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#1
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![]() "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..... |
#2
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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 |
#3
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![]() "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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