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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 |
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 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 |
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. |
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 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 |
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. |
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! =----- |
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. |
On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote: 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. Hi Jerry, In response to this, and your email, your wishes are most likely to evince a deafening silence. If you are trying to survey those who have built one from a population of those who used them, I think you've already hit a saturation of 2 possible users. Your own construction effort would qualify you to expand the population by 50%. As for why a slab style over a cylinder style. That for me is taken on faith that greater minds, in the person of HP Metrologists, figured that one out a long time ago. Myself, I can infer their rationale that perhaps demands more precision and accuracy than you care to pursue; but as any path demands a handcrafted solution, why put in 80% of the effort for a 20% design when 90% effort would double or triple your return? Harkening back to my days at Metrology school, I can imagine that the slab method was chosen because of the inevitable inner line sag that would inject residual SWR into the system. Sagging between two plates would seem to me to be a non-issue. Sagging within a cylinder may not bring enough residual SWR to cause you grief either, but you have to build and test one to discover the error of your presumption if it disappoints you. As for using your scope to eke out the voltage measurements. That is a tantalizing thought, but the big boys accomplish more with less. Simplicity is the keyword, with thinking outside of the box. You are focusing on the literal, absolute voltage measurement when SWR is all a matter of relativity that affords orders of magnitude more sensitivity and resolution (and hence accuracy). Research the Agilent archives for the Metrology papers of the 1960s. The discussion is very accessible (with only the math necessary to perform a real measurement) and focused to the concept and the theory. The writing of that era is a hallmark of clarity. 73's Richard Clark, KB7QHC |
On Mon, 10 May 2004 18:13:00 GMT, Richard Clark
wrote: Sagging between two plates would seem to me to be a non-issue. On further reflection (no pun), I would retract this because line sag would necessarily induce a voltage variation between the probe and the line that did not actually exist (probe tracking is very important, of course). Sag may not introduce residual SWR, but it may appear to. Also, yes, you could use a less than one wavelength long slotted line - provided you had a sliding load to make up for the remaining length. In other words, instead of moving the probe to measure the crests/troughs, you move the load (or both very carefully). When you introduce limitations into design and wish to maintain precision, it necessarily follows that you need more standards to compare against. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes" wrote: 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. Hi Jerry, In response to this, and your email, your wishes are most likely to evince a deafening silence. If you are trying to survey those who have built one from a population of those who used them, I think you've already hit a saturation of 2 possible users. Your own construction effort would qualify you to expand the population by 50%. As for why a slab style over a cylinder style. That for me is taken on faith that greater minds, in the person of HP Metrologists, figured that one out a long time ago. Myself, I can infer their rationale that perhaps demands more precision and accuracy than you care to pursue; but as any path demands a handcrafted solution, why put in 80% of the effort for a 20% design when 90% effort would double or triple your return? Harkening back to my days at Metrology school, I can imagine that the slab method was chosen because of the inevitable inner line sag that would inject residual SWR into the system. Sagging between two plates would seem to me to be a non-issue. Sagging within a cylinder may not bring enough residual SWR to cause you grief either, but you have to build and test one to discover the error of your presumption if it disappoints you. As for using your scope to eke out the voltage measurements. That is a tantalizing thought, but the big boys accomplish more with less. Simplicity is the keyword, with thinking outside of the box. You are focusing on the literal, absolute voltage measurement when SWR is all a matter of relativity that affords orders of magnitude more sensitivity and resolution (and hence accuracy). Research the Agilent archives for the Metrology papers of the 1960s. The discussion is very accessible (with only the math necessary to perform a real measurement) and focused to the concept and the theory. The writing of that era is a hallmark of clarity. 73's Richard Clark, KB7QHC Richard I used to think the HP slab configuration was choosen so as to put the probe in a place of minimum fiels within the line. That might be needed so that the probe introduces a minimum disturbance, or influence. The truth is; I dont really know why slab is prefered. I thpought I could attach some light dielectric supports along the bottom of the center conductor to eliminate the serious sag. I think the diam. ratio for the condoctors is close to 2.3 to 1 for 50 ohms without dielectric loading. Thats something I could evaluate after I decide how to make a slotted line. All thoughts from 'readers' will be appreciated, I'm way out of touch with antenna design these days. Jerry |
"Richard Clark" wrote in message ... On Mon, 10 May 2004 18:13:00 GMT, Richard Clark wrote: Sagging between two plates would seem to me to be a non-issue. On further reflection (no pun), I would retract this because line sag would necessarily induce a voltage variation between the probe and the line that did not actually exist (probe tracking is very important, of course). Sag may not introduce residual SWR, but it may appear to. Also, yes, you could use a less than one wavelength long slotted line - provided you had a sliding load to make up for the remaining length. In other words, instead of moving the probe to measure the crests/troughs, you move the load (or both very carefully). When you introduce limitations into design and wish to maintain precision, it necessarily follows that you need more standards to compare against. 73's Richard Clark, KB7QHC Richard This is a good example of how out of touch I am. I thought I could get all the impedance measuring data from a line only slightly over 1/4 wave long. Even after thinking about it for several minutes, I cant convince myself that the slotted line has to be more than 1/2 wave long. Jerry |
On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
wrote: 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. This month's QEX has an article with some information about slotted lines (used at 2.4 GHz in that article if I remember). Torsten Clay N4OGW |
How 'bout this...'
"Jerry Martes" wrote in message ... "Richard Clark" wrote in message ... On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes" wrote: making a slotted line. Hi Jerry, [...] As for why a slab style over a cylinder style. That for me is taken on faith that greater minds, in the person of HP Metrologists, figured that one out a long time ago. ... 73's Richard Clark, KB7QHC Richard I used to think the HP slab configuration was choosen so as to put the probe in a place of minimum fiels within the line. Jerry Gents. What you are calling "slab line" is more properly called "Stripline". It is a very well defined type of transmission line...that is, it has exact equations (unlike the one-sided Micro-strip). I suspect that Jerry's conclusion is right-on. At least theoretically, there is zero field along a line through the center conductor, parallel to the ground planes. So it appears that you would indeed disturb things the least...as long as your 'probe' exited parallel to the ground planes. Ignore everything but the lower right figure on page 3 of the stripline E field: http://www.altera.com/literature/wp/lvdsboardwp.pdf It is all sort of 'sucked' into the ground planes... The best pix I could find... I think as long as the probe has a small cross section when viewed from the line you will disturb the field the least. Seems to me in the old days, the probe consisted of a 1N21 diode... -- Steve N, K,9;d, c. i My email has no u's. |
On Mon, 10 May 2004 17:14:06 -0500, "Steve Nosko"
wrote: |How 'bout this...' | |"Jerry Martes" wrote in message .. . | "Richard Clark" wrote in message | ... | On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes" | wrote: | making a slotted line. | | Hi Jerry, | [...] As for why a slab style over a cylinder style. That for me is |taken | on faith that greater minds, in the person of HP Metrologists, figured | that one out a long time ago. ... | 73's | Richard Clark, KB7QHC | | Richard | | I used to think the HP slab configuration was choosen so as to put the | probe in a place of minimum fiels within the line. | Jerry | | |Gents. | What you are calling "slab line" is more properly called "Stripline". |It is a very well defined type of transmission line...that is, it has exact |equations (unlike the one-sided Micro-strip). I suspect that Jerry's |conclusion is right-on. At least theoretically, there is zero field along a |line through the center conductor, parallel to the ground planes. So it |appears that you would indeed disturb things the least...as long as your |'probe' exited parallel to the ground planes. |Ignore everything but the lower right figure on page 3 of the stripline E |field: |http://www.altera.com/literature/wp/lvdsboardwp.pdf | |It is all sort of 'sucked' into the ground planes... |The best pix I could find... I think as long as the probe has a small cross |section when viewed from the line you will disturb the field the least. |Seems to me in the old days, the probe consisted of a 1N21 diode... Steve Adam in "Microwave Theory and Applications," pp. 384-385 discusses the H-P slab line slotted section. (No surprise, Adam worked for H-P) He references, "A New Type of Slotted Line Section", Proceedings of the I.R.E., Vol. 38, No. 3 (March 1950). The slab line as shown is a round center conductor between parallel plates. Equations for this type of line are given in the ITT "Reference Data For Radio Engineers" and are different from those for stripline, which uses a flat center conductor. General Radio manufactured a true coaxial slotted line section (Discussed in their "Handbook of Coaxial Microwave Measurements"), but it is much more complicated mechanically than the H-P slab line. As to disturbing the fields with the probe, of course the fields are disturbed. Just like every other electrical measurement I can think of, the device or system under test is *always* disturbed. The question is just a matter of degree. As far a building a coaxial slotted line long enough for the frequency of interest, let me just ask how would you propose to machine a straight, uniform width slot down the length of a round piece of soft copper. And then, how would you support the center conductor without the supports getting in the way of the moving probe. Answer these questions (to start with) and then you'll understand why H-P used slab line. |
"Wes Stewart" wrote in message ... On Mon, 10 May 2004 17:14:06 -0500, "Steve Nosko" wrote: |How 'bout this...' | |"Jerry Martes" wrote in message .. . | "Richard Clark" wrote in message | ... | On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes" | wrote: | making a slotted line. | | Hi Jerry, | [...] As for why a slab style over a cylinder style. That for me is |taken | on faith that greater minds, in the person of HP Metrologists, figured | that one out a long time ago. ... | 73's | Richard Clark, KB7QHC | | Richard | | I used to think the HP slab configuration was choosen so as to put the | probe in a place of minimum fiels within the line. | Jerry | | |Gents. | What you are calling "slab line" is more properly called "Stripline". |It is a very well defined type of transmission line...that is, it has exact |equations (unlike the one-sided Micro-strip). I suspect that Jerry's |conclusion is right-on. At least theoretically, there is zero field along a |line through the center conductor, parallel to the ground planes. So it |appears that you would indeed disturb things the least...as long as your |'probe' exited parallel to the ground planes. |Ignore everything but the lower right figure on page 3 of the stripline E |field: |http://www.altera.com/literature/wp/lvdsboardwp.pdf | |It is all sort of 'sucked' into the ground planes... |The best pix I could find... I think as long as the probe has a small cross |section when viewed from the line you will disturb the field the least. |Seems to me in the old days, the probe consisted of a 1N21 diode... Steve Adam in "Microwave Theory and Applications," pp. 384-385 discusses the H-P slab line slotted section. (No surprise, Adam worked for H-P) He references, "A New Type of Slotted Line Section", Proceedings of the I.R.E., Vol. 38, No. 3 (March 1950). The slab line as shown is a round center conductor between parallel plates. Equations for this type of line are given in the ITT "Reference Data For Radio Engineers" and are different from those for stripline, which uses a flat center conductor. General Radio manufactured a true coaxial slotted line section (Discussed in their "Handbook of Coaxial Microwave Measurements"), but it is much more complicated mechanically than the H-P slab line. As to disturbing the fields with the probe, of course the fields are disturbed. Just like every other electrical measurement I can think of, the device or system under test is *always* disturbed. The question is just a matter of degree. As far a building a coaxial slotted line long enough for the frequency of interest, let me just ask how would you propose to machine a straight, uniform width slot down the length of a round piece of soft copper. And then, how would you support the center conductor without the supports getting in the way of the moving probe. Answer these questions (to start with) and then you'll understand why H-P used slab line. Wes I wonder why such a smart guy like you would ask about how a person would machine a straight slot where a "sraight slot" isnt even needed. Why would you propose use of "soft copper" And, why would you put any supports on the center conductor when it isnt necessary to put them on top, where the probe would.be. Besides, at 137 MHZ, is wouldnt be all that necessary to have the probe moveable continuously. It is aparent to me that either you want to diminish the value of my learning project or you are just not smart. I havent read this antenna news group for very long. Your *help* to me is hopefully not typical. I'm trying to learn. What are you trying to do?? Jerry |
Jerry Martes wrote:
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 Though you don't usually see it for sale, "coax" can be square as well as round. Why build a square line? Because it is a hell of a lot easier to mount and hold on to while you are doing the metal work and easier to build a centering probe carriage on a flat surface. -- Jim Pennino Remove -spam-sux to reply. |
On Tue, 11 May 2004 01:27:50 GMT, "Jerry Martes"
wrote: [snip] Let's establish something first. I used to instruct a fourth year/graduate level microwave measurements lab in which experiment #1 was on the use of slotted lines. | | I wonder why such a smart guy like you would ask about how a person would |machine a straight slot where a "sraight slot" (sic) isnt (sic) even needed. If you don't think the longitudinal slot needs to be straight down the length of the line then let's stop here. If I can convince you that it needs to be then we can proceed. |Why would |you propose (the) use of "soft copper"(?) Perhaps I shouldn't have said "soft" copper; nevertheless, copper isn't the easiest material to machine, and it was you, you should recall, that mentioned the use of copper water pipe. If you have a milling machine with five or six feet of table travel, then good for you, have at it. |And, why would you put any supports on the |center conductor when it isnt (sic) necessary to put them on top, where the probe |would.be.(?) The center conductor needs to be precisely located so as to not have the probe coupling vary as it is moved. This will necessitate a fair number of disks/beads or whatever. To be precise, the center conductor diameter should be reduced (or the outer conductor increased) wherever there is a dielectric support. If you intend to slot the supports, then the missing dielectric will modify the effective dielectric constant. Be sure and calculate that effect. |Besides, at 137 MHZ, is wouldnt be all that necessary to have |the probe moveable continuously. Oh, I didn't know that. Apparently General Radio didn't either; they had a micrometer attached to theirs to measure position to the thousandths of an inch. |It is aparent to me that either you want |to diminish the value of my learning project or you are just not smart. Why thank you! |I havent read this antenna news group for very long. I have, although with some of the inane stuff posted lately, it isn't often. BTW, it's an *amateur radio* group. I didn't know we had a band at 137 MHz. |Your *help* to me is |hopefully not typical. No, I believe in this instance that the advice Richard gave you and what I am trying to give you is atypically better that what you will usually get. |I'm trying to learn. What are you trying to do?? Beats the hell out of me. |
"Wes Stewart" wrote in message ... On Tue, 11 May 2004 01:27:50 GMT, "Jerry Martes" wrote: [snip] Let's establish something first. I used to instruct a fourth year/graduate level microwave measurements lab in which experiment #1 was on the use of slotted lines. | | I wonder why such a smart guy like you would ask about how a person would |machine a straight slot where a "sraight slot" (sic) isnt (sic) even needed. If you don't think the longitudinal slot needs to be straight down the length of the line then let's stop here. If I can convince you that it needs to be then we can proceed. |Why would |you propose (the) use of "soft copper"(?) Perhaps I shouldn't have said "soft" copper; nevertheless, copper isn't the easiest material to machine, and it was you, you should recall, that mentioned the use of copper water pipe. If you have a milling machine with five or six feet of table travel, then good for you, have at it. |And, why would you put any supports on the |center conductor when it isnt (sic) necessary to put them on top, where the probe |would.be.(?) The center conductor needs to be precisely located so as to not have the probe coupling vary as it is moved. This will necessitate a fair number of disks/beads or whatever. To be precise, the center conductor diameter should be reduced (or the outer conductor increased) wherever there is a dielectric support. If you intend to slot the supports, then the missing dielectric will modify the effective dielectric constant. Be sure and calculate that effect. |Besides, at 137 MHZ, is wouldnt be all that necessary to have |the probe moveable continuously. Oh, I didn't know that. Apparently General Radio didn't either; they had a micrometer attached to theirs to measure position to the thousandths of an inch. |It is aparent to me that either you want |to diminish the value of my learning project or you are just not smart. Why thank you! |I havent read this antenna news group for very long. I have, although with some of the inane stuff posted lately, it isn't often. BTW, it's an *amateur radio* group. I didn't know we had a band at 137 MHz. |Your *help* to me is |hopefully not typical. No, I believe in this instance that the advice Richard gave you and what I am trying to give you is atypically better that what you will usually get. |I'm trying to learn. What are you trying to do?? Beats the hell out of me. Well Wes I see it *IS* time to stop this rant. You *are* much more educated than I am. And I see you as also being smug and negative , at least as your help applies to my post. Why would I care if the slot was straight?? That makes no sense to me. I certainly have no reason to dissuade anyone from making the slot straight. But, why? If you are so smart, perhaps you can tell me _why_ something is done rather than just stating "thats the way HP or General Radio did it". What am I missing here?? I think that it would be nice to have the slot straight, I had intended to do that a VERY simple way, on a garage sized Bridgeport. Have you no concept of how to ,machine metal?? I wonder why you even bothered to post to my question when you make wrong statements. Please dont go away till you tell why the slot in a line must be straight if the line is to be used for impedance measurement. Yeah, Wes, There is an amateur radio group thats interested in monitoring weather satellites. Does that bother you?? What advice did Richard give that might help me build a slotted line?? Please take a few minutes to consider the validity of your advice before offering it. You seem to get side tracked easily like the softness of copper and the length of what a mill can slot. Try to focus on the fact that Jerry is trying to learn how to build a slotted line at home. If you have *ANY* helpfull information I will certainly appreciate hearing it. Jerry |
A slotted line is a fundamental Standard of Impedance. It relates impedance
measurements to the absolute standard of length, the International Metre! Its weakest feature is the probe mechanism itself. Its electrical presence is unpredictable. Incalculable. Uncertainty can only be minimised. Or am I 60 years out of date? Does anyone know what is the uncertainty in measuring an impedance of, say, around 100 ohms at 10 MHz, as may be claimed by a National Measurement Standards Laboratory. What may be claimed by an instrument manufacturer in the sales blurb is another matter. ---- Reg, G4FGQ |
Reg
I would sure like to get some information from you. I think you are the kind of guy who would know about how this line might be made to work. I have some 'notions' that may well be wrong, and I'm too easily confused to work this out alone. It seems to me that a "home made" slotted line wouldnt have to be 50 ohms, yet it could accurately measure load impedances. For my purpose, I'd want to try to get the line impedance as close to 50 as practical. But, if the slotted line's impedance was 48 ohms or 52 ohms, the line would give quite good information on load impedance. That would require alot of testing to actually determine the Zo of the line. But, I would like only to get real close. I dont need a lab standard. Since I've gotten so involved with this news group discussion, I'll start building a slotted line useing that 5 foot long piece of copper tube at Home Depot. Still, it would be more satisfying to know more about what I'm attempting. It occurred to me that, if building a slotted line for impedance measurement at 137 MHz was easy, someone would have done it already. So, I'm not real confidant that I'll be successful. I'd thought I might be able to use my Oscilloscope with a sensitive probe for the "voltage measurement". But, since I'm hearing nothing from this group about how slotted lines are home made for VHF impedance measurement, I'm not confidant I'll be successful. Of course it is not you who is out of date. It is me. I've been away from electronics and antennas for the last 35 years. Jerry "Reg Edwards" wrote in message ... A slotted line is a fundamental Standard of Impedance. It relates impedance measurements to the absolute standard of length, the International Metre! Its weakest feature is the probe mechanism itself. Its electrical presence is unpredictable. Incalculable. Uncertainty can only be minimised. Or am I 60 years out of date? Does anyone know what is the uncertainty in measuring an impedance of, say, around 100 ohms at 10 MHz, as may be claimed by a National Measurement Standards Laboratory. What may be claimed by an instrument manufacturer in the sales blurb is another matter. ---- Reg, G4FGQ |
On Tue, 11 May 2004 03:19:12 GMT, "Jerry Martes"
wrote: [snip] |If you |have *ANY* helpfull information I will certainly appreciate hearing it. | Sorry, I can't help you. ps. Before I went into engineering I was a machinist. |
Reg Edwards wrote:
Its weakest feature is the probe mechanism itself. Its electrical presence is unpredictable. Incalculable. Uncertainty can only be minimised. I remember making some measurements on half-inch 75 ohm coax at Texas A&M by simply pushing the braid away from a place on the inner insulation and drilling a small hole to the center conductor. We used an RF VTVM probe. I remember seeing the effects of standing voltage waves but I have no idea what the accuracy was. -- 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! =----- |
FWIW here, using network analyzers may be preferable to slotted lines for RF
system measurements. The paper linked below tells why... http://broadcastengineering.com/ar/b...nts_revisited/ RF Visit http://rfry.org for FM broadcast RF system papers. |
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 |
"Jerry Martes" wrote in message .. .
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? Seems like the postings I've seen here are off on the wrong foot. Yep, there will be problems, but I've learned a lot more by building and experimenting with things that didn't work, than listening to or arguing with people telling me that they won't work. And in fact, a lot of the time they DO work well enough for me to accomplish what I wanted. So I'd like to encourage Jerry to have a go at the copper-pipe slotted line, keeping in mind the pitfalls that have been mentioned and thinking of ways around them. I know I would have no trouble at all making a uniform narrow slot in such a pipe, maintaining a radial angle straightness well under a degree, without using a milling machine. I know I'd have no trouble supporting the 7/8" diameter center conductor to maintain the impedance within a fraction of a percent over the line length. With proper geometry, sag would not be an issue for coupling. (Keeping the line vertical is one way, but not the only way.) Perhaps more important is the initial straightness of the pipes. 3/4"nom pipe is stiff enough that I can see doing this with support only at the ends, so impedance AND velocity factor variation caused by dielectric supports are no longer issues. Will everything go smoothly? Heck no! Will Jerry learn from doing it? You bet! Go for it, Jerry, and please see if you can write up something about your results, to share with others. Listen to the advice you get, but make up your own mind about what will work best for you. You should be able to make very decent estimates of the effects from the problem areas folk have mentioned, through either formulas or measurements or both. You should be able to make tests on your line to see if it's performing like you think it is, by applying known loads and testing at various frequencies, using assorted detectors and probes. Cheers, Tom |
I would sure like to get some information from you.
You don't sound like the CIA so I'll do my best. You are correct. A home-made slotted line can be any value Zo provided you know what it is. But it is prudent to be close to the standard 50 or 75 ohms because of the Zo of any other cables and connectors to be used in conjunction with it. Otherwise you will forever be calculating corrections for impedance mismatches. And how do you know what Zo is? You CALCULATE it from measured dimensions of the line's cross-section. After all, that's why you constructed it in the first place. Uncertainty in Zo depends on cross-section measuring accuracy and, of course, on the accuracy of the calculating formula. The most simple cross-section is circular coaxial for which Zo = 60*Ln(D/d) exactly. But there's difficulty in producing a narrow slot without mechanical distortion. If you can think of a way of doing it that's fine. It depends on what measuring accuracy is required. Fortunately, Zo is relatively insensitive to absolute measurements on the cross section. Quite a big fat, strong, easily-measured line can be made. Note that absolute measurements of the cross-section are not required - only the RATIO D/d is needed. This freedom also applies to all other shapes of cross section. There is no requirement for a length of line to be absolutely straight unless you intend to make an international measurement standard out of it. ;o) The next most simple cross section is a square or rectangular aluminium trough or channel with the open top capped with 2 parallel straight strips spaced apart by the width of the slot which should be no wider than absolutely necessary. The separate parts can be screwed together. A problem is providing support for the inner conductor of a long length of line. Ideally it should be rigid enough to be self-supporting without significant sag. Hard aluminium (Duralumin) alloy will be OK. A single support, half-way along the line, could be made from a THIN rigid plastic plate about an inch long. A square or rectanglar cross section will allow the probe carraige to slide smoothly along the line. The formula for calculating line Zo will be fairly simple. I'm sure it will be available from some reliable source. Just don't even dream of deriving a formula from first principles. Accuracy of determination of Zo should be within a few percent even using a wooden ruler. Departure of the inner conductor itself from a circular cross section, eg., square or moderately rectangular, will not detract from measurement accuracy for the degree of accuracy you say you require. Below 200 or 300 MHz there's no need to be very particular about the coaxial connectors at the ends of the line. You should be more concerned about uncertainty of the impedance of the coaxial cables used in conjunction with it. Can't think of anything else at present. How do you propose to use a slotted line to measure unknown impedances? ---- Reg, G4FGQ I think you are the kind of guy who would know about how this line might be made to work. I have some 'notions' that may well be wrong, and I'm too easily confused to work this out alone. It seems to me that a "home made" slotted line wouldnt have to be 50 ohms, yet it could accurately measure load impedances. For my purpose, I'd want to try to get the line impedance as close to 50 as practical. But, if the slotted line's impedance was 48 ohms or 52 ohms, the line would give quite good information on load impedance. That would require alot of testing to actually determine the Zo of the line. But, I would like only to get real close. I dont need a lab standard. Since I've gotten so involved with this news group discussion, I'll start building a slotted line useing that 5 foot long piece of copper tube at Home Depot. Still, it would be more satisfying to know more about what I'm attempting. It occurred to me that, if building a slotted line for impedance measurement at 137 MHz was easy, someone would have done it already. So, I'm not real confidant that I'll be successful. I'd thought I might be able to use my Oscilloscope with a sensitive probe for the "voltage measurement". But, since I'm hearing nothing from this group about how slotted lines are home made for VHF impedance measurement, I'm not confidant I'll be successful. Of course it is not you who is out of date. It is me. I've been away from electronics and antennas for the last 35 years. Jerry "Reg Edwards" wrote in message ... A slotted line is a fundamental Standard of Impedance. It relates impedance measurements to the absolute standard of length, the International Metre! Its weakest feature is the probe mechanism itself. Its electrical presence is unpredictable. Incalculable. Uncertainty can only be minimised. Or am I 60 years out of date? Does anyone know what is the uncertainty in measuring an impedance of, say, around 100 ohms at 10 MHz, as may be claimed by a National Measurement Standards Laboratory. What may be claimed by an instrument manufacturer in the sales blurb is another matter. ---- Reg, G4FGQ |
Jerry Martes wrote:
"I`m considering building a slotted line for measuring impedance at 137 MHz." The 2nd edition of the RSGB "VHF-UHF Manual" has a home-built "trough line" on page 10.26. Dimensions are given for 50 ohms and the line is expected to be usable down to 145 MHz at its length of 36 to 40 inches. It needs to be at least 1/2-wavelength. Best regards, Richard Harrison, KB5WZI |
Thanks Richard. Thats just the kind of information I need. But, I dont
supose I can get that information off the Internet, ?can I? I'd take a trip to the library to look at something like that. Maybe you can tell me more about where I can find the book or article. Jerry "Richard Harrison" wrote in message ... Jerry Martes wrote: "I`m considering building a slotted line for measuring impedance at 137 MHz." The 2nd edition of the RSGB "VHF-UHF Manual" has a home-built "trough line" on page 10.26. Dimensions are given for 50 ohms and the line is expected to be usable down to 145 MHz at its length of 36 to 40 inches. It needs to be at least 1/2-wavelength. Best regards, Richard Harrison, KB5WZI |
Jerry wrote:
"Maybe you can tell me more about where I can find the book or article." I believe the ARRL and RSGB sell each others books. There is a good chance of finding a copy in the U.S.A. of the RSGB "VHF-UHF Manual". Best regards, Richard Harrison, KB5WZI |
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