Summarized, we were trying to set up APRS relay stations up and down the east coast to cover and service all points along the Appalachian Trail and surrounding areas. Some RF link analysis was performed and is available for viewing at http://www.packetradio.net/events/AT_Golden_Packet_Event/ using the remarkable Radio Mobile program and A LOT of SRTM elevation data.

Of the fourteen stations planned several points were accessible only by foot. The White Rock Cliffs station was no exception.

So an all-call was put out for fit hams to assist in the planning and execution of placing a temporary APRS digipeater system on the cliff face of White Rock Cliffs to yield the northbound and southbound RF links. Some interest was expressed, but, in the end, kx4o and kj4faj were the only two hams to volunteer.

Preparations for Station 6 are outlined at http://www.packetradio.net/events/AT_Golden_Packet_Event/stations/station_06.html. Highlights include:

• Enlist the aid of non-ham hikers to build up the team to safe levels
• Get a trail map of the area
• Obtain a Kenwood TM-700 or 710 APRS VHF/UHF radio
• Build a cool antenna that has some gain and can be hand carried
• Get some modern backpacks with water delivery systems

More Hikers

Several folks at work seemed to be in good shape and expressed interest in outdoor activities. I asked two who are new to the area if they would like to hike and learn about trails west of the Northern Virginia area where we live. Two accepted the challenge: Rob Searle and Jay Gundlach.

Trail Map

White Rock Cliffs is at the end of a short pipe-stem trail off the “Big Blue” trail on the border between West Virginia and Virginia. I am just as unfamiliar with this trail system as my new hiker recruits. I did not hesitate to purchase the trail Map F authored by the PATC.

Obtain an APRS Radio

Thankfully, the Golden Packet organizer loaned me a radio for this event. I like it so much that I am now in the market for a TM-710A.

Portable Antenna with Gain

The Golden Packet event is unique in that each station has to focus their efforts on just two directions: link to the next station north and a link to the next station south. Some are using two beam antennas with a power splitter/combiner. Since we had to hike our equipment to the White Rock Cliff’s site, I wanted to concentrate on a simpler omnidirectional antenna with engineering to realize higher gain in all directions. Several approaches were considered:

• Simple quarter wave GP or half-wave J-Pole – Discussed here
• 5/8ths wave J-Pole – Compared here
• Collinear J-Pole – Compared here
• Collinear 5/8ths wave J-Pole – more copper than it is worth
• 5/4 Wave Vertical – Debunked here
• Rubber Ducky Antenna – Not really considered, but is certainly an option

In the end the collinear half-wave J-Pole was the one that simulated best.

EZNEC Currents in 2M 1/2wave J-Pole

I am thankful for programs like EZNEC that allow one to get a rough order look at new antenna topologies and quit guessing. There seems to be a popular notion antennas that provide a good match to the coax for low SWR perform quite well as a radiator. If you have not already noticed, dispelling this myth is a chief goal of Ham Help Desk.

Anyway, as you look at the Half-Wave Collinear J-Pole antenna in the right figure you can see how the lengths were adjusted until the peak currents were centered in each of the two vertical sections. It is a bit hard to see, but sections #1 and #5 form the two half-wave radiation conductors. By the way, all of these conductors are simulated with 1/2 copper pipe – the material used for this antenna. The bottom sections, #6,7, and 8, form the J of the J-Pole and are essentially a half-wave total length. Sections #2, 3 and 4, form the all important 180° delay section to ensure the top half of the antenna radiates in phase with the bottom (rather than out of phase like the debunked 5/4 Wave antenna).

A future post will go into more detail about this antenna. For now, know that we procrastinated and did nothing for this antenna until the very weekend of the Golden Packet event (a Sunday). All parts were purchased on Saturday morning, pipe cutting and soldering performed on Saturday night, test assembly and tuning done on Sunday morning (real early) and hauled untested to the mountain top Sunday midday. Folks, this is not the way to live your life, but EZNEC predictions seemed to work just fine.

Collinear J-Pole

All the antenna pieces breakdown into backpack size items with the exception of the two radiator elements. These two pieces are carried by hand with ease by the hikers. If desired, the radiator sections can be split with a coupler soldered to one of the pipes to allow all pipes to fit inside a backpack. I decided against this for now as I was not sure how flimsy this contraption would be on a windy mountain top.

Did we really need a complicated antenna with a view like this?

Antenna's view south

KJ4FAJ 'Texting' to GDHill (#8)

Here are kx4o and kj4faj learning how to use the Kenwood TM-700A radio…

KX4O & KJ4FAJ

Rob Searle, the most athletic of our foursome, poses with the antenna…

Rob with Collinear J-Pole

Kudos

“Thank you” cannot be said often enough for the help provided by Rob and Jay during this Golden Packet hiking “mission.” They both are clearly active people. They and my son are in far better shape than me as they often would climb the hill at a faster pace that I could possibly hold. It worked out well though. Jay provided help with navigation using the essential Map from PATC, Rob hauled my pack part of the way up the hill giving me a needed break and both located rock overhang cover in case we needed to hide from the nearby thunderstorms.

Jay Gundlach (bottom) and Rob Searle - Essential Crew atop White Rock Cliffs

Thanks also to the SkyWarn Network working out of the 147.300 repeater who, as soon as they knew we were near local thunderstorms, provided updates to alert us of impending danger. We had lightning nearby, but it never came… too close?!?!?! We were ready to dive into the rock shelter located by Jay and Rob at a moment’s notice.

More Pictures

The view towards Maryland - can you see the upper half of the antenna?

The White Rock Cliffs Golden Packet Crew 2009

Jay, Rob and KJ4FAJ resting during descent - they were actually waiting for me to catch up.

Final Thoughts

• The PATC Trail Map was essential to get us on the right path in the beginning of our ascent – well worth the $6 plus shipping.
• Having “real” hikers as part of the group was essential to bring proper experience to the crew.
• I busted my butt in the Gym the last two months preparing for this, but still have a long way to go when I compare myself to the in shape hiking peers.
• I made several 146.520 MHz simplex contacts plus hit my Warrenton home-town repeater with ease.
• I contacted Bob Bruninga, the organizer of the Golden Packet Test, on two different repeaters and I got confused at first because I was, at first, barely hearing one of the repeaters. I quickly realized I was hearing another repeater on the same frequency much farther away.
• My son and I purchased new backpacks for this event. The new packs offer a place to store a good supply of water fed to the hiker via a convenient tube. Nice.
• One 7 amp-hour gel cell powered the whole event just fine. I was skeptical at first, but am now a believer. We brought two 7 AH cells just in case, but did not need the second one.
• I am reasonably happy with the Hiker Collinear J-Pole, especially after the short build time, but hope to refine it a bit. That’s for another post on HHD.

This was a fun event and I hope to repeat it again next year.

John
de kx4o
Future non-fat ham.

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.

One response included a link to this web page showcasing a collinear J-Pole antenna using two 5/8ths wave antenna elements.

As soon as I saw the site I thought, “Oh no… not another 5/8th wave antenna discovery.” However, to my surprise (and very much unlike the regular 5/8ths J-pole which does not work well at all) the two 5/8ths sections yielded a reasonably symmetrical pattern in both free-space and over real ground at a similar height. Feeding issues aside, at least this design passes the threshold of physics.

So let’s compare the relative merits of the 5/8ths collinear J-Pole by first introducing the contenders…

Three J-Poles for this Simulation

I added a regular J-Pole to compare each collinear design against.

The free-space simulation, below, of the buck0 design does show a high takeoff angle compared with a regular J-Pole and a double 1/2 wave collinear J-Pole often called the Super J-Pole.

Regular and Collinear 1/2 and 5/8 wave antenna patterns.

Freespace EZNEC simulations are often practical, but what we care most about is real-world, just above the Earth, simulations. Below are the same three antennas with their bases about 360 inches above real ground in EZNEC…

Three different J-Poles over real Earth

This is more like it. Note the collinear 5/8 wave J-Pole does, indeed, perform about as well as a regular J-Pole in these circumstances at this particular azimuth. The half-wave collinear J-Pole beats out both antennas by about 2 dB. Here is a closeup of the lobes on the right…

Close up of EZNEC J-Pole Lobes

The buck0 5/8ths wave collinear J-Pole does perform. However, if I take the same #14 wire, use the same cool construction techniques, but make a traditional 1/2 wave collinear J-Pole with the feed-stub, a half-wave section, a quarter wave stub topped off with a final half-wave section, the antenna is a good 2 dB stronger than the double 5/8 j-pole from buck0 in over-Earth simulations at about 3 degree elevations in all directions.

Plus if you build a regular J-Pole with #14 wire you will do about as well as the more complex buck0 design.

Less wire… simpler feed… more gain… who knew.

At least the Collinear 5/8 Wave J-Pole works, but it seems clear with the admittedly simple EZNEC simulations above, your wire investment is better spent on the simple traditional 2m meter J-Pole or the Collinear 1/2 Wave (Super) J-Pole.

Thus, the 2 meter 5/4 wave antenna is bunk… or is it?

That term 5/4 wavelength keeps popping up in antenna discussions which seems to suggest it has some mystical power to achieve gain. Could this be true? Let’s look at some possible thoughts on this particular length:

• The Extended double Zepp antenna is about 5/4 wavelengths long
• Two 5/8 wave antennas put butt to butt are about 5/4 wavelengths long
• Collinear element spacing can result in a total length of around 5/4 wavelength
• Hmmm…

One common theme among these successful longer-than-a-wavelength systems is they are all fed in the center, not at the end.

Let’s compare center feeding vs. end feeding a 5/4 piece of wire.

First let me preface this simulation with some points…

• This model is very very simplistic
• It does not model the transmission line – it should for the complete picture
• It does not include a matching system at all – a real antenna would
• This is just a simulation of putting a power source at the middle or the end of a 5/4 piece of wire above real ground about 20 feet or so and that’s all
• You should consider getting your own copy of EZNEC or equiv. when you are ready to model your complete antenna system – transmission lines and all

So this very simple simulation takes a vertical piece of wire and puts a source at the bottom end or in the middle to compare what happens to the signal plot above real ground.

This view reveals the amazing difference in current distribution for each…

End Fed vs Center Fed 5/4 Wave Antenna

If you are doing this in your own copy of EZNEC remember to select Current Phase from the View Antenna Attributes so you can study the current phase, not just the magnitude.

Note how the End Fed 5/4 wave antenna has two large opposing phases of current. The Center Fed 5/4 wave also has two large current nodes, but they are the same phase… just like a good Collinear Antenna.

If you do not understand why the current phase changes when a long wire is energized with RF put the ARRL Antenna book on your Christmas wish list. It is superb at explaining this.

If you pick up a copy of the ARRL Antenna book and read about collinear antennas, you will find a chart suggesting optimum gain vs. separation of the half-wave elements. You will see the center fed 5/4 wave antenna produces the two primary half-wave in-phase elements at a desirable spacing. The ARRL Antenna book is a must read for any antenna experimenters and designers.

Let’s plot the Center Fed 5/4 Wave Antenna first…

RF of Center Fed 5/4 Wave Antenna

8dBi or so and at a nice desirable low angle perfect for VHF communications.

8dBi??? How can a non-beam antenna have such high gain. Well, remember we are simulating this antenna over real ground and real ground will reflect your signal leading to a maximum towards the horizon. If you simulated this design with “Free Space” selected you would get the more typical 3dB or so gain figures. Because real world situations are important, I usually default my simulations over real ground to ensure I don’t miss something basic. Free Space or over real ground, the point of this article is to highlight the difference in performance between the end-fed and center-fed 5/4 wavelength antenna. For those still wondering, yes a beam will produce still more gain than 8dBi.

Now for the End Fed 5/4 Wave Antenna…

End Fed 5/4 Wave Antenna

Hurl.

The 5/4 wave antenna up about 20 feet at the base and fed at the end produces an almost 7dBi gain, but 45 degrees above the horizon – hardly useful. This high angle gain comes at the expense of the low angle RF which is still more than 1 dBi. It is possible all the hype over 5/4 wave antennas is reinforced by the fact they will still work with weak gain tricking the user into thinking it is a superior antenna just because it has 5/4 wave in its title.

Why is the same antenna so different depending on where we feed it?

The answer lies in how we manage the two primary half-wave current nodes in the antenna.

If we feed the antenna in the center with, say, a balanced transmission line, one line pushes current while the other pulls current which results in the current direction being the same in the two dominate half-wave current nodes at the top and bottom of the antenna.

On the other hand, if we manage to feed the end-fed antenna at the bottom with the same balanced feed (one line connected to the antenna, the other to some sort of counterpoise) the current phase switches every half wavelength resulting in opposite current nodes.

The only way to fix this issue is to feed the antenna in the center or introduce a half-wave 180 degree delay element between the two half-wave current nodes as is done in “double J-Poles” and other Collinear antenna designs.

Comparing both antenna patterns…

End Fed vs. Center Fed 5/4 Wave Antenna

The vertical end-fed 5/4 wavelength antenna may actually work somewhat for VHF use, but is hardly an efficient use of antenna resources.

If you are going to put all the effort into a large cumbersome antenna design you are well advised to:

• Read the ARRL Antenna Book
• Model your antenna in EZNEC – a limited free version is available
• Not believe any web site that promotes amazing performance with any single antenna element longer than 5/8 wavelength
• Model your transmission line, too, when doing your final simulations – you will be surprised how it affects your signal
• Great SWR is great, but has little to do with antenna patterns

Antenna experimentation and design is a fascinating part of our Amateur Radio hobby and well worth the time and expense. However, you will find far more enjoyment in this endeavor if you read up on antenna theory at least a little first and then try your designs in the free version of EZNEC second. This will help you avoid the woodchuck mistakes leading to far more enjoyment of your hobby.

It may not be intuitive at first, but the answer, at least from an antenna pattern standpoint, seems to offer some hope.

It is difficult to calculate the feed-point impedance of a J-Pole perfectly since it is a tapped system. However, the calculations I ran for 430 MHz suggest it is somewhat close to a good match.

Here is the same J-Pole from Cebik’s collection with 146 and 430 MHz…

2 meter j-pole at 146 and 430 MHz.

The currents on the long vertical section act a bit like a long wire pointing straight up as shown in this comparison plot…

2m J-Pole Plot at 146 and 430 MHz

The pattern for 430 MHz, shown in green, is wild and certainly suggests this is not the ideal antenna for the job. However, if you look closely at the low elevation detail…

Close up of 2m J-Pole at 146 and 430 MHz.

…you will see a reasonable lobe of power at a low angle.

This simulation was done with the antenna at 300 AGL by the way.

So can you use a 2 meter J-Pole at 430 MHz and expect reasonable results. EZNEC suggests a cautious yes. However, be real sure about your SWR before you try this. It might not be what you expect.

Unfortunately, one web site promoting a 5/4 wave antenna solution failed to realize you can’t make an antenna longer without eventually hitting a limit.

During commute time repeater discussions my friend was contemplating the use of a 5/4 wavelength vertical for his VHF antenna. He mentioned a web site with the construction details for just such an antenna which results in a nice simple no fuss vertical antenna housed in a PVC enclosure. Great, I thought, but I had some concerns over this whole 5/4 wavelength thing. I told my friend I would work up a simulation to see how good this antenna is. The first step was to find the web site containing the construction details for a 5/4 wave VHF antenna. This was easy resulting in this web site…

The author provides superb details on how to construct the 5/4 wavelength VHF antenna. He wisely predicted the installation into PVC would change the speed of light of conductors within. He revealed the need for a matching network. Good so far. In fact, I bet this antenna design provides a good match to 50 ohm coaxial cable, is of sound construction and will last many years in the elements.

However, that’s only part of the story isn’t it. How will the antenna actually perform.

EZNEC to the rescue…

While it is time consuming to simulate all the wire size and dielectric constant details the candidate 5/4 wave antenna offers, we can make the following assumptions and model accordingly…

• Since there is a parallel line component about 1/4 wavelength long, this is really an end fed full wavelength antenna with the extra 1/4 wave portion acting like an impedance transformer just like a J-Pole
• The existing models of J-Pole antennas from Cebik’s excellent NEC antenna simulation collection provide just what we need to start analyzing the full-wave antenna
• The 5/4 wave “tall” antenna will be compared directly to the 3/4 wave “tall” J-Pole

Both antennas are modeled with 18 AWG wire, used by the web site author for most of the vertical element, with the horizontal portion 300 inches over EZNEC’s “Real/High Accuracy” ground. Here they are with their RF currents shown…

Half vs. Full Wave Antennas

Those in the know will already spot trouble with the full wave radiating element. Those RF currents do not result in more RF radiation in the horizontal plane, but rather help to cancel it. For those of you experimenting with EZNEC be sure “Current Phase” is selected in the View Antenna options so you will see the vector, not just the magnitude, of the currents on the antenna view. You also need to ensure all your wires go in the same direction with end 2 connecting to end 1 of the next wire or the current phase might show incorrect vectors; The RF simulation plots work perfectly with wire direction either way, however.

Here is the full wave plot in red compared with a simple half wave J-pole in blue…

Elevation Plots of Half vs. Full Wave Vertical Antennas

Recall both antennas are simulated with the base 300 inches above ground with the following discoveries:

• The full-wave vertical is at least 7dB worse than the regular J-Pole at 4 degrees elevation – important for base to mobile and most any VHF communications
• The full-wave vertical shows negative 1.67 dBi gain at the terrain hugging low angles
• The full-wave does provide move energy at high angles which might be of benefit for base to air communications

I ran SWR calculations for both antennas and they both offer an excellent match to 50 ohm cable throughout the 2 meter band. For completeness here they are…

End Fed Half Wave (J-Pole) SWR

End Fed Full-Wave SWR (with J-Pole Feed)

What are we to draw from this?

First, great SWR does not a good antenna make.

Second, the full wave antenna does not, by itself, provide any benefit for terrestrial radio use and is, in fact, detrimental.

One can ask, though, how is it taller antenna designs provide better radiation towards the horizon.

The answer is based on the idea of making the RF current peaks in the antenna be in the same phase so the energy, towards the horizon, adds rather than subtracts. Many methods exist to achieve this, but one popular technique is to add a half-wave delay between the two half-wave antenna portions. By doing this we cause the current in the top antenna to be a full 360 degree delayed from the current in the bottom and, thus in phase. This technique results in a type of antenna generally called “Collinear.” The figure below illustrates how this is achieved in the popular Double J-Pole antenna…

Full-Wave J-Pole with Hairpin to make Collinear

Plenty of web sites exist to show how to build a Double J-Pole antenna so we won’t go into that here.

The UHF antenna on your vehicle may well have two or three antenna sections arranged as a collinear antenna with half-wave delay coils between each section to keep each radiating in phase. If we look at the evolution of the vertical antenna…

Evolution of Vertical Antenna

…we see 5/8 wave antennas tend to be the upper limit of length before phasing techniques need to be applied if we are to keep a good signal towards the horizon. Collinear antennas are a tried and true approach for antennas longer than 5/8 wave. Whether a hairpin or helical resonator is used to phase the 1/2 wave antenna “pieces,” improved performance awaits the antenna builder. As we any good thing there are diminishing returns by adding more co-phased antenna pieces.

The 5/4 wave antenna discussed is really a full-wave antenna with a 1/4 wave feed. The lack of an additional half-wave delay element between the two high current portions of the antenna suggest the author has missed an important detail in antenna design which will render the antenna far less useful than anticipated.

It is likely the author confuses his desire for a single wire antenna with dipoles of similar length – the extended double Zepp type is an example. The difference between an end fed piece of wire and a center fed dipole is, again, the current phases. A center fed dipole pushes current in one wire while pulling on the other thereby ensuring each dipole leg has current in phase – assuming the dipole legs diverge from the feed-point of course. This cannot happen in an end fed single wire one wavelength or 5/4 wavelength long without some means to delay the half-wave portions by 180 degrees.

However, the author’s noble attempt at antenna design and excellent construction techniques remind us there are other ways to apply his ideas to realize the suspected intent; A desire for a good sturdy Collinear antenna.

A final note worth considering is this… Can EZNEC or any antenna simulation program provide good enough results to use for antenna comparisons? Of course, but there are limits. If you are trying to see if one antenna is a dB or so better or worse than another you do need to be careful with the assumptions you put into your simulation. However, comparing a half-wave antenna against a full-wave antenna, both end fed with a 1/4 wave section, is well outside almost any margin of error so you can expect the full-wave antenna antenna to be far worse than the half-wave for horizon coverage based on the simulations above.