|
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 |
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 |
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 |
QFH Antenna and 72ohm TV Coax
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! -- |
QFH Antenna and 72ohm TV Coax
On Mon, 14 Aug 2006 14:58:32 GMT, Cecil Moore
wrote: 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? I missed the significance of this problem Cecil. Is it principally a theoretical (being lossless) problem that a Smith chart can solve, or does it have some other significance? Whilst a Smith chart is great for visualising transmission line problems, a great way for visually mapping impedance over a range of frequencies, it isn't the most practical way to solve practical problems when we have access to the computing power commonly available to designers today. Owen PS: I think the problem you have given can be solved with simple trig: find the reactance of the Z02 section using one trig term, find the length of Z01 that would deliver that reactance using one trig term, add that length and the actual length of Z01 section, find the reactance of the Z01 section using one trig term. I could do that in a flash with a scientific hand calculator while you were sharpening your pencil. It is a trivial problem either way, and can only ever be an approximation of a practical problem. -- |
QFH Antenna and 72ohm TV Coax
"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 |
QFH Antenna and 72ohm TV Coax
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 -- |
QFH Antenna and 72ohm TV Coax
Owen Duffy wrote:
Cecil Moore wrote: 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? I missed the significance of this problem Cecil. Is it principally a theoretical (being lossless) problem that a Smith chart can solve, or does it have some other significance? It's just a mental exercise with a hidden significance. This is the type of problem that I would solve with a Smith Chart. How about a solution? What impedance does the source see? The physical length of the stub is 90 degrees. What is the electrical length of the stub in degrees? -- 73, Cecil http://www.qsl.net/w5dxp |
QFH Antenna and 72ohm TV Coax
Cecil Moore wrote:
It's just a mental exercise with a hidden significance. This is the type of problem that I would solve with a Smith Chart. How about a solution? What impedance does the source see? The physical length of the stub is 90 degrees. What is the electrical length of the stub in degrees? KISS But if you did that you couldn't get this thread to last forever. tom K0TAR |
QFH Antenna and 72ohm TV Coax
"Cecil Moore" wrote in message . .. Owen Duffy wrote: Cecil Moore wrote: 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? I missed the significance of this problem Cecil. Is it principally a theoretical (being lossless) problem that a Smith chart can solve, or does it have some other significance? It's just a mental exercise with a hidden significance. This is the type of problem that I would solve with a Smith Chart. How about a solution? What impedance does the source see? The physical length of the stub is 90 degrees. What is the electrical length of the stub in degrees? -- 73, Cecil http://www.qsl.net/w5dxp Hi Cecil Is it posible that the length of the "stubs" change? I'd have thought the length of the stub is always the same. 45 degrees should always be 45 degrees, shouldnt it?? An open circuit, 45 degrees back along a 50 ohm line looks like 50 ohms capacitive. That 50 ohms looks like something like 500 ohms inductive as viewed 45 degrees back along a 600 ohm line. I'd guess your point is that 500 ohms of pure inductive reactance is never seen 90 degrees back from an open, no matter what the Zo of the line Jerry |
QFH Antenna and 72ohm TV Coax
Tom Ring wrote:
KISS Keep It Simple Stupid? But if you did that you couldn't get this thread to last forever. I have kept it as simple as possible. Wonder why nobody has ventured an answer? -- 73, Cecil http://www.qsl.net/w5dxp |
QFH Antenna and 72ohm TV Coax
Jerry Martes wrote:
Is it posible that the length of the "stubs" change? I'd have thought the length of the stub is always the same. 45 degrees should always be 45 degrees, shouldnt it?? An open circuit, 45 degrees back along a 50 ohm line looks like 50 ohms capacitive. That 50 ohms looks like something like 500 ohms inductive as viewed 45 degrees back along a 600 ohm line. I'd guess your point is that 500 ohms of pure inductive reactance is never seen 90 degrees back from an open, no matter what the Zo of the line The point I'm eventually going to make is about loading coils in mobile antennas but let's stick with the above stub example. | 45 deg | 45 deg | Source====Z01=========Z02====open Z01 = 600 ohms, Z02 = 50 ohms If the Z0 were constant and the stub was 90 degrees long, the source would see zero ohms. Yet in our above example the stub is physically 90 degrees long and the source sees +j500 ohms. The above stub is electrically 130 degrees long. There is a 45 degree delay through the Z01 section of stub. There is a 45 degree delay through the Z02 section of stub. There is a 40 degree phase shift at the Z01 to Z02 junction. If we want to turn the above stub into a functional 1/4WL open stub such that the source sees zero ohms, we can remove 40 degrees from the Z02 section. If we make the Z02 section 5 degrees long, the entire stub will be electrically 90 degrees long, and 1/4WL resonant. There will be a 45 degree delay through the Z01 section There will be a 5 degree delay through the Z02 section There will be a 40 degree phase shift at the Z01 to Z02 junction. -- 73, Cecil http://www.qsl.net/w5dxp |
QFH Antenna and 72ohm TV Coax
On Mon, 14 Aug 2006 23:04:30 GMT, Owen Duffy wrote:
PS: I think the problem you have given can be solved with simple trig: find the reactance of the Z02 section using one trig term, Z=-j50*cot(45)=-j50 find the length of Z01 that would deliver that reactance using one trig term, l=acot(50/600)=85.2 add that length and the actual length of Z01 section, find the Z01'=85.2+45=130.2 reactance of the Z01 section using one trig term. I could do that in a X=-j600*cot(130.2)=j507.7 flash with a scientific hand calculator while you were sharpening your pencil. It is a trivial problem either way, and can only ever be an approximation of a practical problem. But you wouldn't get that accuracy from the Smith chart. Owen -- |
QFH Antenna and 72ohm TV Coax
"Cecil Moore" wrote in message m... Jerry Martes wrote: Is it posible that the length of the "stubs" change? I'd have thought the length of the stub is always the same. 45 degrees should always be 45 degrees, shouldnt it?? An open circuit, 45 degrees back along a 50 ohm line looks like 50 ohms capacitive. That 50 ohms looks like something like 500 ohms inductive as viewed 45 degrees back along a 600 ohm line. I'd guess your point is that 500 ohms of pure inductive reactance is never seen 90 degrees back from an open, no matter what the Zo of the line The point I'm eventually going to make is about loading coils in mobile antennas but let's stick with the above stub example. Hi Cecil I sure dont want to get involved with any mobil antenna loading coil discussions, I admit that I'm not qualified. | 45 deg | 45 deg | Source====Z01=========Z02====open Z01 = 600 ohms, Z02 = 50 ohms If the Z0 were constant and the stub was 90 degrees long, the source would see zero ohms. Yeah, but is isnt a line with a constant Zo Yet in our above example the stub is physically 90 degrees long and the source sees +j500 ohms. The above stub is electrically 130 degrees long. I'd disagree with a conclusion that, just because the impedance seen by the source is 500 ohms, the line connecting it to a load is 90 degrees long. There is a 45 degree delay through the Z01 section of stub. There is a 45 degree delay through the Z02 section of stub. There is a 40 degree phase shift at the Z01 to Z02 junction If I disagree, do I have to get involved with some lengthy mathmatical discussion? I'm not skilled enough to argue with you Cecil. I'm not even smart. But, I sure dont see how anyone can conclude there is a phase shift at the junction of two transmission lines. There is a shunt capacitive reactance that results from that abrupt change in dimensions, but I wouldnt have thought it would be enough to result in 40 degrees of phase shift. .. If we want to turn the above stub into a functional 1/4WL open stub such that the source sees zero ohms, we can remove 40 degrees from the Z02 section. If we make the Z02 section 5 degrees long, the entire stub will be electrically 90 degrees long, and 1/4WL resonant. There will be a 45 degree delay through the Z01 section There will be a 5 degree delay through the Z02 section There will be a 40 degree phase shift at the Z01 to Z02 junction. I think I agree with the concept you are using. You are giving the condition where a purely capacitive reactor of 50 ohm impedance is required to be resonated by introducing a series inductor. A short length of higher impedance transmission line will sure do that. The higher the line Zo, the shorter it needs to be to resonate that 50 ohm capacitor. -- 73, Cecil http://www.qsl.net/w5dxp Jerry |
QFH Antenna and 72ohm TV Coax
"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.... |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
Owen Duffy wrote:
On Mon, 14 Aug 2006 23:04:30 GMT, Owen Duffy wrote: PS: I think the problem you have given can be solved with simple trig: find the reactance of the Z02 section using one trig term, Z=-j50*cot(45)=-j50 find the length of Z01 that would deliver that reactance using one trig term, l=acot(50/600)=85.2 add that length and the actual length of Z01 section, find the Z01'=85.2+45=130.2 reactance of the Z01 section using one trig term. I could do that in a X=-j600*cot(130.2)=j507.7 flash with a scientific hand calculator while you were sharpening your pencil. It is a trivial problem either way, and can only ever be an approximation of a practical problem. But you wouldn't get that accuracy from the Smith chart. Thanks very much, Owen. I used the Smith chart to get 85, 130, and 500 above. That's about as good an accuracy as I ever need. Also MicroSmith says the impedance value is j507.2 ohms. The stub, which is 45+45 = 90 degrees physically, is electrically 130 degrees long. There is an ~80 degree shift in the Gamma angle at the 600--50 ohm impedance discontinuity resulting in ~40 degrees being added to the electrical length of the stub by the impedance discontinuity. Let's say we now want to turn that stub into an electrical 1/4WL stub. If we made the two sections the same number of degrees, how many degrees would they occupy? | X deg | X deg | source====Z01=========Z02====open 1/4WL stub where Z01=600 ohms and Z02=50 ohms I get 16.1 degrees for X. A stub that is physically ~1/12WL long is 1/4WL resonant, i.e. a 90 degree phase shift from end to end. Does this remind anyone of a base-loaded mobile antenna? -- 73, Cecil http://www.qsl.net/w5dxp |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
Jerry Martes wrote:
"Cecil Moore" wrote in message Yet in our above example the stub is physically 90 degrees long and the source sees +j500 ohms. The above stub is electrically 130 degrees long. I'd disagree with a conclusion that, just because the impedance seen by the source is 500 ohms, the line connecting it to a load is 90 degrees long. Well, it is physically 90 degrees long because the two physical pieces are physically 45 degrees each. That's a given. However, the +j500 result tells us that it is electrically 130 degrees removed from the open circuit at the far end. There is a 45 degree delay through the Z01 section of stub. There is a 45 degree delay through the Z02 section of stub. There is a 40 degree phase shift at the Z01 to Z02 junction If I disagree, do I have to get involved with some lengthy mathmatical discussion? I'm not skilled enough to argue with you Cecil. I'm not even smart. But, I sure dont see how anyone can conclude there is a phase shift at the junction of two transmission lines. There is an abrupt change in the Gamma angle of the reflection coefficient at the impedance discontinuity. I can show you why on a phasor graphic. Simplified, it goes something like this. Itotal = 21.5*sin(25) = 10*sin(65) where 21.5 is the phasor amplitude of the current in the 50 ohm section at the junction and 10 is the phasor amplitude of the current in the 600 ohm section at the junction. The values must be the same even though the magnitude of Z0, which controls the amplitude of the current, has changed. If those values must be equal and the amplitude changes because the Z0 changed, the only other thing that can change is the phase angle. -- 73, Cecil http://www.qsl.net/w5dxp |
QFH Antenna and 72ohm TV Coax
On Mon, 14 Aug 2006 23:04:30 GMT, Owen Duffy wrote:
On Mon, 14 Aug 2006 14:58:32 GMT, Cecil Moore wrote: 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 I missed the significance of this problem Cecil. Hi Owen, Forgive the slur from the world of fixed sports, the significance is a cross-Atlantic, Australian tag team troll. 73's Richard Clark, KB7QHC |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
"Cecil Moore" wrote in message . .. Jerry Martes wrote: "Cecil Moore" wrote in message Yet in our above example the stub is physically 90 degrees long and the source sees +j500 ohms. The above stub is electrically 130 degrees long. I'd disagree with a conclusion that, just because the impedance seen by the source is 500 ohms, the line connecting it to a load is 90 degrees long. Well, it is physically 90 degrees long because the two physical pieces are physically 45 degrees each. That's a given. However, the +j500 result tells us that it is electrically 130 degrees removed from the open circuit at the far end. There is a 45 degree delay through the Z01 section of stub. There is a 45 degree delay through the Z02 section of stub. There is a 40 degree phase shift at the Z01 to Z02 junction If I disagree, do I have to get involved with some lengthy mathmatical discussion? I'm not skilled enough to argue with you Cecil. I'm not even smart. But, I sure dont see how anyone can conclude there is a phase shift at the junction of two transmission lines. There is an abrupt change in the Gamma angle of the reflection coefficient at the impedance discontinuity. I can show you why on a phasor graphic. Simplified, it goes something like this. Itotal = 21.5*sin(25) = 10*sin(65) where 21.5 is the phasor amplitude of the current in the 50 ohm section at the junction and 10 is the phasor amplitude of the current in the 600 ohm section at the junction. The values must be the same even though the magnitude of Z0, which controls the amplitude of the current, has changed. If those values must be equal and the amplitude changes because the Z0 changed, the only other thing that can change is the phase angle. -- 73, Cecil http://www.qsl.net/w5dxp Hi Cecil Thanks for pointing me toward learning about reflection coefficient. I am really surprised that there is such a large amount of phase shift at that junction. Jerry |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
Jerry Martes wrote:
Thanks for pointing me toward learning about reflection coefficient. I am really surprised that there is such a large amount of phase shift at that junction. W8JI has theorized an even larger phase shift there at the junction of a loading coil and the stinger of a mobile antenna. Unfortunately, he attributes 100% of the phase shift below the stinger to that single junction point, while ignoring the phase shift provided by the loading coil. This thread is my attempt at setting the technical record straight and correcting W8JI's new old wives' tale. This discussion started years ago on QRZ.com where both sides were wrong and nobody was 100% technically correct, including myself. -- 73, Cecil http://www.qsl.net/w5dxp |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
Cecil Moore wrote: W8JI has theorized an even larger phase shift there at the junction of a loading coil and the stinger of a mobile antenna. Unfortunately, he attributes 100% of the phase shift below the stinger to that single junction point, while ignoring the phase shift provided by the loading coil. This thread is my attempt at setting the technical record straight and correcting W8JI's new old wives' tale. This discussion started years ago on QRZ.com where both sides were wrong and nobody was 100% technically correct, including myself. There is nothing that verifies Cecil's theory except Cecil's theory. 73 Tom |
Physically short stubs, was: QFH Antenna and 72ohm TV Coax
On Tue, 15 Aug 2006 13:57:12 GMT, Cecil Moore
wrote: X=-j600*cot(130.2)=j507.7 507.7? Says who? Me and two of my friends, Mr Hewlett and Mr Packard. Thanks very much, Owen. I used the Smith chart to get 85, 130, and 500 above. That's about as good an accuracy as I ever need. Also MicroSmith says the impedance value is j507.2 ohms. I am surprised that Microsmith has accumulated so much error (~0.1%) after so few operations. Owen -- |
| All times are GMT +1. The time now is 06:10 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com