EMRFD Message Archive 9107

Message Date From Subject
9107 2013-09-09 21:24:53 patt896 VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

I tried a J-pole for Aircraft Band.  No gain.  I converted a 6 element FM Yagi to Air Band.  Seems that there are several methods of matching the dipole (flat or folded).


Gamma Match

Coax loop

4:1 current Balun



I'm not certain which of the above works with a straight vs folded dipole and which would be best at 126.0 MHz.







9112 2013-09-10 16:45:27 kb1gmx Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 Folded dipole is about 200ohms if the antenna is a half wavelength up or about 4ft.

 Plain dipole will be around 70 ohms.

 Jpole will have the same gain as a vertical dipole.

 Why a yagi at aircraft band as its line of sight and with typical aircraft altitudes 

that is a considerable distance (100+ miles)?   However if you continue that path

use a folded elelemnt with 4:1 balun (note: the actual impedance will depend 

on element spacing).   Note2: you will have to aim it roughly at the airplane in 

question as it will be directional, off axis the gain of most yagi antennas is 

less than dipole.

Also the polarizati

9117 2013-09-11 06:28:12 Russ Ramirez Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz
On a straight dipole, attaching the coax directly is fine, especially for receive. For a folded dipole, you'll have a 200-300 ohm characteristic impedance, so a 4:1 balun (voltage or current) will do the trick. A current balun is mainly beneficial for choking (blocking) currents on the outside of the coax on transmit, so again for receive it would not matter which type you use or how much power it can handle, so winding on a T-50 or T-80 size core would be fine.


9119 2013-09-11 10:47:48 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

Thx, Allison and Russ.  My original Yagi had a flat dipole, but I'm changing to folded.  The antenna and Funcube Pro work fine for planes at 33K ft, but I'm trying to hear JAX Intl Tower, about 30 miles north.  Don't know if that's possible without my own tower.


Found this:

Ref. Harry Lythall - SM0VPO
For VHF work the most comm

9120 2013-09-11 11:00:58 rlramirez77 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz
Good find. It is a common practice on T/R antennas to use a Transmission Line Transformer such as this for a 4:1 balun. My Mosley 2m beam does this with 75 ohm RG-6, but again for receive purposes, you really don't need the quality (loss characteristics) of RG-213 for the balun and probably not for the lead in either. Be aware that even a 6 dB loss (a huge amount at this frequency, maybe seen on a 200'+ cable run) is only a single S unit difference on receive, so using RG-58 would be fine in most cases.

9121 2013-09-11 11:39:52 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

Good point Russ.  I''ll try this:

Folded Dipole Driven Element of multi-element (yagi) beam.

Center Freq = 126.0 MHz.

2 parallel lengths RG59* 75 ohm coax.

Velocity factor of double-shield Rg-59 is .80

234/126.0 = 1.8571 = 1.8571 feet  x 12 = 22.2852 inch

22.29 * V (.8) = 17.83 inch

Z=52 ohms




9122 2013-09-11 14:31:47 Ed Manuel Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

Also, a 126 mhz receiving preamp would be pretty easy to build and mount at the antenna, eliminating virtually all the feed line loss.  Power can be fed right up the coax with a bias tee.


Ed, N5EM


9123 2013-09-11 15:09:42 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

Just hooked up a PAR Aircraft BPF and AR2 Preamp.  Bought a VHF relay to mount the lot on the mast, fed by an AR2 DC Injector. 


The folded dipole and amp work well.  Still cannot hear JAX tower.  Need to get it all up on the roof.


After I add four more Directors....



9124 2013-09-11 15:45:22 kb1gmx Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 how far are you from JAX tower?

I can say that even with all that gain unless you close or very high 20 miles is unlikely.

They are truly LOS and of your over their horizon you have to be high or nothing.

Part of the reason for this is the roughly 700 channels are not all available and most are 

shared so they are designed to have enough range for aircraft they will talk to and not much excess. 

I am in MA near BOS(21mi) and BED(16mi) and hearing the tower at BOS is 

not possible with a tower at the house. I have terrain bewteen here and there.  BED

however I can hear on a pocket scanner but I'm line of sight to there.   Keep in mind 

the tower  rarely talks to people more than 30 miles out (more like 10 at the big metro 

airports, thats what sector and approach are for) in the air where they have the 

advantage.  If they are that far out approach is usually the active contact 

and may not originate at the airport.  Add to that the antennas used at towers often 

favor  aircraft that are above the horiz

9125 2013-09-11 16:27:30 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 Well, Nuts.  I was afraid of that.  JAX is 30 Miles NW over flat FL terrain, elevation probably 15 ft.


I'll hook up the mast relay so that the preamp is selected only with DC power.


thx, Richard

9126 2013-09-12 12:54:53 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 Found another Yagi design.  Two wavelength boom:

"Either of the above antennas will enhance one's ability to monitor air band activity. ATC ground to air transmissions are often difficult to hear at long distances. Low altitude aircraft are similarly difficult to receive, and require high gain directional antennas for suitable reception. With these antennas (on a rooftop or 30 foot mast), one should expect clear reception of aircraft down to 5000 feet at 200 miles and ATC stations at a range of about 40 miles over flat terrain. Very long range reception is possible if either of these high gain air band antennas are mounted

9127 2013-09-12 13:20:33 radioaustralia123 Re: [emrfd] VHF Dipole Feedpoint Impedance Match Options @126.0 MHz
You may be interested in the Collinear Antenna article that was in the Australian Silicon Chip magazine. Using coax cable for the elements in a plastic pipe. You can purchase the article if required.
Roderick Wall, vk3yc.
9128 2013-09-12 15:03:37 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

Thanks, Roderick.  Article is on order.  A 30 ft vertical would match my stealth 40 meter S9.



9129 2013-09-12 16:37:26 kb1gmx Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 That yagi is based on the Kent Britain WA5VJB cheap yagi and its gain is real at 13db.  There is a yabut

The airband exceeds the band width of most easy yagis, That yagi is limited to about maybe 3-4mhz of the band.

The log periodic is a good wideband antenna but lower in gain for all its size.  At nearly 9.5db for a log is higher than the usual 7 db I often see.

The key is high enough to see a distance D=1.31*sqrt(H) where H is the antenna height in feet and D 

is the distance in miles.  You have to do that for both end to get a height where both can see each other.

Typically airplanes are higher than 2500ft, towers are usually 50-75ft.  This is line of sight and CHF signals do bend some for longs paths so about 5-10% more is reasonable. An example if the tower antenna

is at 75 ft the LOS is about 12miles.  If you antenna is 75ft up LOS is 12 miles and both can see each

other at a distance of 24miles.  For a Cessna 150 at 3500ft visual line of sight on a clear day exceeds 

20 miles (I fly one!). the radio distance to horizon is 78miles.

FYI its not unusual for non tower stations (center, sector and approach) to be remote from the airport.

One cannot assume the signal originates there at the tower, you can find out for sure though.  Also 

allow for most US towers running less than 50W AM and some lower.  

As a pilot I love listening.  I'm also a VHF/UHF op so none of this is strange to me. Going 30 miles is 

doable but it takes height to make it reliable.   Same problem as running a 2M FM mobile to a repeater 

on a hill, one has to have a path to go far.


9130 2013-09-12 18:53:32 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 Got my license under the GI Bill, Cincinnati, 1978 in a Cessna 150.  Haven't flown since the fuel crisis raised aviati

9131 2013-09-12 18:58:11 James Duffey Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz
Richard - I suggest you get a copy of YagiCad, free at

< http://www.yagicad.com >

It will allow you to model Yagis in free space. It is a simple, but effective modeling program utilizing NEC.

You don't say how you converted the FM antenna to the 126 MHz band. But, inputing the dimensions of your converted Yagi into YagiCad should allow you to analyze it and see what the gain is, where it resonates, and the bandwidth. With a simple dipole feed, it will show you what the feedpoint impedance is. It will also calculate a matching technique, gamma, tee, hairpin, or lumped constant to match to 50 Ohms. It is pretty straight forward to use.

One thing to know about feeding Yagis is that the driven element really only serves to launch the electromagnetic wave in the antenna. So, its length is not critical. The easiest way to match a split dipole driven element in a Yagi is to adjust its length until you end up with a 50 Ohm resistive part of the complex impedance and then cancel the residual reactance with an inductance if too short, or a capacitance if too long.

Having said all that, depending on how you converted the FM beam, you may not have an optimum or even near optimum design for 126 MHz. In fact, as the requirements for an FM band beam, primarily wide bandwidth, and your requirements, high gain on a single frequency desire to copy a single air band station, are quite different. I suspect that it may not be an optimum conversion. While YagiCad has an optimizer, it may be best to start with a proven design. The boom length is pretty much the thing that drives the gain. Forget about the number of elements.

If I were you, I would start with a 2M design that uses a boom roughly 144/126 times the boom length you have. Enter that design into YagiCad. Scale to 126 MHz. That should get you a pretty good design. It may require some tweaking, but you can get a good match with the match calculator.

If you want a quick and dirty solution, look up the Cheap Yagis from WA5VJB:

< http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf >

They are cheap and easy to make, and can be implemented without tuning. Enter the dimensions into YagiCad for a 2M design and then scale it for 126 MHz. I suspect that the 3 element will do the job and be manageable in size.

Let us know how this all turns out. You should be able to do what you want without a lot of hassle. - Duffey KK6MC
9132 2013-09-12 21:50:38 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

p.s., Roderick. 

I'm a fan of Peter Temple mystery novels (own the lot) and the ABC Series with Guy Pierce.


Unknown in the US....

9133 2013-09-13 10:14:15 kb1gmx Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

 I'm in the antenna design business....

Collinear antennas are indeed effective for omnidirectional use.  A 5.8DB antenna will be about 20ft tall

and will be useless for overhead as the design concept is to put all the RF on the horizon.  For monitoring ground based stations this may be a winner.  For air based systems... remember the cone of silence

that occurs when you cross a VOR dead center.

Started training in 79, license in 81 after buying the one I own still.  Airplanes and electronics are a great hobby mix.


9138 2013-09-14 12:02:27 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

thx, Duffy.  I used Yagi Calculator by John Drew, VK5DJ: DL6WU style Yagis for VHF/UHF (updated 6 June 2013)  http://www.vk5dj.com/yagi.html .  I'll check out YagiCad.

9139 2013-09-14 12:55:32 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz
9173 2013-09-27 08:39:12 wb8yyy_curt Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

usually the broadcast-band beams are arranged as log periodic to obtain wider bandwidth.  if so, this would be obvious with great diversity in element length.  a bunch of years ago many of converted an FM BC radio antenna for 6m use -- by trimming elements to approx. 'yagi' length (someone had done a eznec type model to develop a recipe).  if your elements can be trimmed to approximate a yagi, I am thinking the revised mutual coupling might allow a direct coax feed (as is used on my converted 2m beam) with only a multiturn coax or other simple balun.  another opti
9192 2013-09-28 06:12:36 patt896 Re: VHF Dipole Feedpoint Impedance Match Options @126.0 MHz

I built the Yagi using a folded dipole radiator and a 1/2 wave coax phasing loop.  Works great with 6 elements.  A ten element version did not improve gain and is 12 feet long.  I'll have to recheck my measurements.


I'm also going to turn my KB9VBR airband J-pole into a Super J.  But I'll bet the Yagi has better performance.


thx, Curt, Richard