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How to Measure a 2M Yagi Impedance?
Hi,
I have done quite a bit of numerical analysis of VHF Yagi's with usually very good agreement between theory and performance at least for gain and pattern. One of the things that seems more difficult to correctly predict however is the input impedance, and thus to design some sort of a match without some sort of subsequent trial and error. In an effort to explore the differences further I have been looking at ways of measuring the impedance directly rather than indirectly by vswr with the matching in place. I have made a number of different return loss bridges, and even tried the technique described in an old Ham Radio Article where you take two VSWR readings with and without an added series resistance. All of course making allowances for coax length etc. The problem I have is all of them give, in some cases wildly, different answers even when used on the same antenna. So my questions a What sort of accuracy can I expect from these sorts of methods? Is there a better way (which doesn't involve large sums of money) to measure antenna impedance at say 146Mhz? Thanks Paul VK3DIP |
"Paul (Home) News" wrote I have done quite a bit of numerical analysis of VHF Yagi's with usually very good agreement between theory and performance at least for gain and pattern. One of the things that seems more difficult to correctly predict however is the input impedance, and thus to design some sort of a match without some sort of subsequent trial and error. In an effort to explore the differences further I have been looking at ways of measuring the impedance directly rather than indirectly by vswr with the matching in place. I have made a number of different return loss bridges, and even tried the technique described in an old Ham Radio Article where you take two VSWR readings with and without an added series resistance. All of course making allowances for coax length etc. The problem I have is all of them give, in some cases wildly, different answers even when used on the same antenna. So my questions a What sort of accuracy can I expect from these sorts of methods? Is there a better way (which doesn't involve large sums of money) to measure antenna impedance at say 146Mhz? Thanks Paul VK3DIP =============================== Paul, Attempts to accurately determine antenna input impedance, using an inherently ambiguous, innacurate SWR meter at the transmitter end of a line of uncertain length and velocity, are doomed to failure. *Never* expect to obtain numbers worthy of serious engineering application. There are far too many uncertainties of unknown magnitudes. The only way of obtaining an accurate measurement is to climb a ladder taking with you an R plus or minus jX hand-held impedance bridge. Can you borrow one ? But why do you wish to know antenna input impedance when you are aleady quite happy with using an inaccurate SWR meter to fiddle a 1-to-1 SWR at the transmitter end. The ultimate objective, of course, is just to obtain a 50-ohm load for the transmitter regardless of what the SWR and antenna impedance might be. ---- Reg, G4FGQ |
The input impedance can be measured reasonably well at ground level.
Align the antenna so that the reflector is 'down' and the last director is 'up'. Ground effects are minimized due to the F/B of the antenna. The antenna is radiating straight 'up'. Next take a 1 wavelength, allowing for velocity factor, coax line and connect it to the antenna feedpoint. Finally, beg, borrow, requisition, pilfer, rustle, etc., an antenna analyzer similar to the MFJ 259B. Connect it to the other end of the 1 wavelength coax. Select the function to read impedance. Dial in your frequency and read the impedance. A one minute job once the antenna, coax and meter are at hand. + + + Paul (Home) News wrote: Hi, I have done quite a bit of numerical analysis of VHF Yagi's with usually very good agreement between theory and performance at least for gain and pattern. One of the things that seems more difficult to correctly predict however is the input impedance, and thus to design some sort of a match without some sort of subsequent trial and error. In an effort to explore the differences further I have been looking at ways of measuring the impedance directly rather than indirectly by vswr with the matching in place. I have made a number of different return loss bridges, and even tried the technique described in an old Ham Radio Article where you take two VSWR readings with and without an added series resistance. All of course making allowances for coax length etc. The problem I have is all of them give, in some cases wildly, different answers even when used on the same antenna. So my questions a What sort of accuracy can I expect from these sorts of methods? Is there a better way (which doesn't involve large sums of money) to measure antenna impedance at say 146Mhz? Thanks Paul VK3DIP |
Hi Paul
Gordon VK2ZAB (and others) published some time ago a complex Z bridge thing for VHF/UHF. It uses transmission lines for tuned elements and is band specific. Try http://www.vhfdx.oz-hams.org/measurements.html And do other google searches using Gordons callsign Cheers Bob VK2YQA Paul (Home) News wrote: Hi, I have done quite a bit of numerical analysis of VHF Yagi's with usually very good agreement between theory and performance at least for gain and pattern. One of the things that seems more difficult to correctly predict however is the input impedance, and thus to design some sort of a match without some sort of subsequent trial and error. In an effort to explore the differences further I have been looking at ways of measuring the impedance directly rather than indirectly by vswr with the matching in place. I have made a number of different return loss bridges, and even tried the technique described in an old Ham Radio Article where you take two VSWR readings with and without an added series resistance. All of course making allowances for coax length etc. The problem I have is all of them give, in some cases wildly, different answers even when used on the same antenna. So my questions a What sort of accuracy can I expect from these sorts of methods? Is there a better way (which doesn't involve large sums of money) to measure antenna impedance at say 146Mhz? Thanks Paul VK3DIP |
Dave, I was replying to the original questioner. But by immediately
following your response with mine and including a comment of yours caused a little confusion. Sorry! I agree your method will work. The problem, a practical one, is obtaining a COAXIAL line length exactly an integral number of 1/2-wavelengths long. There's no way of proving it exept by climbing a ladder and disconnecting the line from the antenna. And it is an exact 1/2-wavelength long at ONE frequency only. But it is required to make measurements over a whole band of frequencies. To shift to other frequencies involves calculations taking Zo into account. But Zo is not accurately known. So then you have to measure line Zo. And so on. And you have to know exactly what you are doing because the 259B does not provide the sign of jX in R+jX. But as I said before, all you want to know is whether or not the transmitter is loaded with 50 ohms. To hell with SWR and antenna input impedance. ;o) ---- Reg, G4FGQ |
Thanks Bob (Bob)
I'm going to make one of those impedance "meters". I sure appreciate having guys like you do all the research work for me. Thanks again. Jerry "Bob Bob" wrote in message ... Hi Paul Gordon VK2ZAB (and others) published some time ago a complex Z bridge thing for VHF/UHF. It uses transmission lines for tuned elements and is band specific. Try http://www.vhfdx.oz-hams.org/measurements.html And do other google searches using Gordons callsign Cheers Bob VK2YQA Paul (Home) News wrote: Hi, I have done quite a bit of numerical analysis of VHF Yagi's with usually very good agreement between theory and performance at least for gain and pattern. One of the things that seems more difficult to correctly predict however is the input impedance, and thus to design some sort of a match without some sort of subsequent trial and error. In an effort to explore the differences further I have been looking at ways of measuring the impedance directly rather than indirectly by vswr with the matching in place. I have made a number of different return loss bridges, and even tried the technique described in an old Ham Radio Article where you take two VSWR readings with and without an added series resistance. All of course making allowances for coax length etc. The problem I have is all of them give, in some cases wildly, different answers even when used on the same antenna. So my questions a What sort of accuracy can I expect from these sorts of methods? Is there a better way (which doesn't involve large sums of money) to measure antenna impedance at say 146Mhz? Thanks Paul VK3DIP |
I want to caution you about using a half or one wavelength line to do
measurements. That's a viable method if the imedance of the antenna is close to the characteristic of the line. If it's not, you'll find that even a surprisingly small line loss -- one that you'd normally consider negligible, can seriously skew your results. The calculation is straightforward presuming you know the loss -- I'm sure Reg's program would be adequate. Do some what-ifs with various antenna impedances and you'll see what I mean. The effect gets worse as the antenna and transmission line impedances get more different, and as the line gets longer. That is, a one wavelength line will have more effect than a half wavelength one. Also, line length becomes more and more critical as the impedance of the antenna and transmission line become more different and as the line gets longer. Again, a little experimentation with the calculations will illustrate what to expect. Even if you carefully account for the transformation of the connecting line or don't use any line at all, you have to be aware of common mode currents and how your test setup differs from your normal rig connection. And finally, even with a perfect lab setup, you'll find that good impedance measurements can be hard to make with amateur equipment. Before you get carried away, make some measurements on the bench with your meter and using good quality loads, or at least RC combinations using chip resistors and capacitors or ones with extremely short leads. Make impedances similar to ones you hope to measure. If you can get values which are accurate enough to suit you, go to the next step and measure the same loads through a transmission line as has been suggested, and see if you're able to extract the actual load value from the measured value with sufficient accuracy. If you get that far, you've partially answered your question about what kind of accuracy to expect, and you're ready to start figuring out how to deal with common mode currents. Decent antenna impedance measurements aren't simple to make, even at HF. They're more difficult at VHF and above. Roy Lewallen, W7EL |
Reg Edwards wrote:
Dave, I was replying to the original questioner. But by immediately following your response with mine and including a comment of yours caused a little confusion. Sorry! I agree your method will work. The problem, a practical one, is obtaining a COAXIAL line length exactly an integral number of 1/2-wavelengths long. The MFJ 259B will measure the length of the line for you before test. Leave it open circuited and connect the other end to the MFJ. It finds the 1/2 wavelength frequency. There's no way of proving it exept by climbing a ladder and disconnecting the line from the antenna. A bench setup with a 1 wavelength line does not require climbing and measuring at the tower. And it is an exact 1/2-wavelength long at ONE frequency only. But it is required to make measurements over a whole band of frequencies. To shift to other frequencies involves calculations taking Zo into account. But Zo is not accurately known. So then you have to measure line Zo. And so on. Agree it is a one frequency measurement. And you have to know exactly what you are doing because the 259B does not provide the sign of jX in R+jX. But, the sign of X is very easy to determine. Increase frequency 'slightly' on the 259B and observe absolute value of X. If X increases it is inductive. If X decreases it is capacitive. BTW the Or is a value at antenna resonance only at one frequency. But, you a very well aware of that. The comment is made for other readers. But as I said before, all you want to know is whether or not the transmitter is loaded with 50 ohms. To hell with SWR and antenna input impedance. ;o) ---- Reg, G4FGQ There is one possibility remaining. If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. Most Ham yagis are not tuned for optimum gain as we all know. There is WAY WAY too much emphasis among today's hams regarding low VSWR. My 75/80 doublet has a VSWR approaching 30:1 and works just FB!! The problem is the absence of output tuning in most of the rigs available today. Oh! for my OLD VIKING II or my Drake 4C. grin Gor Bless and you have my permission to celebrate the USA Father's Day. |
Roy, your caution is well placed. As I've said before, most amateurs and professionals (it seems from these walls) suffer from delusions of accuracy. Their delusions are seldom frustrated by things going wrong after some tedious, supposedly highly accurate, design work has been done. They congratulate themselves on their success and sometimes follow up by writing learned papers incorporating 6 places of decimals about it. But in engineering reallity, especially with Radio, things work simply because any bloody thing will work after a fashion. And if the transmitter is loaded with an actual but unknown impedance between 30 and 80 ohms, such that it works, they continue to remain oblivious to their delusions. I relate this, certainly not to cast ridicule, but with the inention of enhancing the underlying beauty of this intriguing hobby of ours. Perhaps 'suffer from' are the wrong words. In the UK it is Father's Day. So I am about to pour myself another glass from a bottle of special reserve port, a thoughtful present from a loving 'doter'. ---- 73 and 88, Reg, G4FGQ "Roy Lewallen" wrote in message ... I want to caution you about using a half or one wavelength line to do measurements. That's a viable method if the imedance of the antenna is close to the characteristic of the line. If it's not, you'll find that even a surprisingly small line loss -- one that you'd normally consider negligible, can seriously skew your results. The calculation is straightforward presuming you know the loss -- I'm sure Reg's program would be adequate. Do some what-ifs with various antenna impedances and you'll see what I mean. The effect gets worse as the antenna and transmission line impedances get more different, and as the line gets longer. That is, a one wavelength line will have more effect than a half wavelength one. Also, line length becomes more and more critical as the impedance of the antenna and transmission line become more different and as the line gets longer. Again, a little experimentation with the calculations will illustrate what to expect. Even if you carefully account for the transformation of the connecting line or don't use any line at all, you have to be aware of common mode currents and how your test setup differs from your normal rig connection. And finally, even with a perfect lab setup, you'll find that good impedance measurements can be hard to make with amateur equipment. Before you get carried away, make some measurements on the bench with your meter and using good quality loads, or at least RC combinations using chip resistors and capacitors or ones with extremely short leads. Make impedances similar to ones you hope to measure. If you can get values which are accurate enough to suit you, go to the next step and measure the same loads through a transmission line as has been suggested, and see if you're able to extract the actual load value from the measured value with sufficient accuracy. If you get that far, you've partially answered your question about what kind of accuracy to expect, and you're ready to start figuring out how to deal with common mode currents. Decent antenna impedance measurements aren't simple to make, even at HF. They're more difficult at VHF and above. Roy Lewallen, W7EL |
Dave Shrader wrote:
If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
I've been away from Yagis for many years. But, maximum gain requires
maximum radiation which requires maximum current which requires lowest radiation resistance. Twenty years ago, or so, Ro of 15 to 20 ohms was common in high gain Yagis wher Gamma matching was used to raise the impedance to approximately 50 ohms. A slight reduction in gain allows Ro of close to 50 ohms. Kraus, Antennas, McGraw-Hill 1950, Chapter 11 provides the analysis for a simple 2 element 'Yagi' type array. In written terms, the driving point, feed point, resistance, ignoring losses, is the radiation resistance of the driven element minus the ratio of the mutual impedance to the self impedance of the parasitic elements. Far field gain is maximized by a term where the input power is divided by the net impedance of the driven element minus the net impedance contributed by the parasitic elements. Conclusion, maximum gain, in any configuration [3 element, 4 element, etc.], requires lowest Rr produced by highest mutual coupling. I'm not arguing that more gain is produced by the longest boom or the most elements. What I am stating is that for any configuration the gain for that configuration is MAXIMIZED when the Rr is minimized. Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? |
Ian White, G3SEK wrote:
Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? Yes, quite interesting, since a yagi is _not_ resonant in the design frequency range, otherwise it couldn't work. Tom K0TAR |
My, we can sure learn a lot of new things about Yagis from this
newsgroup. Unfortunately, they're not true. I have a very high confidence in the ability of EZNEC to accurately model Yagi antennas. This is due to feedback from several professional customers who have analyzed Yagis with EZNEC and tested the actual antennas on test ranges. Let's take the EZNEC example file NBSYagi.EZ. If you change the driven element (wire 2) length from 2 * 54.875" to 2 * 54.56", you'll find that the feedpoint impedance is 11.53 - j0.0752 ohms -- it's resonant, and it's certainly functioning as a Yagi. The pattern and gain are nearly identical to the original NBS design. Now, change the director (wire 3) length from 2 * 54.313" to 2 * 56". This drops the gain from 9.68 dBi to 8.66 dBi, and lowers the feedpoint resistance from 11.53 ohms to 7.849 ohms. The point of maximum gain is obviously not the point of minimum feedpoint resistance. Anyone having an explanation for why the gain should be greatest when the feedpoint resistance is minimum and why a Yagi can't work when resonant should examine their explanations carefully in order to uncover the flaws that are obviously present in the explanations. Roy Lewallen, W7EL Tom Ring wrote: Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? Yes, quite interesting, since a yagi is _not_ resonant in the design frequency range, otherwise it couldn't work. Tom K0TAR |
I stand corrected Roy
Roy Lewallen wrote: My, we can sure learn a lot of new things about Yagis from this newsgroup. Unfortunately, they're not true. I have a very high confidence in the ability of EZNEC to accurately model Yagi antennas. This is due to feedback from several professional customers who have analyzed Yagis with EZNEC and tested the actual antennas on test ranges. Let's take the EZNEC example file NBSYagi.EZ. If you change the driven element (wire 2) length from 2 * 54.875" to 2 * 54.56", you'll find that the feedpoint impedance is 11.53 - j0.0752 ohms -- it's resonant, and it's certainly functioning as a Yagi. The pattern and gain are nearly identical to the original NBS design. Now, change the director (wire 3) length from 2 * 54.313" to 2 * 56". This drops the gain from 9.68 dBi to 8.66 dBi, and lowers the feedpoint resistance from 11.53 ohms to 7.849 ohms. The point of maximum gain is obviously not the point of minimum feedpoint resistance. Anyone having an explanation for why the gain should be greatest when the feedpoint resistance is minimum and why a Yagi can't work when resonant should examine their explanations carefully in order to uncover the flaws that are obviously present in the explanations. Roy Lewallen, W7EL Tom Ring wrote: Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? Yes, quite interesting, since a yagi is _not_ resonant in the design frequency range, otherwise it couldn't work. Tom K0TAR |
I should have stated that more clearly. What I meant was, none of the
elements of a yagi are resonant, except perhaps the driven element. My point was that the elements except the driven one(s) must be above or below resonance, or the yagi isn't a yagi. I have also seen a commercial yagi with the driven element longer than the reflector, so it likely wasn't remotely near resonance. It was also a very poorly performing commercial yagi, but that's a different matter. tom K0TAR Roy Lewallen wrote: My, we can sure learn a lot of new things about Yagis from this newsgroup. Unfortunately, they're not true. I have a very high confidence in the ability of EZNEC to accurately model Yagi antennas. This is due to feedback from several professional customers who have analyzed Yagis with EZNEC and tested the actual antennas on test ranges. Let's take the EZNEC example file NBSYagi.EZ. If you change the driven element (wire 2) length from 2 * 54.875" to 2 * 54.56", you'll find that the feedpoint impedance is 11.53 - j0.0752 ohms -- it's resonant, and it's certainly functioning as a Yagi. The pattern and gain are nearly identical to the original NBS design. Now, change the director (wire 3) length from 2 * 54.313" to 2 * 56". This drops the gain from 9.68 dBi to 8.66 dBi, and lowers the feedpoint resistance from 11.53 ohms to 7.849 ohms. The point of maximum gain is obviously not the point of minimum feedpoint resistance. Anyone having an explanation for why the gain should be greatest when the feedpoint resistance is minimum and why a Yagi can't work when resonant should examine their explanations carefully in order to uncover the flaws that are obviously present in the explanations. Roy Lewallen, W7EL Tom Ring wrote: Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? Yes, quite interesting, since a yagi is _not_ resonant in the design frequency range, otherwise it couldn't work. Tom K0TAR |
About what I expected. If someone states something truthfull in this
group, no one responds. And it as a group you are all, even Roy, obviously subject to this. No one bothered to even think about what I originally said, or try to see the tongue in cheek. I guess if you can't argue, it's no fun. I don't blame you all for that, but it is interesting to observe. And sad. tom K0TAR Tom Ring wrote: I should have stated that more clearly. What I meant was, none of the elements of a yagi are resonant, except perhaps the driven element. My point was that the elements except the driven one(s) must be above or below resonance, or the yagi isn't a yagi. I have also seen a commercial yagi with the driven element longer than the reflector, so it likely wasn't remotely near resonance. It was also a very poorly performing commercial yagi, but that's a different matter. tom K0TAR Roy Lewallen wrote: My, we can sure learn a lot of new things about Yagis from this newsgroup. Unfortunately, they're not true. I have a very high confidence in the ability of EZNEC to accurately model Yagi antennas. This is due to feedback from several professional customers who have analyzed Yagis with EZNEC and tested the actual antennas on test ranges. Let's take the EZNEC example file NBSYagi.EZ. If you change the driven element (wire 2) length from 2 * 54.875" to 2 * 54.56", you'll find that the feedpoint impedance is 11.53 - j0.0752 ohms -- it's resonant, and it's certainly functioning as a Yagi. The pattern and gain are nearly identical to the original NBS design. Now, change the director (wire 3) length from 2 * 54.313" to 2 * 56". This drops the gain from 9.68 dBi to 8.66 dBi, and lowers the feedpoint resistance from 11.53 ohms to 7.849 ohms. The point of maximum gain is obviously not the point of minimum feedpoint resistance. Anyone having an explanation for why the gain should be greatest when the feedpoint resistance is minimum and why a Yagi can't work when resonant should examine their explanations carefully in order to uncover the flaws that are obviously present in the explanations. Roy Lewallen, W7EL Tom Ring wrote: Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? Yes, quite interesting, since a yagi is _not_ resonant in the design frequency range, otherwise it couldn't work. Tom K0TAR |
Tom, K0TAR wrote:
"What I meant was, none of the elements of a yagi are resonant, except perhaps the driven element." That`s usually right. The reflector is lengthened and directors are shortened to conveniently produce phase relations which determine reinforcement or repression in directions as desired. However, this is not the only way. Commercial broadcast curtain antenna arrays use parasitic elements which have the same length as the driven elements in some instances. Short-circuit stubs repalace drive lines in the parasitic elements, and these are adjusted for the desired phasing instead of adjusting element lengths. Best regards, Richard Harrison, KB5WZI |
Paul, VK3DIP wrote:
"Is there a better way (which doesn`t involve large sums of money) to measure antenna impedance at say 146 MHz?" Use a line of any number of 1/2-wavelengths to connect the antenna to a VHF admittance or impedance bridge complete with signal source and bridge detector (VHF receiver). Measure away and record your results. I agree with most of G4FGQ`s response. You can expect the antenna`s environment to affect its performance and impedance. I suggest the transmission line which is a minimum integral number of 1/2-wavelengths as required to connect your bridge to the antenna as an alternative to Reg`s ladder. A 1/2-wave line repeats the impedance connected to its end. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
That`s usually right. The reflector is lengthened and directors are shortened to conveniently produce phase relations which determine reinforcement or repression in directions as desired. However, this is not the only way. Commercial broadcast curtain antenna arrays use parasitic elements which have the same length as the driven elements in some instances. Short-circuit stubs repalace drive lines in the parasitic elements, and these are adjusted for the desired phasing instead of adjusting element lengths. That's a nice trick. Of course that still means they aren't resonant since you just displaced the "center" of the element. Seems a good way for a broadcaster to be able to adjust the pattern if needed after construction. I seem to remember an HF wire antenna project that used that method to go from driven plus reflector to driven plus director to get a reversible beam. I also remember a set of 5 slopers that were in the ARRL antenna book or handbook that could be steered. Oh well, way off topic here now. cul Tom K0TAR |
Richard Harrison wrote:
Paul, VK3DIP wrote: "Is there a better way (which doesn`t involve large sums of money) to measure antenna impedance at say 146 MHz?" Use a line of any number of 1/2-wavelengths to connect the antenna to a VHF admittance or impedance bridge complete with signal source and bridge detector (VHF receiver). Measure away and record your results. I've been out of town and not following this thread. Here's what I do for HF - knowing the length, VF, and attenuation factor of ladder-line. Trim the laddder-line until the impedance looking into the ladder-line is purely resistive. Draw the corresponding SWR circle on a Smith Chart. Using the line-attenuation factor, draw an SWR circle outside of that one. The antenna feedpoint impedance lies on that outside SWR circle. Calculate the exact electrical length of the length of ladder-line being used and use the Smith Chart to track from the purely resistive feedpoint impedance back to the antenna feedpoint impedance on the largest SWR circle. Of course, the accuracy of the final indirect measurement depends upon the accuracy of all the parameters used in the calculation. My accuracy has always been good enough for what I needed. I've never done it with coax but I assume the same principles apply. -- 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! =----- |
Tom, K0TAR wrote:
"Of course that still means thery aren`t resonant aince you just displaced the "center" of the element." Kraus describes adjustment of the phase between driven and parasitic elements on page 320 of his 1950 edition of "Antennas": "The parasitic element may have a fixed length of 1/2 wavelength, the tuning being accomplished by inserting a lumped reactance in series with the antenna at its center point." In my case, the "lumped reactance" was a tuned stub adjusted to the desired phase difference between parasitic and driven elements as indicated by an RCA WM-30A phase monitor. Best regards, Richard Harrison, KB5WZI |
Cecil, W5DXP wrote:
"Trim the ladder-line until the impedance looking into the ladder-line is purely resistive." Sure. The line is purely resistive at resonant lengths where the power factor is one. No reactance. A 1/2-wave is a resonant length. Charlie Wright, an A.D. Ring and Accociates engineer used to drive our German engineers crazy, telling them that slopes on the autobahn used coble stones because they didn`t know how to pour concrete on an incline. Charlie also got to a group using an RCA WM-30A phase monitor to tune parasiitic elements in a curtain array. Most medium-wave directional stations at the time used a WM-30A as a phase monitor, just as shortwave stations used them for tune-up. Charlie had used the monitor for years and knew it had an underated resistor which sometimes failed. The group had upended the chassis and Charlie offered to help troubleshoot. The Germans acquiesced. Charlie asked for voltage measurements from unrelated parts of the circuit, took out his slide rule and feigned a few calculations. Then, Charlie pointed to the defective resistor and said: "Change that one." The crowd shook its collective heads but complied. The monitor miraculously sprang to life again. Charlie chuckled to himself as he left the incredulous crowd. Best regards, Richard Harrison, KB5WZI |
Cecil, W5DXP wrote:
"Trim the ladder-line until the impedance looking into the ladder-line is purely resistive." Sure. The line is purely resistive at resonant lengths where the power factor is one. No reactance. A 1/2-wave is a resonant length. Charlie Wright, an A.D. Ring and Accociates engineer used to drive our German engineers crazy, telling them that slopes on the autobahn used coble stones because they didn`t know how to pour concrete on an incline. Charlie also got to a group using an RCA WM-30A phase monitor to tune parasiitic elements in a curtain array. Most medium-wave directional stations at the time used a WM-30A as a phase monitor, just as shortwave stations used them for tune-up. Charlie had used the monitor for years and knew it had an underated resistor which sometimes failed. The group had upended the chassis and Charlie offered to help troubleshoot. The Germans acquiesced. Charlie asked for voltage measurements from unrelated parts of the circuit, took out his slide rule and feigned a few calculations. Then, Charlie pointed to the defective resistor and said: "Change that one." The crowd shook its collective heads but complied. The monitor miraculously sprang to life again. Charlie chuckled to himself as he left the incredulous crowd. Best regards, Richard Harrison, KB5WZI |
They can be wonderful engineers, however.....
My 999 story: A major automobile manufacturer tasked their German branch to design a new transmission for a "sporty" car. Prototype arrived at the proving grounds and looked anemic. Transmission was placed into prototype car. Everyone went to see the first use. Driver wound up the engine to red line, and loud 9 9 9 was heard as clutch was engaged and shrapnel was produced. US engineers turned to German engineers and said: "We told you how it would be used - now believe us." An antenna system was used to send data back for analysis. 73 Mac N8TT -- J. Mc Laughlin - Michigan USA Home: "Richard Harrison" wrote in message ... Cecil, W5DXP wrote: snip Charlie Wright, an A.D. Ring and Accociates engineer used to drive our German engineers crazy, telling them that slopes on the autobahn used coble stones because they didn`t know how to pour concrete on an incline. Charlie also got to a group using an RCA WM-30A phase monitor to tune parasiitic elements in a curtain array. Most medium-wave directional stations at the time used a WM-30A as a phase monitor, just as shortwave stations used them for tune-up. Charlie had used the monitor for years and knew it had an underated resistor which sometimes failed. The group had upended the chassis and Charlie offered to help troubleshoot. The Germans acquiesced. Charlie asked for voltage measurements from unrelated parts of the circuit, took out his slide rule and feigned a few calculations. Then, Charlie pointed to the defective resistor and said: "Change that one." The crowd shook its collective heads but complied. The monitor miraculously sprang to life again. Charlie chuckled to himself as he left the incredulous crowd. Best regards, Richard Harrison, KB5WZI |
tom wrote,
About what I expected. If someone states something truthfull in this group, no one responds. And it as a group you are all, even Roy, obviously subject to this. No one bothered to even think about what I originally said, or try to see the tongue in cheek. I guess if you can't argue, it's no fun. I don't blame you all for that, but it is interesting to observe. And sad. tom K0TAR You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH |
tom wrote,
About what I expected. If someone states something truthfull in this group, no one responds. And it as a group you are all, even Roy, obviously subject to this. No one bothered to even think about what I originally said, or try to see the tongue in cheek. I guess if you can't argue, it's no fun. I don't blame you all for that, but it is interesting to observe. And sad. tom K0TAR You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH |
Tdonaly wrote:
You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH I've been here long enough to know that it's mostly that a lot of people here like to argue. And while I may need to work on my comm skills, it doesn't change that fact at all. Like I said, sad. tom K0TAR |
Tdonaly wrote:
You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH I've been here long enough to know that it's mostly that a lot of people here like to argue. And while I may need to work on my comm skills, it doesn't change that fact at all. Like I said, sad. tom K0TAR |
tom wrot,
Message-id: Tdonaly wrote: You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH I've been here long enough to know that it's mostly that a lot of people here like to argue. And while I may need to work on my comm skills, it doesn't change that fact at all. Like I said, sad. tom K0TAR What's wrong with wanting to argue? Argument, sometimes even violent argument, has been a hallmark of Western science for a long time. People who take everything at face value, without question or disagreement, end up believing the strangest things. 73, Tom Donaly, KA6RUH |
tom wrot,
Message-id: Tdonaly wrote: You can think of it this way, or more probably, you need to work on your communication skills. 73, Tom Donaly, KA6RUH I've been here long enough to know that it's mostly that a lot of people here like to argue. And while I may need to work on my comm skills, it doesn't change that fact at all. Like I said, sad. tom K0TAR What's wrong with wanting to argue? Argument, sometimes even violent argument, has been a hallmark of Western science for a long time. People who take everything at face value, without question or disagreement, end up believing the strangest things. 73, Tom Donaly, KA6RUH |
Tdonaly wrote:
What's wrong with wanting to argue? Argument, sometimes even violent argument, has been a hallmark of Western science for a long time. People who take everything at face value, without question or disagreement, end up believing the strangest things. 73, Tom Donaly, KA6RUH Arguing endlessly on the same old subjects knowing the opposition won't budge is what annoys me. And that's what goes on here very often. I have no issues with a discussion that actually goes someplace. tom K0TAR |
Tdonaly wrote:
What's wrong with wanting to argue? Argument, sometimes even violent argument, has been a hallmark of Western science for a long time. People who take everything at face value, without question or disagreement, end up believing the strangest things. 73, Tom Donaly, KA6RUH Arguing endlessly on the same old subjects knowing the opposition won't budge is what annoys me. And that's what goes on here very often. I have no issues with a discussion that actually goes someplace. tom K0TAR |
Tdonaly wrote:
What's wrong with wanting to argue? What' wrong indeed? Arguments are the cornerstone of logic. My dictionary says a definition of "argument" is "3. a process of reasoning". -- 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! =----- |
Tdonaly wrote:
What's wrong with wanting to argue? What' wrong indeed? Arguments are the cornerstone of logic. My dictionary says a definition of "argument" is "3. a process of reasoning". -- 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! =----- |
Dave Shrader wrote:
Ian White, G3SEK wrote: Dave Shrader wrote: If the Yagi is to be tuned for MAXIMUM gain, and that is the objective, then Ro will be the lowest value at resonance. That's an interesting assertion. Do you have further evidence for it? (Apologies for the delay in replying to this, Dave. I've been away from the computer for two weeks.) I've been away from Yagis for many years. But, maximum gain requires maximum radiation which requires maximum current which requires lowest radiation resistance. Twenty years ago, or so, Ro of 15 to 20 ohms was common in high gain Yagis wher Gamma matching was used to raise the impedance to approximately 50 ohms. A slight reduction in gain allows Ro of close to 50 ohms. Kraus, Antennas, McGraw-Hill 1950, Chapter 11 provides the analysis for a simple 2 element 'Yagi' type array. In written terms, the driving point, feed point, resistance, ignoring losses, is the radiation resistance of the driven element minus the ratio of the mutual impedance to the self impedance of the parasitic elements. Far field gain is maximized by a term where the input power is divided by the net impedance of the driven element minus the net impedance contributed by the parasitic elements. Conclusion, maximum gain, in any configuration [3 element, 4 element, etc.], requires lowest Rr produced by highest mutual coupling. This is stretching a simplified theoretical case, way beyond the point where it ceases to apply. I agree that the maximum *theoretical* gain - ignoring losses - is achieved when the element currents are as high as possible, and the feedpoint resistance is as low as possible. This also requires that the element spacing is as close as possible... which leads to the interesting conclusion that a compact beam should have more gain than a full-sized one! In practice, of course, this doesn't happen. The reason is that losses can *never* be ignored in this particular problem. As the element currents rise and the feedpoint impedance drops, the I^2*R losses in the elements and the matching losses to 50R rapidly overtake any theoretical increase in gain. This means that high-gain beams with deliberately high element currents are only a theoretical curiosity. The underlying theory has a valid place in academic textbooks such as Kraus, but it isn't relevant to practical antenna engineering. (Even superconducting elements and matching circuits wouldn't make such antennas practical.) Also, it isn't correct to apply generalizations about 2- and 3-element yagis to a long, multi-element yagi. In particular, the first 2 or 3 elements of a long yagi cannot be considered in isolation from all the other elements. It is true that gain optimization in multi-element yagis tends to reduce the feedpoint impedance towards 15-20R, but this is a remote side-effect of all the other design parameters. A low feedpoint impedance certainly isn't a desirable design aim in itself, because it leads to significant matching losses and a reduction in the SWR bandwidth. Numerous designers have found that when they are getting close to a gain-optimized design, it is usually possible to raise the feed impedance back towards 50R by inserting an additional first director with a very close spacing ahead of the driven element. (This technique may have been developed after you ceased to take a close interest in yagi design, Dave.) The close-spaced first director is mostly an impedance-changing device, and it has relatively few side-effects on the overall gain and pattern. With a multi-element yagi, it is usually possible to take out most of these side-effects in the next round of optimization. The result is a yagi that can be fed directly from 50R coax (through a balun) which eliminates matching losses and greatly improves the SWR bandwidth. If the re-optimization is done well, any decrease in gain is almost undetectable in simulation, and completely undetectable on the air. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
As a sidebar, we found while testing 432 beams at Central States, that
our older beams, only a couple years, seemed low in gain. We ScothBrighted the elements, Al welding rod, as I remember, with a hobby brass driven element and T match, and got 3 or 4 10th's more on a 17 foot beam I designed for EME than I had on the 1st range test. Sorry about the looong sentence. tom K0TAR Ian White, G3SEK wrote: I agree that the maximum *theoretical* gain - ignoring losses - is achieved when the element currents are as high as possible, and the feedpoint resistance is as low as possible. This also requires that the element spacing is as close as possible... which leads to the interesting conclusion that a compact beam should have more gain than a full-sized one! In practice, of course, this doesn't happen. The reason is that losses can *never* be ignored in this particular problem. As the element currents rise and the feedpoint impedance drops, the I^2*R losses in the elements and the matching losses to 50R rapidly overtake any theoretical increase in gain. This means that high-gain beams with deliberately high element currents are only a theoretical curiosity. The underlying theory has a valid place in academic textbooks such as Kraus, but it isn't relevant to practical antenna engineering. (Even superconducting elements and matching circuits wouldn't make such antennas practical.) Also, it isn't correct to apply generalizations about 2- and 3-element yagis to a long, multi-element yagi. In particular, the first 2 or 3 elements of a long yagi cannot be considered in isolation from all the other elements. It is true that gain optimization in multi-element yagis tends to reduce the feedpoint impedance towards 15-20R, but this is a remote side-effect of all the other design parameters. A low feedpoint impedance certainly isn't a desirable design aim in itself, because it leads to significant matching losses and a reduction in the SWR bandwidth. Numerous designers have found that when they are getting close to a gain-optimized design, it is usually possible to raise the feed impedance back towards 50R by inserting an additional first director with a very close spacing ahead of the driven element. (This technique may have been developed after you ceased to take a close interest in yagi design, Dave.) The close-spaced first director is mostly an impedance-changing device, and it has relatively few side-effects on the overall gain and pattern. With a multi-element yagi, it is usually possible to take out most of these side-effects in the next round of optimization. The result is a yagi that can be fed directly from 50R coax (through a balun) which eliminates matching losses and greatly improves the SWR bandwidth. If the re-optimization is done well, any decrease in gain is almost undetectable in simulation, and completely undetectable on the air. |
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