Anatomy of a low frequency aviation radio beacon

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Hammy eyes

Has the ever happened to you?

Imagine driving through the countryside with the XYL where she admires the bucolic farmland passing by…

Bucolic Culpeper County, Virginia

Alas your XYL is the long suffering wife of an amateur radio operator with hammy vision superpowers…

Distant view of Culpeper MSQ Low Frequency Beacon Transmitter.

“Hmm, what is this” you murmur to yourself.  You turn right onto a gravel road leading to a vineyard, but by sheer coincidence you pass by your real quarry…

View of entire tower and transmitting electronics of Culpeper MSQ beacon.

“Okay honey, I will be just a few minutes having a good look at this thing” you say with clinched teeth.  You grab your camera with long lens and proceed to study of one of the few remaining LF beacons still in use for aviation.

Culpeper Non Directional Beacon (NDB)

NDB aeronautical chart symbol
NDB aeronautical chart symbol

This navigation aid is five miles south-south-west of Culpeper Regional Airport providing a runway center line fix for inbound aircraft. Airnav.com confirms this beacon is still in use and provides details…

  • Frequency: 351 kHz
  • Morse ID: MSQ
  • Power: 25 watts

Authorities have been decommissioning these beacons aplenty, but this one still is heard where I live 13 miles north.  The car’s AM radio confirms some desense so it clearly has energy in its near-field.

There use to be a beacon on the airport premises, but that is long gone as are a great many others in Virginia.  For whatever reason, MSQ still exists.

Medium Frequency Beacon

In the US, airway beacons operate between 190 and 535 kHz so technically straddle the 1000 m wavelength line between LF and MF bands.  Such long wavelengths present an economic and engineering challenge to the transmitting station designer and builder.  An eyeball survey suggests the tower height at this station is about 100 feet (~30 m).  With this in mind, the design challenge becomes familiar to mobile amateur operators attempting 80 and 40m on their vehicles, but at a much larger scale.

The design challenge for electrically short monopole antennas
The design challenge for electrically short monopole antennas

Compared to the exemplar Marconi 1/4 wave monopole, the NDB station antenna is minute.  As the above height comparison reveals, the beacon facility has a very electrically-short radiator.  At 351 kHz frequency, we’re dealing with a wavelength of over a half mile (.85 km) so any small antenna system that is going to radiate this 25 watts effectively deserves a study.

Starting at the bottom

Transmitting equipment of MSQ low frequency Culpeper aviation beacon.
Transmitting equipment of MSQ low frequency Culpeper aviation beacon.

Other than the guy wire footings, everything is nicely contained within the small fence.  This is for security to be sure, but, as we will see below, safety as well.

Transmitter and insulated tower supports for low frequency MSQ Culpeper beacon.
Transmitter gear and insulated tower supports for low frequency MSQ Culpeper beacon.

My guess is the transmitter gear is housed in the right yellow box next to the ac power feed.  The large diameter conduit connects the transmitter to the left yellow box I assume is nothing more than a passive antenna matching system.

The tower appears to be of simple galvanized steel construction with impressive insulator supports.

Insulated tower supports for low frequency MSQ Culpeper beacon.
Insulated tower supports for low frequency MSQ Culpeper beacon.

How about a closer look.

Insulating tower posts.
Insulating tower posts.

The feedpoint

Have a look at… THIS!!!

Transmitter with its high voltage terminal supply energy to the insulated tower.
Transmitter with its high voltage terminal supplies energy to the insulated tower.

Many of you are probably thinking “This is for just 25 watts!?!”  Indeed an electrically short antenna requires a bit of Smith chart extremes in the matching department that involve high voltages as we will see in the analysis below.  Hams being hams, we focus on details like terminals so let’s do just that.

Transmitter antenna terminal

She’s a beaut. The copper wire gently bends up to the tower connection point where it simply clamps onto the galvanized lattice.

Transmitter wire and insulated tower supports for low frequency MSQ Culpeper beacon.

And that’s it!  Not really all that extravagant.

Hmmm, what about lightning, surge. etc.

The tower is electrically isolated from ground and energized by the copper wire from the yellow box.  Assuming the guying system does not ground the tower, we should wonder how electrical charge buildup or lightning events find a way to earth.  If you were paying attention to the photos above of the insulator terminal, you see a piece of metal behind it. Here are better views…

Feedpoint of MSQ LF Beacon
Feedpoint of MSQ LF Beacon with air gap dissipation.
Air gap ground point at transmitter output

The gap is pretty large at around an inch or so.  I assume some sort of static drain is built into the transmitter/matching system leaving this gap to provide last effort shunting only for, I assume, lightning events.  This is just a guess, but I think a reasonable one.

Warning label near antenna terminal

No kidding: I have little doubt this sign is to be taken seriously.

Going up the tower – insulated guy wires

This tower has three sets of guy wires. The bottom and middle set look like this.

Lower insulated guy wires.

It’s hard to tell in the photo, but the galvanized wire loop around the tower legs ends in a bolt shared with what appears to be non-conductive line.   Here’s a close up of the guy on the right.

Lower insulated guy wires.
Lower insulated guy wires.

The attachment guys are marginally connected electrically, being just wrapped a bit around the tower leg.  However, the remaining length past the bolt appears non conductive.  Breaking up or otherwise ensuring guy wires do not interfere with the operation of an active vertical radiator makes perfect sense.

Top of the tower – non-insulated guy wires

At the top of the tower we find something completely different.

Upper non-insulated guy wires.
Upper non-insulated guy wires.

We see the same galvanized wire wrapping around the tower posts, but there are some extra wires involved. Let’s have a closer look…

Upper non-insulated guy wires.
Upper non-insulated guy wires showing jumpers.

It’s a little difficult to see, but an extra pigtail of galvanized wire is spliced into the main guy and then clamped securely to the tower lattice.  It’s clear the designer meant for these guy wires to have a secure electrical connection to the top of the tower while providing mechanical support.

Antenna top hat

Here’s a view up the tower showing all three sets of guys.

View of tower showing lower and mid insulated guy wires along with connected top guy wires for a top hat.

We see the lower and middle guy wires with a darker guy material plus those transition clamps close to the tower.  The electrically connected galvanized top guy wires go out a bit and appear to terminate at transition points at the red arrows (below).  Assuming these transition points insulate these wires from the remainder of the mechanical guying system, we have three spokes of an antenna top hat.

View of tower showing lower and mid insulated guy wires along with connected top guy wires for a top hat.
View of tower highlighting the end points of the top hat guy wires.

So far all the rules of operating an electrically short antenna are in play with this particular beacon transmitting station.

Transmitter/Matcher/Tower grounding

Copper wires from the yellow boxes and what appears to be numerous radials tie together and clamp to one tower leg.

Transmitter and tower grounding.
Transmitter and tower grounding.

Here is a closer view of that big bundle of assumed radials.

Transmitter and tower grounding to radials.
Transmitter and tower grounding to radials.

That’s about all I can tell from these photos.  I usually see radials head straight into the ground rather than through the weather cap we see here.  Perhaps this protects the radial wires from the weed trimmer making it a bit easier for cleaning crews to do their work.

Guy wire ground termination

Eventually the guy wires re-connect with galvanized wire well away from the tower and terminate at the tie points with the usual grounding system.

Guy wires for “hot” low frequency beacon tower.

Electrical analysis of this 351 kHz beacon

We can perform a basic NEC analysis with these guesstimates…

  • Tower with nine sections including the lowest one comprising the insulators about 90 feet (~27 meters) tall.
  • Top hat wires made from 45 degree guys about 60 feet (~18 meters) length.
  • Assume 25 watts is available at the transmitter output insulator at whatever impedance this point really is.
  • Assume the transmitter grounds to some earth rod plus radial field – Just tie to MiniNEC ground for now.
DANGER

The analysis below is ultra basic and only models the approximate lengths of perfectly conducting radiators of small diameter.  It’s only here to show the purpose of the top hat feature and highlight the large voltage at the feedpoint.  All that said, let’s proceed.

NEC Structure for low frequency beacon

NEC Antenna structure for LF/MF beacon with current magnitudes.

Notice the top hat wires do a good job of of moving the low current, high voltage tips away from the tower top to allow the vertical monopole to maintain good current magnitude along the entire height.  Hence the purpose of a top hat.

NEC Parameters for low frequency beacon

NEC Antenna parameters for LF/MF beacon

This is the voltage across the source near the bottom of the vertical wire (little magenta circle).  Both the Voltage and Impedance values confirm this antenna is very capacitive.  Despite the best efforts of the top hat, this antenna is still electrically short.  A conjugate antenna matching system, the big yellow box, is essential.  With only 25 watts, this simulation suggests the reactive voltage on the tower is very high.

NEC gain pattern for low frequency beacon

Bear in mind there are many parameters, specifically ones involving ground modeling and efficiencies, that play havoc with the actual real gain of this antenna system.  The pattern below shows about a 3 dBi gain, but don’t hold me to this as I used the MiniNEC ground with zero ohms to earth.  The pattern shape is probably a good representation, however.

NEC antenna pattern and gain for LF/MF beacon

There is, of course, a ground wave element to consider as well, but this doesn’t matter much for this system’s primary aviation use.  It does matter for my use of this beacon in testing of SDRs, antennas and such from my home.  This beacon is ideal for checking MF reception.  I hope it stays on the air.

A much better NEC analysis is possible with time.  All I want to point out for now is the high voltage.

E-field measurements

Because I can, I made several spot measurements around the LF beacon along the gravel road at various distances from the radiation tower.  I also generated the e-field profile from the NEC4 simulation for comparison.  The graph below is for probe measurements and the simulation at 2.5 meters above ground level.

Comparing simulated and measured e-fields.
Comparing simulated and measured e-fields

My measured electric field values roughly follow what NEC4 predicts.  The type of ground used for simulation changes the results considerably.  For the purposes of this article, and given the rudimentary NEC model with its many variables, this is a reasonable correlation.  For those with NEC4 and a desire to play around with your own simulations, here is the NEC file with a .txt added.  Remove the .txt before simulation.

Key takeaways

The aviation non-directional LF/MF beacon is a good study of radio engineering.  Despite its size, the 100 foot or so tower is an electrically small antenna problem that requires some thought to achieve a design win.  In today’s world, cell phones have the similar problem of housing electrically small antennas for cellular, WiFi and soon 5G use.  The problem for most tech aficionados is they never see and appreciate just what goes into making these devices’ compact antennas work.  The aviation beacon is really no different, but is on a macro scale that provides a great view into the techniques one needs to prevail.  Things we learn from this beacon survey include:

  • Electrically small is electrically small no matter how you scale it.
  • Top hats are the tried and true solution for increasing efficiency if and when you have the space.
  • High voltage and capacitive reactance are the norm for electrically small antennas.
  • Even though this beacon is only 25 watts, thousands of volts exist at the transmitter system antenna terminal and the tower above its insulating spacers. Forget the one hand rule… this requires a no hand rule… in fact just keep yourself far away from such installations.
  • The primary radiator is galvanized steel tower with three galvanized steel top hat wires.  No copper weld or anything fancy.
  • This system is rugged, simple and plays (or played) an important role in aviation.

Perspective for 630 and 2200 meter transmit

This beacon provides perspective for those who dare to transmit on these new low bands with electrically small antennas at almost any power level.  Various stories of burned up insulators for the uninitiated suggest the trouble taken by the designers of this NDB station towards handling high voltage were prudent.  May the lessons taught by these engineers and technicians inspire our use of 630 and 2200 meters.

Conclusion

Non-directional beacons have been an important navigation aid to the aviation world for many decades.  The transmitting stations use AM modulation, or a close equivalent, with audio tone Morse identifier.  Here is a snippit kindly provided by K2PI.

It does one thing and it does it well.  Progress is moving more of these stations to the scrap heap as satellite navigation push ground based systems off the air.  However, as a pilot, the shuttering of the ground based systems in the face of easily jammed satellite offerings gives me pause, but that’s another story.

If you would like to see and appreciate an electrically small antenna in large form, go see, BUT DO NOT TOUCH, your local LF or MF non-directional beacon before its gone forever.

Oh, in case anyone might wonder, I never broke the plane of the fence to get these photos.  Good cameras make it possible.

Further reading list

  1. https://en.wikipedia.org/wiki/Non-directional_beacon#Monitoring_NDBs
  2. https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html
  3. http://www.faraim.org/aim/aim-4-03-14-33.html
  4. http://www.tashkoo.com/Products/TASHKOO_NDB_Antenna.html

Revisions

  • 2020-05-29 – Added e-field measurements and a refined NEC4 model. Added comments about 630 and 2200 meters.

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