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#1
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50 ohm - 5 foot long - Slotted line.
I'm condidering building a slotted line for measuring impedance at 137
MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Jerry |
#2
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Jerry Martes wrote:
I'm condidering building a slotted line for measuring impedance at 137 MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Try a Lecher wire system described in 50's ARRL Handbooks. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#3
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"Cecil Moore" wrote in message ... Jerry Martes wrote: I'm condidering building a slotted line for measuring impedance at 137 MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Try a Lecher wire system described in 50's ARRL Handbooks. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- Cecil I thought lecher line measurements used a balanced line. I'm not smart enough to know how to compensate for any irregulariities associated with open wire measurements of an unbalanced load. And, it seems that it would be easy enoughto make a 'good enough' slotted line. But, since I havent found referance to any home built coaxial slotted lines, maybe its not as easy as I think. Jerry |
#4
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Jerry Martes wrote:
"Cecil Moore" wrote in message ... Jerry Martes wrote: I'm condidering building a slotted line for measuring impedance at 137 MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Try a Lecher wire system described in 50's ARRL Handbooks. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- Cecil I thought lecher line measurements used a balanced line. I'm not smart enough to know how to compensate for any irregulariities associated with open wire measurements of an unbalanced load. And, it seems that it would be easy enoughto make a 'good enough' slotted line. But, since I havent found referance to any home built coaxial slotted lines, maybe its not as easy as I think. Jerry About the only difficulty in doing it (besides cost) I can think of is it would be hard to find a 5 foot long tube you could cut a slot in and not have it distort due to relieved stress. Use a square U channel for a bottom and 2 pieces of L to make the top. Look up the equation for the impedance of square coax, get brass tubing (which comes in increaments of 1/32") for the center conductor, make 2 end plates for connectors, screw the thing together. Probably should use plastic screws for the top so they don't have to be flush on the inside. Thought about making one myself for grins and giggles, but bought a MFJ analyzer instead; lot more portable. -- Jim Pennino Remove -spam-sux to reply. |
#5
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Jerry Martes wrote:
I thought lecher line measurements used a balanced line. Yep, it does. I make all my measurements on the balanced portion of my line and Smith Chart extrapolate to find out what is going on in the coax. The approach depends on what you need to accomplish. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#6
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On Mon, 10 May 2004 03:12:05 GMT, "Jerry Martes"
wrote: I'm condidering building a slotted line for measuring impedance at 137 MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Jerry Hi Jerry, It is not all that hard to do in fact. HP solved that problem long ago by thinking outside of the box by thinking box. As with other test equipment issues, it is simply a matter of planning and testing with very simple methods. For those whose greatest physical effort in Ham radio is sliding a credit card across the showroom display counter, this is called "work." Basically you construct your coaxial line with the usual interior line, but unlike the conventional expectations, you do not try to emulate the outer portion as a cylinder. You construct the outer portion as two parallel conductive planes (appropriately shorted to the connector shells at each end (hence the allusion to box): view HP 805C Slotted Line picture on ebay at: http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0 Where the two planes stand apart, you insert a probe to measure the potential along the line. The depth of the line within and in between the two parallel surfaces insures the line isolation (no leakage) as well as preserving the line characteristic Z. However, anyway that you look at it (even the lecher line suffers from this) you run the risk of over coupling and throwing the measurement into confusion (very simple to make errors). The problem is the probe will introduce its own SWR and gum up the works if it lacks sensitivity. I won't bother too much with dimensions here, but instead offer a formula for such a structu Zc = (138/sqrt(e))·log(4h/pi·d) where e: dielectric constant (= 1 for air) d: interior line diameter h: wall separation You will want to build it long enough to be more than a wavelength of course. You will also need to calibrate it to determine the residual SWR it presents to the system (this will reveal construction errors). Off hand, I would suggest that the walls be roughly a 2 to 4 cm apart and at least a 20 cm wide (larger wouldn't hurt). Build one quick and dirty to get your gross mistakes out of the way without spending too much time on them. I can guarantee no one here could build it right the first time (including yours truly). Once you've got the first pass attempt on the bench, then we can talk about how to use it right. ;-) 73's Richard Clark, KB7QHC |
#7
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"Richard Clark" wrote in message ... On Mon, 10 May 2004 03:12:05 GMT, "Jerry Martes" wrote: I'm condidering building a slotted line for measuring impedance at 137 MHz. I find no referances to home made lines thru my quick Google search. Does anyone know of any publication that show how someone has already worked out the problems of contructing one? Jerry Hi Jerry, It is not all that hard to do in fact. HP solved that problem long ago by thinking outside of the box by thinking box. As with other test equipment issues, it is simply a matter of planning and testing with very simple methods. For those whose greatest physical effort in Ham radio is sliding a credit card across the showroom display counter, this is called "work." Basically you construct your coaxial line with the usual interior line, but unlike the conventional expectations, you do not try to emulate the outer portion as a cylinder. You construct the outer portion as two parallel conductive planes (appropriately shorted to the connector shells at each end (hence the allusion to box): view HP 805C Slotted Line picture on ebay at: http://web.ask.com/redir?bpg=http%3a...html&qte=0&o=0 Where the two planes stand apart, you insert a probe to measure the potential along the line. The depth of the line within and in between the two parallel surfaces insures the line isolation (no leakage) as well as preserving the line characteristic Z. However, anyway that you look at it (even the lecher line suffers from this) you run the risk of over coupling and throwing the measurement into confusion (very simple to make errors). The problem is the probe will introduce its own SWR and gum up the works if it lacks sensitivity. I won't bother too much with dimensions here, but instead offer a formula for such a structu Zc = (138/sqrt(e))·log(4h/pi·d) where e: dielectric constant (= 1 for air) d: interior line diameter h: wall separation You will want to build it long enough to be more than a wavelength of course. You will also need to calibrate it to determine the residual SWR it presents to the system (this will reveal construction errors). Off hand, I would suggest that the walls be roughly a 2 to 4 cm apart and at least a 20 cm wide (larger wouldn't hurt). Build one quick and dirty to get your gross mistakes out of the way without spending too much time on them. I can guarantee no one here could build it right the first time (including yours truly). Once you've got the first pass attempt on the bench, then we can talk about how to use it right. ;-) 73's Richard Clark, KB7QHC Richard I'm pretty sure I'll try to make it coax instead of "slab" like HP does it. I saw some 2 inch copper pipe at Home Depot today. They sell it in 5 foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for the slotted coax. I thought I might be able to 'get away with' using my scope for detecting the line voltages. I am wide open for learning that my approach has extremely difficult to solve problems. It all seems fairly easy right now. I was hoping to get a response from someone who either had built a line or knew of a publication on the pitfalls associated with making a slotted line. Jerry |
#8
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On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote: [Richard's good stuff snipped] You can lead 'em to water, but you can't make 'em drink. | | I'm pretty sure I'll try to make it coax instead of "slab" like HP does |it. I saw some 2 inch copper pipe at Home Depot today. They sell it in 5 |foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for |the slotted coax. I thought I might be able to 'get away with' using my |scope for detecting the line voltages. I am wide open for learning that my |approach has extremely difficult to solve problems. It all seems fairly |easy right now. | I was hoping to get a response from someone who either had built a line or |knew of a publication on the pitfalls associated with making a slotted line. |
#9
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Wes Maybe I missed your point. I am trying to learn how to build a slotted line for impedance measurements at 137 MHz. It would be real helpfull to me to see how someone has built one at home. I'd sure appreciate hearing about how you did it or what you know about how I could build one here in my garage. In fact, I'd like to know more about why a person (HAM type) would use the slab line configuration in preferance to coaxial. Jerry "Wes Stewart" wrote in message ... On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes" wrote: [Richard's good stuff snipped] You can lead 'em to water, but you can't make 'em drink. | | I'm pretty sure I'll try to make it coax instead of "slab" like HP does |it. I saw some 2 inch copper pipe at Home Depot today. They sell it in 5 |foot lengths for about $22.00. I'm OK with using only 1/2 wavelength for |the slotted coax. I thought I might be able to 'get away with' using my |scope for detecting the line voltages. I am wide open for learning that my |approach has extremely difficult to solve problems. It all seems fairly |easy right now. | I was hoping to get a response from someone who either had built a line or |knew of a publication on the pitfalls associated with making a slotted line. |
#10
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Jerry Martes wrote:
In fact, I'd like to know more about why a person (HAM type) would use the slab line configuration in preference to coaxial.... One advantage of slab line is that when you drop something like a set screw into the line, it falls right on through. (Been there, done that). I've used the HP slab-line instrument and obtained very good, repeatable results with it. Others have commented that the big advantage is ease of fabrication, and I have to agree. Cutting a slot in a water pipe seems like a lot of trouble for minimum gain. Another alternative might be a trough line. This is a slab line with bottom side closed off. The advantage this would provide is a means of supporting the center conductor by means of a longitudinal, dielectric strip between it and the bottom plate of the line. A long piece of Teflon fiberglass would be ideal for this. The line dimensions would have to be adjusted to maintain a 50-ohm Zo. Anyway, it's an interesting project. Good luck with it. Jim, K7JEB Glendale, AZ |
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