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Old August 14th 06, 03:49 PM posted to rec.radio.amateur.antenna
Lee Lee is offline
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Posts: 36
Default QFH Antenna and 72ohm TV Coax


"Walter Maxwell" wrote in message
...
On Mon, 14 Aug 2006 07:44:10 GMT, "Lee"

wrote:

I`m building a QFH NOAA weathersat antenna and wish to use low loss t/v

coax
which is
72ohm; but is more manageable and less lossy at VHF than the 50ohm RG58
specified
over a 100ft run!.... ( RF pre-amps aint cheap! ).......

What are the pros and cons... can i use CT100 72/75ohm..... i believe i

can
but, will the impedence affect
the 4 turn choke ????...


Should read `Choke Balun`....sorry, my typo...

I`m following a design by Bill Sykes G2HCG & Bob Cobey G0HPO
which calls for RG58 with a four turn choke balun....


Thanks.

Lee......de G6ZSG.....

Hi Lee,

Consider this: If your QFH has a 50-ohm terminal impedance, the mismatch

is only
1.44: 1. Therefore, the loss due to the mismatch is 0,14 dB,

insignifiant--use
the 72-ohm line and forget the miniscule mismatch.

Even if the mismatch was 2:1, the reflection loss is only 0.51 dB.

Concerning the 4-turn choke, nothing happens to the matching operation,

because
nothing inside the coax changes due to the coiling of the coax.


I`ll try making the feed RG58 with the choke balun and then feed that with
72ohm...

Thanks....

Lee......de G6ZSG.....


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Old August 14th 06, 04:05 PM posted to rec.radio.amateur.antenna
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Default QFH Antenna and 72ohm TV Coax

Lee wrote:
I`ll try making the feed RG58 with the choke balun and then feed that with
72ohm...


No need to do that. Just use 4 turns of the 72 ohm coax.
--
73, Cecil http://www.qsl.net/w5dxp
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Old August 14th 06, 04:32 PM posted to rec.radio.amateur.antenna
Lee Lee is offline
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Posts: 36
Default QFH Antenna and 72ohm TV Coax


"Cecil Moore" wrote in message
om...
Lee wrote:
I`ll try making the feed RG58 with the choke balun and then feed that

with
72ohm...


No need to do that. Just use 4 turns of the 72 ohm coax.


Thanks Cecil, this is my second QFH !!!
the first worked a treat as designed with RG58, but 100feet is a bit
lossy.....
a preamp under pmr towers is asking a bit too much hence tv coax.....

Lee...de G6ZSG

--
73, Cecil http://www.qsl.net/w5dxp



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