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measuring antenna resonance with an 8405a
I would like to know if the follow technique will work to measure resonance of
an antenna. I know have a working HP 8405a. It has been a struggle to repair it and to align it, however it now appears to be working. My plan is to connect a bi-directional coupler to the 8405a. The coupler will be terminated in a 50 Ohm terminator. The phase will be read for this configuration. The next step is to replace the 50 Ohm terminator with a direct connection to an unknown antenna. I will then change the frequency until the phase angle matchs the 50 Ohm phase angle. Does this make sense? What are the draw backs? How close will this method get compared to a noise bridge? Thanks - Dan kb0qil |
measuring antenna resonance with an 8405a
I would like to know if the follow technique will work to measure resonance
of an antenna. I know have a working HP 8405a. It has been a struggle to repair it and to align it, however it now appears to be working. My plan is to connect a bi-directional coupler to the 8405a. The coupler will be terminated in a 50 Ohm terminator. The phase will be read for this configuration. The next step is to replace the 50 Ohm terminator with a direct connection to an unknown antenna. I will then change the frequency until the phase angle matchs the 50 Ohm phase angle. Does this make sense? What are the draw backs? How close will this method get compared to a noise bridge? Thanks - Dan kb0qil Dan, the phase angle of the reflection coefficient of a matched load is unimportant, and is likely to be anything. Specifically; if it is a dot on the Smith Chart center -- who cares? Typically the return loss of a good quality load could well be 40dB. Again, if you are measuring beyond the frequency range of the coupler you must determine its directivity. It is very likely that you are reading the phase of poor directivity and not the 50 ohm load. Also if you are using the 8405A, do you have all the accessories? In particular the BNC ( + 50 ohm thru) probe attachments which should be connected directly to your coupler ports. If you require N - BNC adapters to connect to the coupler, then as long as you have the same structures on each port, any phase error will be cancelled out. It is also good practice to attach a high quality 20 dB pad to the input of the coupler (thus ensuring your source return loss approaches 40 dB) -- you cannot always be certain that your source, plus coax, provides a good match. Note if the unit cannot lock with the 20 dB pad, then at least use a 10 dB pad. Another test is to connect both probes to the same source via a BNC "T" adapter. Do you show zero phase error? Interesting you have an 8405A, it is many years since I used one. I see them priced from $250.00 to a fully refurbished unit from Tucker (www.tucker.com) for $2,500.00. Regards, Frank |
measuring antenna resonance with an 8405a
On Mon, 05 Dec 2005 21:18:33 -0800, dansawyeror
wrote: I would like to know if the follow technique will work to measure resonance of an antenna. I know have a working HP 8405a. It has been a struggle to repair it and to align it, however it now appears to be working. My plan is to connect a bi-directional coupler to the 8405a. The coupler will be terminated in a 50 Ohm terminator. The phase will be read for this configuration. The next step is to replace the 50 Ohm terminator with a direct connection to an unknown antenna. I will then change the frequency until the phase angle matchs the 50 Ohm phase angle. Does this make sense? Sort of, but not exactly. As I described in another thread, the pitfall is the fact that the phase lengths of the coupler are different. You could set the phase offset to zero at frequency X but when you change to frequency Y, there is a built-in error. The correction for this is to use a line stretcher that (more-or-less) equalizes the phase lengths with respect to frequency. What are the draw backs? See above. How close will this method get compared to a noise bridge? With a little care you can make measurements that leave noise bridge measurements in the dust... not even close. With -lots- of care, you can approach network analyzer accuracy... more than is needed for practical antenna measurements. You need to see H-P App note AN77-3. When I get time---and with my dial-up connection---I mean lots of time, I will try to post a 13 MByte .pdf on qsl.net, if that's working. |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 07:45:36 -0700, Wes Stewart
wrote: On Mon, 05 Dec 2005 21:18:33 -0800, dansawyeror wrote: I would like to know if the follow technique will work to measure resonance of an antenna. I know have a working HP 8405a. It has been a struggle to repair it and to align it, however it now appears to be working. My plan is to connect a bi-directional coupler to the 8405a. The coupler will be terminated in a 50 Ohm terminator. The phase will be read for this configuration. The next step is to replace the 50 Ohm terminator with a direct connection to an unknown antenna. I will then change the frequency until the phase angle matchs the 50 Ohm phase angle. Does this make sense? Sort of, but not exactly. As I described in another thread, the pitfall is the fact that the phase lengths of the coupler are different. You could set the phase offset to zero at frequency X but when you change to frequency Y, there is a built-in error. I should have added that you don't do this with a 50 ohm load, but a short circuit instead. You want a big reflection with a known phase for a calibration standard. |
measuring antenna resonance with an 8405a
Sort of, but not exactly. As I described in another thread, the
pitfall is the fact that the phase lengths of the coupler are different. You could set the phase offset to zero at frequency X but when you change to frequency Y, there is a built-in error. I should have added that you don't do this with a 50 ohm load, but a short circuit instead. You want a big reflection with a known phase for a calibration standard. The first step is to establish the accuracy of the test equipment. A short/open at the output port of the directional coupler should produce a frequency independant phase shift between the forward and reverse coupled port. If you are not getting 180/0 deg, then there is something fundamentally wrong with the test set up. Connecting both probes of the 8405A to the same source should establish if any errors exist in the vector voltmeter. Using the BNC/probe adapters is essential to maintain repeatability in your measurements. Reference HP's 778D coupler at http://cp.literature.agilent.com/lit.../5952-8133.pdf : " Impedance Measurements The 778D is also well suited for measurements of impedance when used with the Agilent 8405A vector voltmeter. The technique is described in Application Note 77-3, Measurement of Complex Impedance, available at your nearest Agilent sales office. Again, a reflectometry technique is used. With the vector voltmeter, however, both magnitude and phase angle of the reflection coefficient can be measured. This setup is shown in Figure 3. Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart to provide impedance of such devices as antennas or other passive components. "It is many years since I have seen AN77-3, but I seem unable to find a copy on the web. Frank |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 16:48:32 GMT, "Frank"
wrote: Sort of, but not exactly. As I described in another thread, the pitfall is the fact that the phase lengths of the coupler are different. You could set the phase offset to zero at frequency X but when you change to frequency Y, there is a built-in error. I should have added that you don't do this with a 50 ohm load, but a short circuit instead. You want a big reflection with a known phase for a calibration standard. The first step is to establish the accuracy of the test equipment. A short/open at the output port of the directional coupler should produce a frequency independant phase shift between the forward and reverse coupled port. Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. If you are not getting 180/0 deg, then there is something fundamentally wrong with the test set up. Connecting both probes of the 8405A to the same source should establish if any errors exist in the vector voltmeter. Using the BNC/probe adapters is essential to maintain repeatability in your measurements. Reference HP's 778D coupler at http://cp.literature.agilent.com/lit.../5952-8133.pdf : " Impedance Measurements The 778D is also well suited for measurements of impedance when used with the Agilent 8405A vector voltmeter. The technique is described in Application Note 77-3, Measurement of Complex Impedance, available at your nearest Agilent sales office. Again, a reflectometry technique is used. With the vector voltmeter, however, both magnitude and phase angle of the reflection coefficient can be measured. This setup is shown in Figure 3. Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart to provide impedance of such devices as antennas or other passive components. "It is many years since I have seen AN77-3, but I seem unable to find a copy on the web. Frank The pertinent pages are he http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote: Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. With respect, I think you two guys are talking about different kinds of couplers. Wes, your earlier description makes is clear that samples in the coupler you described are located at equidistant from their nearest ends, but not in the centre, so at different positions in the coupler. I think the coupled lines type of coupler for lower frequency use might fit this category. Frank assumes a coupler where the samples for both ports are taken at the same physical location on the main line. I think a crossed waveguide coupler might fit this category (depending on the way the coupling holes are implemented). Owen -- |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 19:18:50 GMT, Owen Duffy wrote:
On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart wrote: Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. With respect, I think you two guys are talking about different kinds of couplers. Wes, your earlier description makes is clear that samples in the coupler you described are located at equidistant from their nearest ends, but not in the centre, so at different positions in the coupler. I think the coupled lines type of coupler for lower frequency use might fit this category. Frank assumes a coupler where the samples for both ports are taken at the same physical location on the main line. May be, but his link: http://cp.literature.agilent.com/lit.../5952-8133.pdf shows exactly what I'm talking about in Figure 3. Note the line stretcher on one sample port. Because I haven't seen the inside of one of these I don't know where the coupled arms reside with respect to each other but the fact that H-P shows the line stretcher tells me that they must have some (unavoidable, if not purposeful) asymmetry. They claim 4 degree phase tracking but it's unclear to me whether they mean the coupled arms with respect to the main line or to each other. I think a crossed waveguide coupler might fit this category (depending on the way the coupling holes are implemented). Owen |
measuring antenna resonance with an 8405a
"Owen Duffy" wrote in message ... On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart wrote: Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. With respect, I think you two guys are talking about different kinds of couplers. Wes, your earlier description makes is clear that samples in the coupler you described are located at equidistant from their nearest ends, but not in the centre, so at different positions in the coupler. I think the coupled lines type of coupler for lower frequency use might fit this category. Frank assumes a coupler where the samples for both ports are taken at the same physical location on the main line. I think a crossed waveguide coupler might fit this category (depending on the way the coupling holes are implemented). Owen The type of coupler I am thinking of is indeed where the coupling lines for forward and reflected are in the same physical region. The point I was making refers particularly to the HP 778D; where it is specified in: http://cp.literature.agilent.com/lit.../5952-8133.pdf "Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart". Indicating that there is no significant phase error over the nominal bandwidth of the coupler. It is certainly something I have always taken for granted, but in recent years have been spoiled by Agilent's VNAs. Even Matthaei, Young, and Jones' classic text does not get into specifics of the phase response of TEM couplers. If I get a chance in the near future, I will run a phase response of an HP778D coupler, and/or also run an Eagleware simulation of a co-planar WG coupler. Just read Wes' comments on the line stretcher on the above pdf. I must admit I also wondered about that, but assume it is intended as a cal adjustment for a short/open standard. Even VNAs, in their non calibrated state, show a pretty good dot on the right/left of the Smith Chart for an open/short in the lower hundreds of MHz. Incidentally I have tried to access the http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf which is unbelievably slow, but I keep getting error messages saying the file is corrupted. I am using a 2.5 Mb ADSL, which is normally pretty fast. Frank |
measuring antenna resonance with an 8405a
"Wes Stewart" wrote in message ... On Tue, 06 Dec 2005 19:18:50 GMT, Owen Duffy wrote: On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart wrote: Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. With respect, I think you two guys are talking about different kinds of couplers. ...Owen's coupler location discussion... May be, but his link: http://cp.literature.agilent.com/lit.../5952-8133.pdf shows exactly what I'm talking about in Figure 3. Note the line stretcher on one sample port. Because I haven't seen the inside of one of these I don't know where the coupled arms reside with respect to each other but the fact that H-P shows the line stretcher tells me that they must have some (unavoidable, if not purposeful) asymmetry. They claim 4 degree phase tracking but it's unclear to me whether they mean the coupled arms with respect to the main line or to each other. I have done this type of measurement and the coupler is somewhat un-important in regard to where on the line its samples are taken from. The line stretcher does the compensation to place both samples at the same point on the main line. It is set in a calibration process to get to that point (seems to mee only a short is required). Memory is starting to fade here, but if you want the samples to be "at the plane of the DUT" then you either must use two line stretchers or use the vector voltmeter capability to compensate for the difference in sample location vs. DUT location. If I recall correctly, this is simply a phase offset and the old HP vector voltmeter has such an offset capability. Look at it this way. You want the FWD and REF phases to be at the DUT. To do this you must put an equal (electrical) length of line in all three sections of line - the desired, the FWD sample and the reverse sample. I admit the reflected part has an intuitive glitch that I can't resolve at this time since it is always longer, but I know this works. You can sweep the set-up and the phase of the two samples will sit right there on that of a short . This rells you that the samples are "at the DUT". Right?? 73, Steve, K,9.D;C'I |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart
wrote: May be, but his link: http://cp.literature.agilent.com/lit.../5952-8133.pdf shows exactly what I'm talking about in Figure 3. Note the line stretcher on one sample port. Because I haven't seen the inside of one of these I don't know where the coupled arms reside with respect to each other but the fact that H-P shows the line stretcher tells me that they must have some (unavoidable, if not purposeful) asymmetry. They claim 4 degree phase tracking but it's unclear to me whether they mean the coupled arms with respect to the main line or to each other. I could be wrong, but I think that 4 deg seems part of the tolerance of the phase alignment of the two sample ports wrt each other over the frequency range. Perhaps this is a coupler where the ports are approximately in phase, and perhaps the line strether is to adjust phase of forward and reflected ports to create a new reference plane where it is needed or convenient to the measurement. Owen -- |
measuring antenna resonance with an 8405a
"Frank" wrote in message news:6Pnlf.138047$y_1.50980@edtnps89... The type of coupler I am thinking of is indeed where the coupling lines for forward and reflected are in the same physical region. The point I was making refers particularly to the HP 778D; where it is specified in: http://cp.literature.agilent.com/lit.../5952-8133.pdf "Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart". Indicating that there is no significant phase error over the nominal bandwidth of the coupler. The old vector voltmeter has a phase offset ability to compensate. Just read Wes' comments on the line stretcher on the above pdf. I must admit I also wondered about that, but assume it is intended as a cal adjustment for a short/open standard. See my previous post on this. It IS used to get both samples "At the measurement plane." 73, Steve, K,9.D;C'I |
measuring antenna resonance with an 8405a
Wes,
The file reports an error. Can you load it? Thanks very much. Dan Wes Stewart wrote: On Tue, 06 Dec 2005 16:48:32 GMT, "Frank" wrote: Sort of, but not exactly. As I described in another thread, the pitfall is the fact that the phase lengths of the coupler are different. You could set the phase offset to zero at frequency X but when you change to frequency Y, there is a built-in error. I should have added that you don't do this with a 50 ohm load, but a short circuit instead. You want a big reflection with a known phase for a calibration standard. The first step is to establish the accuracy of the test equipment. A short/open at the output port of the directional coupler should produce a frequency independant phase shift between the forward and reverse coupled port. Maybe I misunderstand, but what you seem to be saying is that if I put Probe A at point X on a transmission line and Probe B at point X+Y (Y0), the phase difference with respect to frequency will not change. The directional coupler is no different, it is sampling the main line at two physically different locations. If you are not getting 180/0 deg, then there is something fundamentally wrong with the test set up. Connecting both probes of the 8405A to the same source should establish if any errors exist in the vector voltmeter. Using the BNC/probe adapters is essential to maintain repeatability in your measurements. Reference HP's 778D coupler at http://cp.literature.agilent.com/lit.../5952-8133.pdf : " Impedance Measurements The 778D is also well suited for measurements of impedance when used with the Agilent 8405A vector voltmeter. The technique is described in Application Note 77-3, Measurement of Complex Impedance, available at your nearest Agilent sales office. Again, a reflectometry technique is used. With the vector voltmeter, however, both magnitude and phase angle of the reflection coefficient can be measured. This setup is shown in Figure 3. Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart to provide impedance of such devices as antennas or other passive components. "It is many years since I have seen AN77-3, but I seem unable to find a copy on the web. Frank The pertinent pages are he http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf |
measuring antenna resonance with an 8405a
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measuring antenna resonance with an 8405a
The type of coupler I am thinking of is indeed where the coupling lines
for forward and reflected are in the same physical region. The point I was making refers particularly to the HP 778D; where it is specified in: http://cp.literature.agilent.com/lit.../5952-8133.pdf "Data can be read from the two meters of the vector voltmeter and transferred directly to a Smith Chart". Indicating that there is no significant phase error over the nominal bandwidth of the coupler. The old vector voltmeter has a phase offset ability to compensate. Just read Wes' comments on the line stretcher on the above pdf. I must admit I also wondered about that, but assume it is intended as a cal adjustment for a short/open standard. See my previous post on this. It IS used to get both samples "At the measurement plane." 73, Steve, K,9.D;C'I Ok, Steve, makes sense. Anyway, have re-visited Matthaei et al, and also "Foundations for Microstrip Circuit Design", by T. C. Edwards (Edwards does appear to contain an error referring to "j-omega", which should simply be "omega"). Both texts agree on the expression for coupling on a single section, quarter wave, TEM mode, coupled transmission line. Also realized that the equation does contain a frequency parameter. Running an analysis in MathCAD, for a 20 dB coupler with design center at 150 MHz, produces some interesting results: Coupling at 100 MHz, and 200 MHz = -22.5 dB; Maximum phase error -- at band edges = +/- 2.5 degrees. Increasing the frequency range of analysis from 50 to 250 MHz shows a dramatic drop in coupling amplitude, at these frequency limits, to 32 dB. Phase error, however, does not seem to be effected very much; peaking at +/- 2.8 degrees, and following what appears to be a sinusoidal curve. From experience I know that coupler directivity degrades significantly beyond the design bandwidth. Edwards does state: " Values of directivity, on microstrip, beyond 12 to 14 dB are difficult to achieve". Co-planar structures are much better, and can easily be analyzed with Genesys' 2.5D EM simulations. Without access to HFSS, or similar FEM programs, I doubt directivity could be calculated for coaxial structures. The main problem, with operation of a coupler beyond its design bandwidth, appears to be its loss of coupling. This, combined with degradation of directivity, would certainly account for very large phase errors. Frank |
measuring antenna resonance with an 8405a
On Tue, 06 Dec 2005 23:13:35 GMT, Owen Duffy wrote:
On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart wrote: May be, but his link: http://cp.literature.agilent.com/lit.../5952-8133.pdf shows exactly what I'm talking about in Figure 3. Note the line stretcher on one sample port. Because I haven't seen the inside of one of these I don't know where the coupled arms reside with respect to each other but the fact that H-P shows the line stretcher tells me that they must have some (unavoidable, if not purposeful) asymmetry. They claim 4 degree phase tracking but it's unclear to me whether they mean the coupled arms with respect to the main line or to each other. I could be wrong, but I think that 4 deg seems part of the tolerance of the phase alignment of the two sample ports wrt each other over the frequency range. I tend to agree, but as someone who used to write specs and then sit back and watch vendors try to skirt them, I'm always dubious. :-) Perhaps this is a coupler where the ports are approximately in phase, and perhaps the line strether is to adjust phase of forward and reflected ports to create a new reference plane where it is needed or convenient to the measurement. I think what's being missed in this discussion is this: The 8405 has a phase offset adjustment that can make -any- relative phase read 180 degrees on the meter. So you could put a short on 100 feet (30.48m) and adjust the phase offset to make the meter read 180 deg. This would be fine until you changed frequency then you're in trouble. That same effect is the reason for adding a line stretcher; it's to make that 180 degree relationship track with respect to frequency. Paragraph 2 of the document I provided explains this nicely. I've built reflectometers (many times) where dual couplers were not available and two back-to-back singles were used. (Sometimes, three were used with the added one in a feedback loop used to improve the source match of the generator) I can guarantee you that the two coupled arms didn't phase track and that is the general case that I was trying to put forth at the outset of this discussion. |
measuring antenna resonance with an 8405a
Wes,
Thank you. Two couplers configured "in to in" from a tee, to set up isolated "in phase" coupler signals, do seem to maintain very close to a relationship across frequencies. This was observed with an oscilloscope After much scrounging around to make the connectors a constant length and number the 8405a does not maintain a constant phase across the same inputs. The 20kc IF output shows a significant phase change across the same frequency range. Its back to the drawing board to re-tune the 8405a. Somewhere in the process I missed something. The phase angle should not vary that much. Dan Wes Stewart wrote: On Tue, 06 Dec 2005 23:13:35 GMT, Owen Duffy wrote: On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart wrote: May be, but his link: http://cp.literature.agilent.com/lit.../5952-8133.pdf shows exactly what I'm talking about in Figure 3. Note the line stretcher on one sample port. Because I haven't seen the inside of one of these I don't know where the coupled arms reside with respect to each other but the fact that H-P shows the line stretcher tells me that they must have some (unavoidable, if not purposeful) asymmetry. They claim 4 degree phase tracking but it's unclear to me whether they mean the coupled arms with respect to the main line or to each other. I could be wrong, but I think that 4 deg seems part of the tolerance of the phase alignment of the two sample ports wrt each other over the frequency range. I tend to agree, but as someone who used to write specs and then sit back and watch vendors try to skirt them, I'm always dubious. :-) Perhaps this is a coupler where the ports are approximately in phase, and perhaps the line strether is to adjust phase of forward and reflected ports to create a new reference plane where it is needed or convenient to the measurement. I think what's being missed in this discussion is this: The 8405 has a phase offset adjustment that can make -any- relative phase read 180 degrees on the meter. So you could put a short on 100 feet (30.48m) and adjust the phase offset to make the meter read 180 deg. This would be fine until you changed frequency then you're in trouble. That same effect is the reason for adding a line stretcher; it's to make that 180 degree relationship track with respect to frequency. Paragraph 2 of the document I provided explains this nicely. I've built reflectometers (many times) where dual couplers were not available and two back-to-back singles were used. (Sometimes, three were used with the added one in a feedback loop used to improve the source match of the generator) I can guarantee you that the two coupled arms didn't phase track and that is the general case that I was trying to put forth at the outset of this discussion. |
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