Folded Dipole Airband Polarisation?

Tapet

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Sep 12, 2020
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Ok I am a complete Newb but here goes. I brought a cheap RTL-SDR play I have managed to get it going ok, sort of and now making a folded dipole for Airband centred on 124Mhz. Just wondering what is the best Polarisation to mount it. Good thing is I am close to an International airport just off to the side of the usual appoach flight path. They touch down just less than a K from my house! (Good thing I love aircraft :) I guess most aircraft would be horizontaly polarised or are they? But guess many air ground comms would be vertical. I have spotted that some antennas near the end of the runway are horizontal.

Which might be the best way to go?

I intend to use a TV 300 to 75 ohm balun and RG 11 coax from my C band Sat TV experimenting days. Would soldering a 50 ohm PL 259 or 50 ohm N connector on the end cause much loss?

On the Calculator I used to make it- Dipole The conection side has one leg slightly longer than the other. Why is that? I would of thought it would be slap bang in the middle.

Many thanks
 

chief21

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All airband VHF/UHF voice comms are vertically polarized. The aviation industry employs many types of specialized radio equipment. The antennas you see near the runway are not related to the voice frequencies.

There are coax adapters available that can be used to connect the 300-to-75 ohm balun to a PL259 or N connector.
 

iMONITOR

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I ran across this which gives a lot of info on antenna polarization:

 

dlwtrunked

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All airband VHF/UHF voice comms are vertically polarized.
...
Actually although almost all airband VHF/UH comms are vertically polarized, not "all" FAA voice comms are transmitted as such. See the two antennas to the left in the attached photo from FAA Buena Vista site in Virginia used by Washington Center of circular polarized antennas used for voice comms. This is atop a mountain and done this way to help communicate with lower aircraft who's antenna polarity might not well match a vertical polarized signal from the mountain top.
 

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majoco

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Just like mine....the initial construction had a metal clamp at the outer end joining the elements to the horizontal shaft but it was pointed out that this would change the impedance from 300ohms to 2 x 75ohm elements. The plastic insulator was fitted ( it has a short across the elements ) but I really didn't notice any difference!
 

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Tapet

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Sep 12, 2020
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Thanks for your replies and informative links.
Might still go for the for folded dipole. Although the off centre fed dipole looks good and easy to make. I'll have a crack at it later.
Probably going to get more fun out making antennas than anything I going to hear from them. :)
 
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