|
QFH Antenna and 72ohm TV Coax
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 ????... Thanks. Lee......de G6ZSG..... |
QFH Antenna and 72ohm TV Coax
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 ????... Thanks. Lee......de G6ZSG..... It will work perfectly, because the TV coax is good HF quality. Why do you use a 4 turn choke? w. |
QFH Antenna and 72ohm TV Coax
"Helmut Wabnig" *_.-_- wrote in message ... On Mon, 14 Aug 2006 07:44:10 GMT, "Lee" wrote: It will work perfectly, because the TV coax is good HF quality. Why do you use a 4 turn choke? w. Sorry, typo!.........Balun!! Thanks, thought so. Lee.....de G6ZSG...... |
QFH Antenna and 72ohm TV Coax
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 ????... 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. Walt, W2DU |
QFH Antenna and 72ohm TV Coax
Please leave me out of this. I'm mentally handicapped!
I don't know, never did know, how to use an old fashioned, mid-20th century Smith Chart. ---- Reg. "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 ????... 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. Walt, W2DU |
QFH Antenna and 72ohm TV Coax
"Lee" wrote in message k... 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 ????... Thanks. Lee......de G6ZSG..... Hi Lee There are alot of affordable amplifiers designed for TV that you could use at the base of your QFH. You might consider building your owm amplifier to fit in the base of the QFH. I wouldnt recomend the use of a pre-amp at the antenna for NOAA satelite station. They often cause more problems than they solve. All Electronics has alot of ferrite tubes that can be used to fit over the coax so you wouldnt need the "4 turn choke". Jerry |
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 ????... 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. Walt, W2DU |
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..... |
QFH Antenna and 72ohm TV Coax
"Jerry Martes" wrote in message news:hU%Dg.12256$hH1.9718@trnddc08... "Lee" wrote in message k... 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 ????... Thanks. Lee......de G6ZSG..... Hi Lee There are alot of affordable amplifiers designed for TV that you could use at the base of your QFH. You might consider building your owm amplifier to fit in the base of the QFH. I wouldnt recomend the use of a pre-amp at the antenna for NOAA satelite station. They often cause more problems than they solve. All Electronics has alot of ferrite tubes that can be used to fit over the coax so you wouldnt need the "4 turn choke". `4 turn Choke Balun`.....typo.... Jerry Thanks Jerry, i`ll give it some thought as i`m right under some pmr towers which breaks through a little from 150megs pagers and a preamp may worsen things... Lee....de G6ZSG.... |
QFH Antenna and 72ohm TV Coax
Reg Edwards wrote:
I don't know, never did know, how to use an old fashioned, mid-20th century Smith Chart. Reg, I'm curious how you would solve this stub problem without a Smith Chart. | 45 deg | 45 deg | Source====Z01=========Z02====open Stub sections are lossless. Z01 = 600 ohms and is 45 degrees long. Z02 = 50 ohms and is 45 degrees long. What is the impedance looking into the stub from the source? -- 73, Cecil http://www.qsl.net/w5dxp |
| All times are GMT +1. The time now is 03:24 AM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com