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#160430 - 03/28/06 11:15 AM Slightly OT: Radios  
PEdoubleNIZZLE  Offline
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
Hi, I wasn't sure whether to post it here, or in the theory area, but here are my questions:

1) I recently bought a VHF receiver, and it says to use an antenna with a length 1/4 of the wavelength. What if I used one that was 1/2 the wavelength, or full wavelength?

2) (Quarter wavelenght would be something a little over 2 feet for VHF Air, I believe.) If I used an arbitrarily large antenna, say 20 feet, Would I get any better reception?

3) What would be the difference between using the antenna vertically or horizontally?

Any help, would be appreciated, even if you only give me a website (Teach a man to fish...). I tried looking it up online already, but I got really basic stuff that I've known since I was 12.


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#160431 - 03/29/06 12:10 AM Re: Slightly OT: Radios  
Joined: Mar 2004
Posts: 251
Kingwood, TX USA
Most radios have a 50 Ohm antenna input. At VHF freq. a 1/4 wave antenna matches this. A 1/2 wave antenna would need a matching coil added to bring it to a 50 ohm impedance that the radio needs. The better the match, the more effective the antenna. Although not important on a receiver, if you transmit on a mismatch antenna, you risk damage to the radio. As far as horizontally or vertically, ideally it should match the transmitting antenna. Most mobile radio antennas are vert. polarized. Antenna height above ground is far more important for reception at VHF freq. than lenght of the antenna. Also use a good feedline to feed the antenna. Has some antenna information Robert

#160432 - 03/29/06 09:12 PM Re: Slightly OT: Radios  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
If this VHF set is only a reciever, you can pretty much use what you like on it.
However, something that is resonant at the frequency you want would be good.
I can't remember if Aeronautical signals are Vertically or Horizontally-Polarised, however, there is a 20dB loss in signal if you have the wrong polarisation.
I have an idea that it's Horizontal (ie: with the elements flat, as opposed to perpendicular to the ground).
If you are going to use any sort of dipole antenna, you'll need a balun to match the antenna to your coaxial cable, without one you'll lose signal.
Hope this helps. [Linked Image]

#160433 - 04/03/06 05:35 AM Re: Slightly OT: Radios  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
I do know that Helicopters use Horizontal Polarisation.

#160434 - 04/12/06 07:51 PM Re: Slightly OT: Radios  
dereckbc  Offline
Joined: Oct 2003
Posts: 156
Tulsa, OK
Back in my RF Engineer days I used to be a rubber necker ( a jerk with a scanner listening in on phone calls or what ever). One good set up is a YAGI antenna with two rotors. One rotor that turns a full 360% to cover the compass for diretional pointing, and another 90% rotor for polarization between vert-horz.

But that is a bit elaborate, what you can do on the fairly cheap is buy an omni directional circular polarization antenna. No rotor needed and works for either vert or horz polarization. You could even make one out of copper tubing. ARRL has lots of designs

#160435 - 04/12/06 07:53 PM Re: Slightly OT: Radios  
dereckbc  Offline
Joined: Oct 2003
Posts: 156
Tulsa, OK
Trumpy said:
I do know that Helicopters use Horizontal Polarisation.

Not on a steep turn.

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