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).
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.RussK0WFS 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.
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
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.
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.
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 Hi,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.