point impedance is something north of 200 ohms (or something highish). It does not take many radials to create a ground resistance lower than that. However, 60, 80 and 160 meters are a different story. I will add more radials.

Performance – Good enough
With all the simulations I performed in EZNEC…

…I was not expecting more out of this antenna than possible. I knew it would be pointless for 10 and 15 meters. It knew it would provide good results for 20 meters. It would be close to a 1/4 wave 40 meter vertical. It would provide some results on 80 meters. It would stink on 160, yet still provide “something” to use for this difficult band.

In short, it has met all my expectations quite well. I now have one antenna that provides access to the major low sunspot HF bands while sacrificing 15, 12 and 10 meters during this sunspot low. This was the plan for the 2009 Virginia QSO Party and that’s exactly how it worked out.

Comparisons
I had a 75 meter turnstile antenna in addition to this 43 footer during the March 2009 QSO Party. The turnstile out performed the 43 foot vertical in almost every way on 80/75 meters. However, I did use the vertical to pull out one of our roving mobiles about 250 miles away during the QSO Party – I transmitted to him on the turnstile and received on the vertical; I would not have been able to work him without the vertical in the mix. For those wondering, I use the 8 port DX Engineering remote antenna switch – the smaller one – at the end of 240 feet of LMR-400 coax for my ‘antenna yard’ switcher.

40 meters was a bit of a toss up. Most of the time I used the vertical. However, I occasionally tried the 80 meter turnstile on 40 and it did work.

20 meters was tough due to a big DX contest the same weekend as the Virginia QSO Party. I had a tough time competing with the big guns. However, casual operating later revealed the 43 foot vertical does work pretty well on 20 meters.

Attempts at 15 and 10 meter contacts with the vertical are laughable. Zero success, but that’s not a surprise, thus I am not worried about it.

Conclusion
I am very happy to have this antenna. Except for certain times of the year, it is the only HF antenna I have available operating year round. For a single simple antenna (remember no moving parts), it provides at least some coverage on 160-20 meters with best results on 40 and 20 meters. Good enough for the moment.

However…

Those sunspots are coming back and I will lust after a solution for 17-10 meters very soon. My next project will likely be the hex beam as a simple solution for those bands. I will need some kind of support for this antenna which adds to the complexity.

When you compare the steps required to raise even a modest antenna like the hex beam, it is quite obvious no antenna solution will be as simple as the 43 foot vertical even after I took the time to paint it black.

The 43 foot antenna provides, for me at least, a very good value for the money, real-estate and simplicity. I would do it again and humbly suggest it to anyone who needs a single, simple, reliable, stealthy antenna and/or wants an additional “choice” for their antenna farm.

See you on the bands…
73

Ever since the 2009 QST article many folks, myself included, became aware of the simple elegance of the Hexagonal Beam (also known as Hexbeam generally and Hex-Beam® by Traffie Technology).

Indeed the simulated and reported performance of these Hex Beam antennas appears to be quite reasonable for the size. The “hex-beam” group on Yahoo has been buzzing away with discussion.

Originally an organization called “Traffie Technology” offered Hex-Beam products.

Another vendor came along with parts and kits for DIY hex beams at HexKit.com.

Then the QST article in early 2009, written by K4KIO, highlighted a different topology inspired by G3TXQ where the size of the frame and arrangement of the wires differ from the classic Hex-Beam to yield more broadband performance on each of the 20-10 meter bands with the burden of slightly larger size.

Each Hexbeam “style” is covered in superb detail at G3TXQ’s web site…

So the hex beam nuts had two choices of hex beam products: The Classic Hexbeam from Traffie Technologies and the Broadband Hexbeam created by G3TXQ and offered by K4KIO.

One of my favorite amateur radio companies, DX Engineering, came out with their own kit version of the broadband hex beam design called the Hexxagonal Beam.

So now we have four major suppliers of hexbeam kits and/or parts…

• Traffie Technology with the HEX-BEAM®
• HexKit
• K4KIO with the G3TXQ Broad band Hexagonal Beam
• DX Engineering with their HEXXAGONAL BEAM (a version of the G3TXQ)

The various names are humorous. Only Traffie Technology has a registered trade mark on HEX-BEAM®. DX Engineering is trying to claim trade mark on HEXX and HEXXOGONAL using Copyrights which is a bit bizarre. K4KIO discusses trademarks/copyrights on his site very briefly. Two of the three web sites reveal a surprising lack of understanding of what Copyrights and Trademarks are. Oh well.

With four competitors in the Hex Beam supply biz, I suggest the Hex Beam has arrived. These companies know you are out there and are competing for your antenna dollars. Traffie Technology offers the original smaller hex beam where both the driver and reflector are in the shape of a W; Traffie’s web site has a tech section which may suggest the Traffie version is not the classic, but something slightly different and more broad banded. K4KIO has the terrific QST article bringing attention to the G3TXQ broadband design. DX Engineering provides an excellent engineering background to the G3TXQ design stemming from their years of many well engineered antenna products; They have a spiffy new approach to the center base plate.

Look at each of their web sites and you will see the points they make in an attempt to win you over. This has better engineering… This has fewer parts… This has longer history… etc.

Note about Antenna Simulations
Note that some of the suppliers above, and many amateur radio antenna manufacturers in general, argue their antenna designs are not possible to simulate correctly in today’s NEC and other programs. Traffie mentions this on his tech page. Arrow Antennas told me the same thing for something as simple as their 440 MHz Yagi beam claiming no simulation works for their antenna.

Simulation has its limits and the lower cost versions of the available NEC programs often don’t have enough elements to properly model up something like the hex beam antenna, but it can be done with the more capable versions.

If an antenna manufacturer suggests simulations don’t match reality they either don’t understand reality, don’t understand simulations or are hiding something. No modern antenna engineering is performed without simulations (and of course real testing) these days. Antennas far more complex than any seen in amateur circles, including the hex beam, are successfully designed on the computer first with great success. Any manufacturer thinking their design can fool a competently modeled simulation of same is suspicious.

This does NOT mean these manufacturers don’t produce good quality results as hard core experimentation can and does work. Edison was weak on theory, but strong on trying everything experimentally; Edison experimented his way to success on several projects. However, Tesla was strong on theory. Tesla’s AC power system won the battle against Edison’s DC power system even before the first wire was laid down.

The point is Antenna Engineers howl at the idea any amateur antenna has supernatural powers that thwart decades old simulation abilities.

Its all good news though
The good news is the Hex Beam directional antenna in almost any form gives good performance in a compact package. In the end, we are the real winners. Go get you one…

“Why don’t hams like vertical dipole antennas.

See:

http://i31.tinypic.com/160udqb.gif

Elevated Vertical Dipole

Best regards.
Tom
KE6YNH, 73
San Diego, CA

However, there appears to be some kind of mystical attraction to the 5/8 wave radiator. Several of the J-Pole designs attempt to make the radiator 5/8 wave in length and adjust the phasing stub to make for a good match.

Let’s compare the two approaches using the models available from the late Larry Cebik’s NEC collection. Here are the contenders each using 3/8 inch diameter copper pipe…

1/2 Wave (left) vs. 5/8 Wave J-Pole

The current magnitudes resulting from the simulation are shown to reveal just how current flows in the conductors and, just as important, their polarity.

J-Pole fanciers already understand the 1/4 wave stub in the bottom part of the antenna, the J, have roughly identical and opposite currents which tend to cancel any radiation effects. This is very similar to ladder line when and if the currents are equal and opposite. Of course, where the 1/4 wave stub meets the bottom of the 1/2 wave radiating element current is not zero or no power would travel up to the radiator. This results in a slight imbalance in two currents.

The plot below shows the azimuth plot of the signals from both antennas. The 1/2 wave plot is at about 2.8 degrees above the horizon while the 5/8 wave peaks around 2.6 degrees. Each antenna is simulated with their base 360 inches over real ground.

1/2 vs. 5/8 Wave J-Pole Azimuth Plot

Both antennas exhibit asymmetry due to their 1/4 wave phasing stub small imbalance. The 1/2 wave J-Pole beats the 5/8 wave J-Pole by about 2 dB.

Here is the elevation plot of both antennas along the worst case azimuth bearing…

1/2 vs. 5/8 Wave J-Pole Elevation Plot

The asymmetry of a J-Pole antenna really comes to light here with a clear bias towards one side of the antenna. Note the 5/8 wave J-Pole has more energy at higher elevations at a cost of energy on the horizon where we want it.

So what are we to think about the extra copper and less balanced approach offered by the 5/8ths Wave J-Pole antenna?

My conclusion is you are better off sticking with tradition and build the 1/2 Wave J-Pole antenna.

A reasonable question is why isn’t 5/8 wave better. 5/8 wave is about the limit a vertical radiator can be to peak up gain towards the horizon. However, it assumes you have a very robust ground system to work against. A J-Pole has no such ground plane.

Another problem comes from the fact the 5/8 wave j-pole has that unbalanced current in the J part of the antenna. Thus the length of the antenna, electrically, is really something other than 5/8 of a wavelength. Such is not the case with a 5/8 whip over a good ground system.

5/8 Wave antennas have their place, but the term is used far too loosly in amateur radio circles. The J-Pole is one good example where deviating from tradition yields a functional antenna, but one with less performance than the simpler of the two antennas.

If you are considering building a simple J-Pole great! Build a 1/2 Wave J-Pole and don’t forget the balun.

Turnstile antennas are provide circular polarization straight up and at large angles from zenith. Originally I was going to just connect the two antennas together at my switch so I could select one, the other or both. The electrical delay would be provided by one longer feed line. However, one attribute considered essential by the scarce resources on the Internet insisted on switchable polarization: RHCP to LHCP and back by simple inverting the connection to ONE of the antenna feed lines.

With these thoughts I changed the plan a bit. Instead of using my switch and two very long feed lines, I now have one single feed line from the switch to the base of my 50 foot high antenna mast. Here I will provide a relay box with three relays, one coaxial cable in and two coaxial cables out. The two output coaxial cables will each feed one 75 meter dipole on the antenna mast. One cable is ninety electrical degrees longer than the other.

Here is a schematic of the relay box…

Schematic Diagram for Relay Controller

All this will be going into a weather proof enclosure with cables directly connected to the relays where noted.

What about perfect matching John?

I am not too worried about the perfect 1:1 SWR for this setup. In fact I don’t worry much about SWR under 3:1. The impedance presented to the radio side coaxial cable will be the two antennas in parallel although I admit I am not quite sure how these signals combine if they are ninety degrees different in phase. I forgot to look at that in EZNEC. I do remember it was not too high.

In another post I will show some photos of the resulting assembly.

In particular one member has a magnificent full wave 160 meter loop up around 50 feet or so. He is contemplating using it for NVIS on 80 meters. He desires to lower it to about 15 feet to improve the NVIS characteristics.

It is true lowering a dipole will focus more energy straight up while reducing the energy towards the horizon. This is a tried and true technique on 80 and sometimes 40 meter NVIS and offers a potential added benefit of less sensitivity to far away thunderstorm noise. This is a method of diminishing returns; Lowering the antenna favors the sky more, but the overall gain is reduced. In other words, less signal is focused in a better NVIS favoring pattern.

Full wave loops are quite different as this EZNEC simulation suggests. Here is a simple four sided loop with 128 foot sides and fed near one corner – just like my friend’s 160 meter loop.

It is possible almost any antenna sends a little energy towards the sky for NVIS communications, but to fully take advantage of this capability, your antenna design should flood the sky with energy including straight up. This will ensure all possible angles of refraction/reflection are covered.

It seems clear from this very basic, but perfectly reasonable, EZNEC simulation a full wave 160 meter loop running in the 80 meter band is a good antenna for DX work, but leaves much to be desired for NVIS even if you lower it towards the ground like you do with a dipole. At 40 meters NVIS performance is even worse, but WOW the DX capabilities are nice even while the azimuth pattern is a four lobed cloverleaf restricting this nice low-angle gain to four general directions.

The proof of a 160 meter full wave loop antenna’s DX performance is highlighted by my friend’s achievement with his; He just finished 80 meter WAS… Phone… QRP… using, in part, this antenna. Wow.

…But I want NVIS
The practical solution is to keep your 160 meter antenna where it is and add a new simple 80 meter dipole 50 feet or less in height. This pair will team up nicely to get you want you need for 80 meter NVIS in addition to DX capabilities. Keep the 80 meter dipole well away from the 160 meter loop.

The sister web site to Ham Help Desk published several simulations of dipole discussing 40 meters and NVIS at…

Dipoles that are not too high are a quick answer for NVIS.

Other Techniques
Quadrature feeding two orthogonal dipoles, as discussed elsewhere on Ham Help Desk, yields circular and selectable polarization. This technique leverages what the ionosphere sounding folks use to probe the ionosphere at HF frequencies. This is what I am trying this year for the Virginia QSO Party. We will see how well it works.

Conclusion:
Big Full Wave Loops provide good NVIS at their native full wave frequency, but become more like DX antennas at higher frequencies. Loop antennas provide impressive performance at higher frequencies and are, thus, a great no fuss and simple antenna. However, other than the base full wave frequency, this antenna is less than ideal for NVIS above its base full wave frequency.

The above EZNEC plots detail the difference in gain you can expect with the loop antenna at different heights. For loops it seems higher is better. Other expensive improvements include laying a network of ground wires in a crisscross pattern underneath the loop. The simulations for this are not shown, but do reveal a real benefit in the total energy performance. I don’t have an unlimited supply of wire… do you? We will try to stick with the more practical antennas here at Ham Help Desk.

If 80 meters NVIS is your goal, consider a dipole or a full wave 80 meter loop rather than a 160 meter loop.

Some have the big bad beam high in the sky with an 80 meter dipole added.

Others have smaller beams and more wire antennas.

Others go mobile on VHF and HF.

Still others go expedition and construct antennas that lend themselves towards portability.

I am considering added a 75 meter turnstile antenna to my station.

My club is very active in the Virginia QSO Party. All members have supplied their logs for analysis with Cabrillo Analyzer resulting in these graphs of QSOs over time categorized by band…

VAQP QSOs 2006

VAQP QSOs 2007

VAQP QSOs 2008

80 meter contacts are shown in dark purple. It is quite obvious the band that brings in the points is King 80.

During our sunspot minimum we figure concentrating on 80 and 40 antennas for VAQP 2009 is essential. 20 and 15 are important too. 10 is almost pointless except for some local contacts.

Last year my 75 meter inverted V worked just fine. The apex was about 50 feet up with the wire tips dropping to within ten feet of the ground. This plus several other antennas worked OK. NVIS was most certainly the rule for 80 and 40 this contest.

For 2009 I am wondering if there is something more I can do with 75 and 80 meters. I read some notes about ionospheric sounding techniques that sometimes uses turnstile antennas to transmit their ping upwards and most always use turnstile dipole sets to receive the reflected ping from the ionized layers.

Is this a technique I can use to improve my 75 meter antenna’s performance?

If I create two inverted v antennas arranged orthogonally in EZNEC, but not quite touching at the top…

75 meter dual inverted V

.

…and feed feed each like a normal dipole with 90 degrees phase difference between them…

…we get…

75 meter CP pattern for turnstile

Research on the Internet reveals little information except for the occasional “Wow, so and so was sure loud with his 75 meter turnstile.” This is enough for me to at least try this.

The ionosphere folks suggest providing a way to switch the handedness of the polarization by simply inverting the connections of one of the feeds. If I have enough time I will put together a relay box that can select one, the other, both in RHCP or both in LHCP. It will be an experiment.

Listen for kx4o during the weekend of March 20 on 80 meters and tell me if you think my turnstile antenna is worth the effort.