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Palomar noise bridge
I recently got myself a noise bridge to do some quantitative measurements on
my antennas. Lets say I am a bit disappointed. The small dials of the unit make all my antennas to appear very similar indeed and I am wondering if the unit is broken (not according to the test I did with a known load) or if I am expecting way too much. What do real people use to do impedance measurements on their antennas ( and please do not suggest anything from MFJ). 73 Uwe |
Uwe
I'd like to hear more about MFJ, and why you dont want to hear more about them. I was thinking that they were a pretty good way of measuring impedance. Jerry "Jaggy Taggy" wrote in message ... I recently got myself a noise bridge to do some quantitative measurements on my antennas. Lets say I am a bit disappointed. The small dials of the unit make all my antennas to appear very similar indeed and I am wondering if the unit is broken (not according to the test I did with a known load) or if I am expecting way too much. What do real people use to do impedance measurements on their antennas ( and please do not suggest anything from MFJ). 73 Uwe |
I agree with Jerry. Why not use an MFJ. I have two of their oldest
units one for HF and one VHF, the models 207 & 208. Bought mine a long time before he put the to measuring devices together. They measure the resonant freq. and that is what you want to know right? Mine have worked fine for years and still do. His newer instruments are just great. Wish I could justify buying the new MFJ-259B. I had the chance to play with one and it is terrific. If you can't determin everything about your antenna with that, well, you are just a lost soul in a techie world. Friends still will borrow one or the other of my ancient units to check their new construction or to find how far off their antennas have become from the original design. MFJ makes affordable, accurate, devices for Hams and as long as Martin F. Jue is running the place they allways will. Butch Magee KF5DE Diamondhead, MS Jerry Martes wrote: Uwe I'd like to hear more about MFJ, and why you dont want to hear more about them. I was thinking that they were a pretty good way of measuring impedance. Jerry "Jaggy Taggy" wrote in message ... I recently got myself a noise bridge to do some quantitative measurements on my antennas. Lets say I am a bit disappointed. The small dials of the unit make all my antennas to appear very similar indeed and I am wondering if the unit is broken (not according to the test I did with a known load) or if I am expecting way too much. What do real people use to do impedance measurements on their antennas ( and please do not suggest anything from MFJ). 73 Uwe |
On Wed, 30 Mar 2005 00:48:17 GMT, Jaggy Taggy
wrote: I recently got myself a noise bridge to do some quantitative measurements on my antennas. Lets say I am a bit disappointed. The small dials of the unit make all my antennas to appear very similar indeed and I am wondering if the unit is broken (not according to the test I did with a known load) or if I am expecting way too much. Hi Uwe, The Cadillac of Bridges is the General Radio 1606-A. However, it requires you to have a source and detector. It is only a Bridge, but it is a precision Bridge. The circuit is quite simple, but its triple shielding and isolation are old-world craftsmanship. These things sold at a price equivalent to 6 months wage for a bench tech. 6 months wages currently will buy you equipment that could do what was only possible in a standards lab back then - but this is still in the region of 5 figures. You will need a stable source capable of 1 to 10mW power. You will need a well shielded communications receiver to act as a detector. You will also need a steady hand to balance the bridge (which may be part of your problem with the noise bridge). Nulls are often more than 100 dB deep. If you lack sufficient shielding for the detector, this will degrade the nulls considerably. If you lack stability, you will never find the null. The MFJs of the world are quick and dirty, but when they reach their limits you can only guess because they will as easily feed you a bogus reading when they are out to lunch. The GR 1606 has limits too, but they are obvious by dial indication and a top end of 60MHz usage (a more practical top end, however, is 30MHz). 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Wed, 30 Mar 2005 00:48:17 GMT, Jaggy Taggy wrote: I recently got myself a noise bridge to do some quantitative measurements on my antennas. Lets say I am a bit disappointed. The small dials of the unit make all my antennas to appear very similar indeed and I am wondering if the unit is broken (not according to the test I did with a known load) or if I am expecting way too much. Hi Uwe, The Cadillac of Bridges is the General Radio 1606-A. However, it requires you to have a source and detector. It is only a Bridge, but it is a precision Bridge. The circuit is quite simple, but its triple shielding and isolation are old-world craftsmanship. These things sold at a price equivalent to 6 months wage for a bench tech. 6 months wages currently will buy you equipment that could do what was only possible in a standards lab back then - but this is still in the region of 5 figures. You will need a stable source capable of 1 to 10mW power. You will need a well shielded communications receiver to act as a detector. You will also need a steady hand to balance the bridge (which may be part of your problem with the noise bridge). Nulls are often more than 100 dB deep. If you lack sufficient shielding for the detector, this will degrade the nulls considerably. If you lack stability, you will never find the null. The MFJs of the world are quick and dirty, but when they reach their limits you can only guess because they will as easily feed you a bogus reading when they are out to lunch. The GR 1606 has limits too, but they are obvious by dial indication and a top end of 60MHz usage (a more practical top end, however, is 30MHz). 73's Richard Clark, KB7QHC Richard As you know, I built that slotted line that works fairly well for load impedance measurements at 2 meters, and can be used at 100 MHz for FM antenna work. But, it is big and ugly. I do like the concept MFJ uses even though the results are surely less accurate than a good bridge. If I get an urge to include a HF antenna or other device whose impedance I want to determine, I might buy a MFJ. But, if a decent bridge ever becomes available at one of the HAM swap meets I'll sure pick up anything thats affordable. I am impressed that computer programs seem to have made impedance *measurement* unnecessary to most HAMS. I sure wish I was enough smarter to be able to manipulate the computer so I could get confidant about computer program results. That really impresses me when I read about how accurately antenna impedance can be predicted for various changes in parameters. When you guys arent argueing about stuff I cant understand, I sure learn alot from this Antenna Group. Thanks Jerry |
On Wed, 30 Mar 2005 08:16:42 GMT, "Jerry Martes"
wrote: As you know, I built that slotted line that works fairly well for load impedance measurements at 2 meters, and can be used at 100 MHz for FM antenna work. But, it is big and ugly. Hi Jerry, Imagine how ugly it would be for 20M. You could use yours there too, but you would have to study how to do what is called "load shifting." Conceptually it is quite simple, but as you have learned, the art of construction is where the accuracy is. I do like the concept MFJ uses even though the results are surely less accurate than a good bridge. There are so many ways to lose accuracy that you should look at the point spread. MFJ (and the other manufacturers) will probably suit 90% of users' expectations. If I get an urge to include a HF antenna or other device whose impedance I want to determine, I might buy a MFJ. But, if a decent bridge ever becomes available at one of the HAM swap meets I'll sure pick up anything thats affordable. Affordable generally runs in the neighborhood of $300. Myself, I would invest in a swept frequency system that does vector analysis and build heads that perform the SWR determination. This is software/hardware at its best. Flexibility is the hallmark of an instrument that will be kept being used. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Wed, 30 Mar 2005 08:16:42 GMT, "Jerry Martes" wrote: As you know, I built that slotted line that works fairly well for load impedance measurements at 2 meters, and can be used at 100 MHz for FM antenna work. But, it is big and ugly. Hi Jerry, Imagine how ugly it would be for 20M. You could use yours there too, but you would have to study how to do what is called "load shifting." Conceptually it is quite simple, but as you have learned, the art of construction is where the accuracy is. I do like the concept MFJ uses even though the results are surely less accurate than a good bridge. There are so many ways to lose accuracy that you should look at the point spread. MFJ (and the other manufacturers) will probably suit 90% of users' expectations. If I get an urge to include a HF antenna or other device whose impedance I want to determine, I might buy a MFJ. But, if a decent bridge ever becomes available at one of the HAM swap meets I'll sure pick up anything thats affordable. Affordable generally runs in the neighborhood of $300. Myself, I would invest in a swept frequency system that does vector analysis and build heads that perform the SWR determination. This is software/hardware at its best. Flexibility is the hallmark of an instrument that will be kept being used. 73's Richard Clark, KB7QHC Richard Your thoughts on sweep frequency for impedance measurement is very interesting. If I could figure out a way to build the "sensors" I'd probably build a sweep system for 2 meters. Although I could handle building a sweep frequency generator, I wouldnt know how to convert the "incident and reflected" to an impedance. I suspect that, if sweep frequency impedance measurements was simple enough for Me to do, someone would already have done it and published in one of the HAM magazines. Jerry Jerry |
Jerry Martes wrote:
As you know, I built that slotted line that works fairly well for load impedance measurements at 2 meters, and can be used at 100 MHz for FM antenna work. A pick-up loop will do the same thing for ladder-line. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Wed, 30 Mar 2005 14:45:12 GMT, "Jerry Martes"
wrote: Your thoughts on sweep frequency for impedance measurement is very interesting. If I could figure out a way to build the "sensors" I'd probably build a sweep system for 2 meters. Hi Jerry, Quite simple. Radio Shack sells SWR meters for 2M so you can take a lead for the sensors from there. Although I could handle building a sweep frequency generator, I wouldnt know how to convert the "incident and reflected" to an impedance. Software would do that. I suspect that, if sweep frequency impedance measurements was simple enough for Me to do, someone would already have done it and published in one of the HAM magazines. I'm sure that's already been done, at least in the ads in back. I am NOT discussing what might be, but what has been done. There is a kit, or kit design on the net, it's been mentioned here, I just haven't rummaged up the link yet. 73's Richard Clark, KB7QHC |
"Cecil Moore" wrote in message ... Jerry Martes wrote: As you know, I built that slotted line that works fairly well for load impedance measurements at 2 meters, and can be used at 100 MHz for FM antenna work. A pick-up loop will do the same thing for ladder-line. -- 73, Cecil http://www.qsl.net/w5dxp Cecil I've always ignored the use of twin lead for impedance measurement. But, it makes sense. I may try building something like that, just for fun. I certainly havent thought this "twin lead" line thru but it seems that one could be easily built from a length of PVC pipe with wires attached to its sides. This seems too easy and relatively accurate for "home in the garage" use. I'd bet I can find some info on the Internet for FM and TV frequency use. Thanks for jogging my mind. Jerry |
I still remember a demonstration of transmission line phenomena given by
Dick Silberstein, W0YBF, at the Denver Radio Club more than 30 years ago. He had made a "slotted line" (more accurately a trough line) from a wire tightly suspended in the "V" of a piece of angle aluminum or steel. It worked very well. It was pretty large -- I believe the frequency he used was around 50 MHz. You could use a smaller piece of angle material or suspend it deeper in the valley for a higher frequency. Roy Lewallen, W7EL Jerry Martes wrote: Cecil I've always ignored the use of twin lead for impedance measurement. But, it makes sense. I may try building something like that, just for fun. I certainly havent thought this "twin lead" line thru but it seems that one could be easily built from a length of PVC pipe with wires attached to its sides. This seems too easy and relatively accurate for "home in the garage" use. I'd bet I can find some info on the Internet for FM and TV frequency use. Thanks for jogging my mind. Jerry |
"Richard Clark" wrote in message ... On Wed, 30 Mar 2005 14:45:12 GMT, "Jerry Martes" wrote: Your thoughts on sweep frequency for impedance measurement is very interesting. If I could figure out a way to build the "sensors" I'd probably build a sweep system for 2 meters. The ARRL handbooks of yore have several designs. Google "monimatch". Although I could handle building a sweep frequency generator, I wouldnt know how to convert the "incident and reflected" to an impedance. Software would do that. No it wouldn't, unless you could figure out some way to get vector magnitude and phase from the two channels. I suspect that, if sweep frequency impedance measurements was simple enough for Me to do, someone would already have done it and published in one of the HAM magazines. Perhaps not the ham magazines; I know I've written at least three articles in Kitplanes magazine as to how to do it for aircraft antenna VSWR. Jim |
On Wed, 30 Mar 2005 08:42:14 -0800, "RST Engineering"
wrote: Although I could handle building a sweep frequency generator, I wouldnt know how to convert the "incident and reflected" to an impedance. Software would do that. No it wouldn't, unless you could figure out some way to get vector magnitude and phase from the two channels. Hi All, I have since researched the archives to find my reference: http://users.adelphia.net/~n2pk/ What can be done, what has been done, and what might be inferred is revealed at this page. 73's Richard Clark, KB7QHC |
Roy That trough configuration is probably more practical than the copper tube coax thing I made. I dont have any current interest in 50 MHz impedance measurement. But, I'll sure consider that "angle trough" method for any low cost tools for impedance measurements. I realize now that a slotted line even for 137 MHz is too large to have around in my garage. It requires too much room to set up the equipment. It is kinda interesting to consider something made from PVC so it is "disposable". I dont know how I'd handle the transitions to and from the coax. But, it seems like a guy could use a couple 4:1 baluns to get from 50 ohm coax to 200 ohm twin lead. I'm pretty sure I could find a couple long lengths of copper wire or tube that I could attach along the sides of some 1 inch PVC to make a 200 ohm line. That would be about 20 feet long for 6 meters. Wow, thats big. Maybe I should just get back to learning EZNEC. You guys actually get good impedance predictions for almost any antenna. Jerry "Roy Lewallen" wrote in message ... I still remember a demonstration of transmission line phenomena given by Dick Silberstein, W0YBF, at the Denver Radio Club more than 30 years ago. He had made a "slotted line" (more accurately a trough line) from a wire tightly suspended in the "V" of a piece of angle aluminum or steel. It worked very well. It was pretty large -- I believe the frequency he used was around 50 MHz. You could use a smaller piece of angle material or suspend it deeper in the valley for a higher frequency. Roy Lewallen, W7EL Jerry Martes wrote: Cecil I've always ignored the use of twin lead for impedance measurement. But, it makes sense. I may try building something like that, just for fun. I certainly havent thought this "twin lead" line thru but it seems that one could be easily built from a length of PVC pipe with wires attached to its sides. This seems too easy and relatively accurate for "home in the garage" use. I'd bet I can find some info on the Internet for FM and TV frequency use. Thanks for jogging my mind. Jerry |
Richard I'm way behind on the learning curve. I had read about that Vector Network Analyzer last year. it looked too complicated for me to build. And, in my ignorance, I thought that system was what the MFJ used. The Smith Chart plots from his VNA data really impresses me. I could possible get interested in building something like that for VHF, if I understood it. Jerry "Richard Clark" wrote in message ... On Wed, 30 Mar 2005 08:42:14 -0800, "RST Engineering" wrote: Although I could handle building a sweep frequency generator, I wouldnt know how to convert the "incident and reflected" to an impedance. Software would do that. No it wouldn't, unless you could figure out some way to get vector magnitude and phase from the two channels. Hi All, I have since researched the archives to find my reference: http://users.adelphia.net/~n2pk/ What can be done, what has been done, and what might be inferred is revealed at this page. 73's Richard Clark, KB7QHC |
On Wed, 30 Mar 2005 17:44:45 GMT, "Jerry Martes"
wrote: The Smith Chart plots from his VNA data really impresses me. I could possible get interested in building something like that for VHF, if I understood it. Hi Jerry, Take the plunge, that is the best way to learn. You already exhibit positive motivation and that's the hard part of learning. 73's Richard Clark, KB7QHC |
On Wed, 30 Mar 2005 09:19:40 -0800, Richard Clark
wrote: [snip] Hi All, I have since researched the archives to find my reference: http://users.adelphia.net/~n2pk/ What can be done, what has been done, and what might be inferred is revealed at this page. This is the greatest thing since sliced bread. I've built one for ~$250.00. Here's a photo taken during construction and checkout: http://www.qsl.net/n7ws/ANA_1.jpg (350K) Currently it's limited to 60 MHz although variants are in the works to increase this. Another minor limitation is that the software is DOS-based although Windows software is under development by other users. Xp users, like me, have a couple of hoops to jump through to access the parallel port but it's no big deal. The accuracy, however, is lab quality and is comparable to commercial network analyzers costing tens to hundreds of $K like I used to use, although it's achieved at the expense of measurement speed. Also see: http://groups.yahoo.com/group/N2PK-VNA/ A higher cost alternative with much reduced dynamic range is he http://radio.tentec.com/Amateur/vna |
On Wed, 30 Mar 2005 11:15:41 -0700, Wes Stewart
wrote: This is the greatest thing since sliced bread. Hi Wes, I can think of no greater commendation from an authority. Thanx for the insights offered. 73's Richard Clark, KB7QHC |
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On Thu, 31 Mar 2005 12:28:46 GMT, Jaggy Taggy
wrote: Richard, what do you mean by "If you lack sufficient shielding for the detector, this will degrade the nulls considerably." The source is the built in noise generator in the bridge. My detector is my Icom communications receiver, connected via coax to the bridge. What additional shielding would be needed?? Hi Uwe, This is entirely revealed by practice. Simply answer how wide is the null? If you don't know how wide it should be, then chances are you don't know how well your system is connected. As I pointed out elsewhere, I can obtain nulls that are 100dB deep. This is one simple measure of width; although not in terms of width, it is very distinctive and correlates with the same consideration. Far more frequently my nulls are 120+dB deep (down into the receiver noise from a signal level of 1mW). The problem with a noise bridge is power. To observe that same depth of null requires a noise source of several Watts (not sure of the actual figure now, but retail sources don't come close). If you balance your bridge and you run out of signal before you run out of balance, then your settings are at the wrong Z. What appeared to be a balanced null was actually the lack of source power to indicate you were off-set. For a noise bridge, you have to keep adjusting even when the noise seems to have been nulled. You keep going until you find the noise signal climbing again, and then split the difference between the two edges of the silent span of readings. If that is too wide, you could have quite a time of this. Now, consider if your shielding is not tight and you could hear your source along a path that was not through the bridge = you never hear a null, or you get a very very broad null that still contains noise/signal. A receiver's shielding that is good to -100 to -120dB is exceptional. In our hobby it is not exactly commonplace, but it is found in good equipment. However, how good your equipment is can only be determined through actual testing as shielding is also a function of grounding, and there everyone is an expert but few are well grounded. 73's Richard Clark, KB7QHC |
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