Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]() "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.... |
#2
![]() |
|||
|
|||
![]()
On Mon, 14 Aug 2006 14:53:53 GMT, "Lee"
wrote: 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, It is easy to build a preamp with high gain and low noise figure and it will exhibit superb performance on a test bench in a shielded room on a signal generator. In a real world environment, you are unlikely to realise the full sensitivity of the receiver due to: - external noise; and - intermodulation products generated within your receiver (preamp). It is harder to build a preamp with low intermodulation distortion, and one method of reducing the results of that intermodulation distortion is front end filtering to reduce the level of undesired signals reaching the non-linear devices. Front end selectivity costs much more money than a low NF preamp transistor or gasfet. Whilst wideband preamps are available at low cost, it is quite likely that they will actually degrade your receiver performance. It may even be that adding an external filter will improve your S/N ratio. An interesting test to perform is to note the S/N ratio, add a small attenuator to the receiver input, and again measure the S/N ratio. If the S/N ratio improves, it is an indicator that you have significant intermodulation distortion and front end filtering may improve the sensitivity. I listened last night and could hear NOAA 14 on a hand held scanner (IC-R20) with a 130mm long rubber duckie off my 2m transceiver. It wasn't good enough for pictures, but it could be heard... so it shouldn't take a lot of receiver sensititivity to decode it well. (BTW, I could not hear the bird using a 200mm whip on the scanner... to much noise from intermod products). I know you asked about coax and you are seeking a low loss coax situation, coax loss might be less important that adequate receiver front end filtering so that you can realise most of its potential in the presence of other strong signals. In the absence of that, coax loss might actually improve S/N! Owen PS: I recently performed some tests on the new Icom IC-7000 on 144MHz to determine the usable sensitivity on a wideband antenna, and although the specified sensitivity is -126dBm, the sensitivity when connected to a Diamond D-130 at this location was -96dBm, that is 30dB poorer than spec, and the main contibution was IMD within the IC-7000. Putting a 10dB attenuator inline improved the sensitivity by 14dB! -- |
#3
![]() |
|||
|
|||
![]() "Owen Duffy" wrote in message ... On Mon, 14 Aug 2006 14:53:53 GMT, "Lee" wrote: 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, It is easy to build a preamp with high gain and low noise figure and it will exhibit superb performance on a test bench in a shielded room on a signal generator. In a real world environment, you are unlikely to realise the full sensitivity of the receiver due to: - external noise; and - intermodulation products generated within your receiver (preamp). It is harder to build a preamp with low intermodulation distortion, and one method of reducing the results of that intermodulation distortion is front end filtering to reduce the level of undesired signals reaching the non-linear devices. Front end selectivity costs much more money than a low NF preamp transistor or gasfet. Whilst wideband preamps are available at low cost, it is quite likely that they will actually degrade your receiver performance. It may even be that adding an external filter will improve your S/N ratio. An interesting test to perform is to note the S/N ratio, add a small attenuator to the receiver input, and again measure the S/N ratio. If the S/N ratio improves, it is an indicator that you have significant intermodulation distortion and front end filtering may improve the sensitivity. I listened last night and could hear NOAA 14 on a hand held scanner (IC-R20) with a 130mm long rubber duckie off my 2m transceiver. It wasn't good enough for pictures, but it could be heard... so it shouldn't take a lot of receiver sensititivity to decode it well. (BTW, I could not hear the bird using a 200mm whip on the scanner... to much noise from intermod products). I know you asked about coax and you are seeking a low loss coax situation, coax loss might be less important that adequate receiver front end filtering so that you can realise most of its potential in the presence of other strong signals. In the absence of that, coax loss might actually improve S/N! Owen PS: I recently performed some tests on the new Icom IC-7000 on 144MHz to determine the usable sensitivity on a wideband antenna, and although the specified sensitivity is -126dBm, the sensitivity when connected to a Diamond D-130 at this location was -96dBm, that is 30dB poorer than spec, and the main contibution was IMD within the IC-7000. Putting a 10dB attenuator inline improved the sensitivity by 14dB! -- Hi Owen Not that it makes alot of difference, but, you could have been hearing NOAA 17 on 137.62. Its coordinates may have been different from where you were anticipating while orienting your 200mm scanner whip. And, as you probably know you'll need about 30 KHz minimum if you want to produce images from the NOAA satellites. I think the IC R20 selectivity is either too narrow or too wide for producing APT images even when the signal strength is adequate. But, you probably knew that. Jerry |
#4
![]() |
|||
|
|||
![]()
On Tue, 15 Aug 2006 00:17:40 GMT, "Jerry Martes"
Not that it makes alot of difference, but, you could have been hearing NOAA 17 on 137.62. Its coordinates may have been different from where you were anticipating while orienting your 200mm scanner whip. And, as you probably know you'll need about 30 KHz minimum if you want to produce images from the NOAA satellites. I think the IC R20 selectivity is either too narrow or too wide for producing APT images even when the signal strength is adequate. But, you probably knew that. It was actually NOAA 15 that I heard (sorry for the typo) and I was listening to the APT signal on 137.5MHz in USB mode. I am pretty confident it was the bird, the doppler shift changed direction at the right moment. I was more interested in the strength, I couldn't find a ready source of link budget calcs on the 'net. Owen -- |
#5
![]() |
|||
|
|||
![]() "Owen Duffy" wrote in message ... On Mon, 14 Aug 2006 14:53:53 GMT, "Lee" wrote: 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, It is easy to build a preamp with high gain and low noise figure and it will exhibit superb performance on a test bench in a shielded room on a signal generator. In a real world environment, you are unlikely to realise the full sensitivity of the receiver due to: - external noise; and - intermodulation products generated within your receiver (preamp). It is harder to build a preamp with low intermodulation distortion, and one method of reducing the results of that intermodulation distortion is front end filtering to reduce the level of undesired signals reaching the non-linear devices. Front end selectivity costs much more money than a low NF preamp transistor or gasfet. Whilst wideband preamps are available at low cost, it is quite likely that they will actually degrade your receiver performance. It may even be that adding an external filter will improve your S/N ratio. An interesting test to perform is to note the S/N ratio, add a small attenuator to the receiver input, and again measure the S/N ratio. If the S/N ratio improves, it is an indicator that you have significant intermodulation distortion and front end filtering may improve the sensitivity. I`ll consider your advice very carefully Owen, Thanks.... I listened last night and could hear NOAA 14 on a hand held scanner (IC-R20) with a 130mm long rubber duckie off my 2m transceiver. It wasn't good enough for pictures, but it could be heard... so it shouldn't take a lot of receiver sensititivity to decode it well. (BTW, I could not hear the bird using a 200mm whip on the scanner... to much noise from intermod products). Yes, i`ve done that also....and the handy was quite strong too but the scanner front end was awful ..... AOR2002..... Ugh!!!.. I know you asked about coax and you are seeking a low loss coax situation, coax loss might be less important that adequate receiver front end filtering so that you can realise most of its potential in the presence of other strong signals. In the absence of that, coax loss might actually improve S/N! Surprisingly, i get quite a good picture using my Yaesu 857D also!!! ( with attenuator in, of course).... Owen PS: I recently performed some tests on the new Icom IC-7000 on 144MHz to determine the usable sensitivity on a wideband antenna, and although the specified sensitivity is -126dBm, the sensitivity when connected to a Diamond D-130 at this location was -96dBm, that is 30dB poorer than spec, and the main contibution was IMD within the IC-7000. Putting a 10dB attenuator inline improved the sensitivity by 14dB! -- I have the Icom IC-E90 which has good audio on the NOAA`s too ... also on the ducky... Lee.....G6ZSG.... |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|