Questions on ANOTHER home-made antenna LOL

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kny2xb

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Dec 19, 2002
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Clearwater, FL
Good Evening/Morning;

DISCLAIMER: I didn't want to tack onto someone else's post, but if I should have, I know that this will be moved

I would like to build and mount a couple of base antennas using the SO-239 connector quarter-wave ground-plane design

One will be for receiving/scanning a mix of VHF: aviation, marine, MURS and rail

I would also like to build one to receive and transmit on the GMRS frequencies

My questions are:

1 Would it be worth using an N female chassis connector and cable instead of the SO-239 for the GMRS antenna?

2 Since the designs that I've read about are usually used at VHF/2 meter frequencies, the element lengths are in the 19-20 inch range

At UHF frequencies, is it feasible to build:

a half-wave using a 12.06 inch/306 mm vertical with 12.66 inch/322 mm radials

or a five-eighths wave using a 15.07 inch/383 mm vertical with 15.83 inch/402 mm radials

[I hope that I got the measurements correct]

I'm figuring a center frequency of 465.1375 MHz just in case anyone ever puts up a GMRS repeater

If this has been covered before and I missed it, I apologize and I'll say: OOPS! my bad

If not, I appreciate any answers, advice, feedback, etc before I dive into this
 

prcguy

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I don't think an N connector is needed for your antennas, an SO-239 may have a fraction of a dB loss at GMRS frequencies when used in a closed 50 ohm cable system but when used to launch an antenna it should make no difference at all.

If you make anything other than a 1/4 wave ground plane with all elements 1/4 wave long you will need some complex matching, you just cant stick a 1/2 wave elements on a connector because a half wave end fed has a several thousand ohm impedance. For simplicity start with an 18" vertical element and 18" radials for your VHF range and 6" elements for UHF.

If you want to progress to a 1/2 wave vertical element, look up a J-pole, which has a 1/4 wave matching section to bring the several thousand ohm impedance down to 50 ohms. A 5/8 wave vertical radiator can usually be matched by making it look like a 3/4 wave which can be close to 50 ohms but has an undesirable pattern with upward lobes and a null at the horizon. Cut the vertical radiator to 5/8 wave and use a loading coil at the base to make it 1/8 wave or so longer until it matches. You would use 1/4 wave ground radials in all cases.
prcguy
 

kny2xb

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Joined
Dec 19, 2002
Messages
171
Location
Clearwater, FL
Thank You for the prompt reply

At first, I presumed that upgrading to a half or five eighths wave by adjusting the antenna dimensions would be a good thing

BUT, I respect and value the knowledge that the scannists and hams here on RR have, so thats why I posted here first before diving in and possibly wasting my time going in the wrong direction

Thank You prcguy
 
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