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using an HP 8405A to measure SWR ?
This is the next chapter in the antenna measuring saga. Today's adventure is
trying to measure SWR with an HP 8405A Vector Voltmeter. The measuring setup is a standard signal generator feeding a tee. One branch feeds the A input of the 8405A and the other a 20 db directional configured in reverse, that is feeding the out port. The assumption is the reflected signal will be read correctly on the in port. The in port is then terminated or connected to an antenna. An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees and 55 db down from the input. (I assume 20 of that is the coupler so I am subtracting 20 from that reading - is that a correct assumption.) Using the method to read the SWR on an antenna produces values very close to those from several SWR meters used as controls. When the output is terminated in 25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5 db. (The B channel won't calibrate to within 1/2 a db so readings are estimates) Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument this is pretty close to an SWR of 2:1. Is there an error in this logic? If the instrument is working well enough to perform this calculation then it should support more complicated measurements. Thanks - Dan |
using an HP 8405A to measure SWR ?
On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
wrote: This is the next chapter in the antenna measuring saga. Today's adventure is trying to measure SWR with an HP 8405A Vector Voltmeter. As I see it, your instrument with accessories (directional coupler, attenuator, etc) can be used to measure the complex reflection coefficient (Gamma) at the measurement plane. Reading the mail, it seems have already worked out how to do that, and to calculate Z given gamma. If you want to determine SWR, you don't need the argument of Gamma, you just need its magnitude rho. SWR=(1+rho)/(1-rho) where rho=|Gamma|. (You are of course throwing away part of the measurement data, the part that is the real power of the VVM. You could measure rho with a directional coupler, calibrated attenuator and xtal detector or a bolometer power meter... but they won't give you the phase data that the VVM does.) The return loss of VSWR=2 is 9.5dB, not 4.5 as you suggest. You need to review some of these simple relationships. You also seem not to be properly including the directional coupler in the calibration loop, why the tee as described, doesn't the 8405A have a tee probe? Is using a VVM an overkill for determining SWR? Probably, especially if the complexity results in errors. Owen -- |
using an HP 8405A to measure SWR ?
On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy wrote:
Reading the mail, it seems have already worked out how to do that, and to calculate Z given gamma. Should read: Reading the mail, it seems have already worked out how to do that, and to calculate Z given Gamma (with a captial G). -- |
using an HP 8405A to measure SWR ?
Where are these equations? I have searched the ARRL handbook for Gamma and did
not find it. This is also the first reference to rho. I was planning on the Z equation next, yes I remember that one from earlier in the week. Owen Duffy wrote: On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy wrote: Reading the mail, it seems have already worked out how to do that, and to calculate Z given gamma. Should read: Reading the mail, it seems have already worked out how to do that, and to calculate Z given Gamma (with a captial G). -- |
using an HP 8405A to measure SWR ?
On Sat, 31 Dec 2005 22:12:47 -0800, dansawyeror
wrote: Where are these equations? I have searched the ARRL handbook for Gamma and did not find it. This is also the first reference to rho. I am at my holiday place, and don't have reference manuals here, so I can't give you specific references. I am confident that the main formulas you need will be in both the ARRL Handbook and the ARRL Antenna Handbook (though some versions of the ARRL publications contain an incorrect formula for Gamma / rho). They will be in any text on transmission lines. Because I don't have the Geek font available here, I write Gamma to mean the uppercase Greek symbol gamma (G). Textbooks should use the symbol. "Gamma" (meaning the uppercase Greek Gamma symbol) is the complex reflection coefficient and Gamma=(Zl-Zo)/(Zl+Zo) where Zl and Zo are complex quantities. rho (lowercase Greek rho) is the magnitude of Gamma, rho=|Gamma|. VSWR=(1+rho)/(1-rho). You can rearrange these expressions to suit your needs. For example, Zl=Zo*(1+Gamma)/(1-Gamma). Be careful, there are different notations and meanings in different texts. You need a reference! Owen -- |
using an HP 8405A to measure SWR ?
|
using an HP 8405A to measure SWR ?
dansawyeror wrote:
Where are these equations? I have searched the ARRL handbook for Gamma and did not find it. This is also the first reference to rho. Some texts, like ITT's "Reference Data for Radio Engineers" simply define rho to be complex as does Ramo and Whinnery. rho = Eref/Efor = -Iref/Ifor = (Z-Z0)/(Z+Z0) = (Y0-Y)/(Y0+Y) = |rho| /_ 2*psi where psi is the electrical angle to the nearest voltage maximum looking back toward the generator from the load. Walter C. Johnson uses 'k' for the reflection coefficient. -- 73, Cecil http://www.qsl.net/w5dxp |
using an HP 8405A to measure SWR ?
On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
wrote: This is the next chapter in the antenna measuring saga. Today's adventure is trying to measure SWR with an HP 8405A Vector Voltmeter. The measuring setup is a standard signal generator feeding a tee. One branch feeds the A input of the 8405A and the other a 20 db directional configured in reverse, that is feeding the out port. The assumption is the reflected signal will be read correctly on the in port. The in port is then terminated or connected to an antenna. It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there. An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees and 55 db down from the input. (I assume 20 of that is the coupler so I am subtracting 20 from that reading - is that a correct assumption.) No and no. Although an open isn't as bad a reference as some folks think (in coax anyway), a better reference is a short. Then you know that the reflection is 100% -180 deg. (Where "" means "angle of") An open has a bit of fringing capacitance and a tiny bit of radiation so it strays (sorry) from 100% 0 degrees. The other thing you need to do is normalize all future readings to 0 dB. Because, as Owen notes, you're throwing away phase information in your quest for SWR only data, then start thinking in terms of return loss. Return loss is measured (or calculated) against a 0 dB reference. So from the beginning, when you get a reading with a short (or open) that ratio becomes your 0 dB reference. Using the method to read the SWR on an antenna produces values very close to those from several SWR meters used as controls. When the output is terminated in 25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5 db. (The B channel won't calibrate to within 1/2 a db so readings are estimates) Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument this is pretty close to an SWR of 2:1. Are you saying that it you step the input power to the "B" channel by 1 dB, it measures a 1/2 dB change, or that with the same signal applied to A and B they only agree within 1/2 dB? If it's the former, then stop here and fix the instrument. If it's the latter, you don't care; you are measuring ratios. Is there an error in this logic? If the instrument is working well enough to perform this calculation then it should support more complicated measurements. There are errors someplace. A 2:1 SWR is a return loss of 9.55 dB. |
using an HP 8405A to measure SWR ?
On Sat, 31 Dec 2005 22:12:47 -0800, dansawyeror
wrote: Where are these equations? I have searched the ARRL handbook for Gamma and did not find it. This is also the first reference to rho. Rho is mentioned on page 5 of AN77-3. Unfortunately, as I have written a long time ago in other threads, Hewlett-Packard's authors (and divisions) could not agree among themselves on a standard notation for complex reflection coefficient. The same holds true for the rest of the literature. My personal (and others') choice is to use upper case Gamma for the complex reflection coefficient and lower case rho for the magnitude of Gamma. If this was standard then there would be no confusion. |
using an HP 8405A to measure SWR ?
In article ,
Wes Stewart wrote: The measuring setup is a standard signal generator feeding a tee. One branch feeds the A input of the 8405A and the other a 20 db directional configured in reverse, that is feeding the out port. The assumption is the reflected signal will be read correctly on the in port. The in port is then terminated or connected to an antenna. It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there. The 8405A manual indicates the use of a power divider, and then a pair of equal-value pads. One side goes to the probe T for the A (reference) probe and thence to the termination, and the other goes to the probe T for the B probe and thence to the device-under-test. The manual is quite clear that the A and B probes need to be connected to points which are isolated from one another. You really don't want the oddities of the load connected to the B side to affect the voltage/phase of the reference signal seen by the "A" probe - it'd certainly wreck the measurement. Using a power divider, and resistive pads for isolation is one way to do this. Using a pad followed by a dual directional coupler (as in the experiment page to which Owen posted a link - thanks!) is another. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
using an HP 8405A to measure SWR ?
|
using an HP 8405A to measure SWR ?
On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy wrote:
On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror wrote: why the tee as described, doesn't the 8405A have a tee probe? Dan, my reason for this question is that you do not seem to have a load independent sample of the forward wave (if you are referencing measurements to the A probe). Ideally you would use a dual directional coupler, otherwise, you need to isolate the A sample from load impedance variations and reflections using a largish attenuator for example. If you are working the ratios out entirely from the B probe, eg B probe measurements on s/c and unknown load, then the A measurement becomes unimportant. Note that while this approach can give you enough info to measure rho and calculate SWR, it will not permit phase measurements. This approach dumbs the instrument down to a single channel RF voltmeter. Owen -- |
using an HP 8405A to measure SWR ?
The VVM appears to do a great job of reading 'open circuit' at 180 degrees and
50 Ohms at 0 degrees, however it does not appear to read shorts accurately. Shorts read at about 150 degrees instead of -180. They also degrade the incoming signal to the point where is has significant noise on the monitor output line. Is there a technique for doing this? Thanks - Dan This is the next chapter in the antenna measuring saga. Today's adventure is trying to measure SWR with an HP 8405A Vector Voltmeter. The measuring setup is a standard signal generator feeding a tee. One branch feeds the A input of the 8405A and the other a 20 db directional configured in reverse, that is feeding the out port. The assumption is the reflected signal will be read correctly on the in port. The in port is then terminated or connected to an antenna. It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there. An open termination reads 180 degrees and a 50 Ohm termination reads 0 degrees and 55 db down from the input. (I assume 20 of that is the coupler so I am subtracting 20 from that reading - is that a correct assumption.) No and no. Although an open isn't as bad a reference as some folks think (in coax anyway), a better reference is a short. Then you know that the reflection is 100% -180 deg. (Where "" means "angle of") An open has a bit of fringing capacitance and a tiny bit of radiation so it strays (sorry) from 100% 0 degrees. The other thing you need to do is normalize all future readings to 0 dB. Because, as Owen notes, you're throwing away phase information in your quest for SWR only data, then start thinking in terms of return loss. Return loss is measured (or calculated) against a 0 dB reference. So from the beginning, when you get a reading with a short (or open) that ratio becomes your 0 dB reference. Using the method to read the SWR on an antenna produces values very close to those from several SWR meters used as controls. When the output is terminated in 25 Ohms (2 x 50 Ohm terminators on a tee) the reflected signal reads about 24.5 db. (The B channel won't calibrate to within 1/2 a db so readings are estimates) Subtracting the 20 db bias yields 4.5 db. Within the error of the instrument this is pretty close to an SWR of 2:1. Are you saying that it you step the input power to the "B" channel by 1 dB, it measures a 1/2 dB change, or that with the same signal applied to A and B they only agree within 1/2 dB? If it's the former, then stop here and fix the instrument. If it's the latter, you don't care; you are measuring ratios. Is there an error in this logic? If the instrument is working well enough to perform this calculation then it should support more complicated measurements. There are errors someplace. A 2:1 SWR is a return loss of 9.55 dB. |
using an HP 8405A to measure SWR ?
In article , Wes Stewart wrote: The 8405A manual indicates the use of a power divider, and then a pair of equal-value pads. One side goes to the probe T for the A (reference) probe and thence to the termination, and the other goes to the probe T for the B probe and thence to the device-under-test. Actually, my manual does not show this. Although I have the full kit of a resistive tee, two 50 ohm "N" sampling tees and appropriate terminations, I don't believe Dan does. The manual is quite clear that the A and B probes need to be connected to points which are isolated from one another. I've read this someplace, but again my version of the manual (unless I'm really missing something) doesn't say it. The BAMA copy mentions it in paragraph 3-14. Later text indicates that attaching the two probes to a single point is an appropriate way to set phase-zero. Nevertheless, the directional coupler provides the isolation between probes. I see the issue, and I think I was conflating two different sorts of measurement regimes. The splitter/isolator/pad arrangement I was referring to appears on page 3-3 of the 8405A manual available at BAMA. It's what's appropriate for doing an in-line test of a transmission line or other network, where you want to see the effect of the network itself and can measure (via probe B) at the network's output. Page 3-4 shows a somewhat similar hookup, which doesn't include the resistive pads... I presume because the device-under-test (an amplifier) is assumed to have high isolation as part of its design. Neither of these hookups wouldn't work for measuring an antenna, since you can't measure at the antenna's output. Instead, using a directional coupler provides the necessary isolation, and (as you point out) lets you determine the incident and reflected signals accurately. I've got to clarify this a bit if I can... If you have the full set of parts per figure 11 in AN77-3 and you are using them as shown, then with equal loads on the two ends, the circuit is essentially a resistive Wheatstone bridge in balance with the null detected by the difference between probes A and B. In this case, the "incident" signal -is- measured by the A probe and the effects to the source by a changing load are incorporated into the measurement. In the case at hand, at least as I imagine it, there is no longer an nice tidy resistive Wheatstone bridge, but some cabling and a directional coupler in the mix. In this case, the generator is no longer the "source", the source is the signal at the input to the coupler. It is my belief (unless I change my mind later) that a sample derived from a resistive divider remote from the input to the directional coupler is not a true measure of the incident signal. Hmmm. In the general case, I believe you're correct. I suspect that the setup shown in the 8405A manual sets up a specific special case, though. The diagrams and text seem to be defining a case in which: - there is a physical and electrical symmetry in the T arrangement - that is, the power splitter is symmetrical, and the pair of attenuator pads between the splitter and the (A probe tap) and (device under test) are matched. The manual makes a point of this issue. - The pads being used are matched to the system's transmission line impedance, so that any reflected signal coming back from the DUT/coupler sees a proper termination by the source (the pad and signal generator, in this case) and is not re-reflected. In this particular situation, I believe that the incident signals reaching the DUT (the input to the coupler, in this case) and the "A" probe, would be identical... would they not? The proper termination of the reflected wave will mean that it won't re-reflect off of the generator and alter the incident wave. The "A" probe signal (off on its side of the "T") and a signal read out via the incident-wave tap on the directional coupler ought to be the same, once the coupling coefficient is taken into account... no? -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
using an HP 8405A to measure SWR ?
I do not have the 8405a adapter kit or power splitter. I made "probe to bnc"
adapters with 3/4 inch copper plumbing stock. The threads on the back of the probe are standard thread so it is easy to make an adapter and simply screw the probes in. This allows the A and B inputs to be connected to the directional coupler with bnc. The measuring setup is a signal generator feeding a tee. One side of the tee connects to the A channel input. The other side feeds the 'output side' of the coupler. The B port is connected to the coupled signal port. The load is connected to the coupler input. This should read the reflected signal. The signal generator is adjusted to read 0 dbm on the A port for all readings. (This is not possible for the 'shorted' readings). I do not have a pad between the generator and the tee. However, the above setup appears to support consistent readings. Dan Wes Stewart wrote: On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt) wrote: In article , Wes Stewart wrote: The measuring setup is a standard signal generator feeding a tee. One branch feeds the A input of the 8405A and the other a 20 db directional configured in reverse, that is feeding the out port. The assumption is the reflected signal will be read correctly on the in port. The in port is then terminated or connected to an antenna. It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there. The 8405A manual indicates the use of a power divider, and then a pair of equal-value pads. One side goes to the probe T for the A (reference) probe and thence to the termination, and the other goes to the probe T for the B probe and thence to the device-under-test. Actually, my manual does not show this. Although I have the full kit of a resistive tee, two 50 ohm "N" sampling tees and appropriate terminations, I don't believe Dan does. The manual is quite clear that the A and B probes need to be connected to points which are isolated from one another. I've read this someplace, but again my version of the manual (unless I'm really missing something) doesn't say it. Nevertheless, the directional coupler provides the isolation between probes. You really don't want the oddities of the load connected to the B side to affect the voltage/phase of the reference signal seen by the "A" probe - it'd certainly wreck the measurement. Using a power divider, and resistive pads for isolation is one way to do this. Actually, you do want to measure (include) the effects of the load on the source. Although you ideally want the source to be unchangable with respect to changes in the load (good source match). That is the point of my suggestion to pad the source at the input to the coupler. (Just as does paragraph 2.3 of Owen's reference) But if the load -does- pull the source you damn sure want to know it and account for it. The desired ratio is that between the incident signal and the reflected signal. Sampling somewhere off in isolated space via a tee and additional padding does -not- yield the incident signal. Using a pad followed by a dual directional coupler (as in the experiment page to which Owen posted a link - thanks!) is another. My point exactly. With the dual coupler the forward coupled arm -is- measuring the incident signal -regardless- of what effects load pull have on the source output. |
using an HP 8405A to measure SWR ?
Owen,
Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Thanks - Dan Owen Duffy wrote: On Sun, 01 Jan 2006 03:25:23 GMT, Owen Duffy wrote: On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror wrote: why the tee as described, doesn't the 8405A have a tee probe? Dan, my reason for this question is that you do not seem to have a load independent sample of the forward wave (if you are referencing measurements to the A probe). Ideally you would use a dual directional coupler, otherwise, you need to isolate the A sample from load impedance variations and reflections using a largish attenuator for example. If you are working the ratios out entirely from the B probe, eg B probe measurements on s/c and unknown load, then the A measurement becomes unimportant. Note that while this approach can give you enough info to measure rho and calculate SWR, it will not permit phase measurements. This approach dumbs the instrument down to a single channel RF voltmeter. Owen -- |
using an HP 8405A to measure SWR ?
"dansawyeror" wrote in message . .. Owen, Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Thanks - Dan Dan Are you open to trying to assemble something to measure the reflection coefficient rather than to put the project on hold? The HP 41952A Transmission/Reflection Test Set uses only one directional coupler. It uses a power splitter at the input with a pad to level the outputs from the "Fwd" and "Rev" ports. I can scan some info from the HP 41952 and E-mail them to you if you have interest in building something. I would think it would be fairly easy to build devices for HF if you already have a decent directional coupler. Jerry |
using an HP 8405A to measure SWR ?
Jerry,
Thanks, the antenna handbook has a design for a bi-directional coupler that looks like it will work. At this point I think the next step is to separate the input to the A port from the reflected signal. Putting a pad in the circuit to increase isolation affected the reflected reading. I am also sorting through those readings. Dan Jerry Martes wrote: "dansawyeror" wrote in message . .. Owen, Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Thanks - Dan Dan Are you open to trying to assemble something to measure the reflection coefficient rather than to put the project on hold? The HP 41952A Transmission/Reflection Test Set uses only one directional coupler. It uses a power splitter at the input with a pad to level the outputs from the "Fwd" and "Rev" ports. I can scan some info from the HP 41952 and E-mail them to you if you have interest in building something. I would think it would be fairly easy to build devices for HF if you already have a decent directional coupler. Jerry |
using an HP 8405A to measure SWR ?
|
using an HP 8405A to measure SWR ?
In article ,
Wes Stewart wrote: Bingo. Didn't seem like an "Electrical feature" to me :-) Yeah. I'm not sure just why there would be substantial interaction if the two points are connected to the same test point, since the rated impedance is pretty high even without 10:1 isolators. But I still submit that when you separate the bridge, insert a DC and some cabling, you lose the symmetry and the signal measured by the A probe is not necessarily the same as the signal incident at the input to the DC. Close maybe, but not something I would rely on. The signal on the other side of the T-and-attenuator setup wouldn't be the same as the signal at the input to the DC, certainly, since the signal at the input of the DC would be affected by the reflected signal. I don't disagree with you there. What I suggest, though, is that the signal on the "A" probe (at the other side of the T from the DC), and a signal as seen at the output of the DC's "forward" coupler line, ought to be very closely correlated. They'd differ by the coupler's coupling factor, of course, and there's be a bit of phase shift from the coupler (dependent on the coupler line length and the frequency). However, the loading at the coupler output from the load (or the calibration short) ought not to affect the signal appearing at the 'forward' tap on the coupler. Remember, when doing the calibration there is a 100% reflection. This can have a huge perturbing effect on the incident signal at the coupler input if the source is not well matched. Agreed, and I don't suggest that measuring the incident at the coupler input is a good idea. That's why I originally suggested a pad right at the coupler input, especially if there is some cabling between the generator (or power splitting tee) and the DC. Agreed. No. The B probe, in the single directional coupler arrangement, is not measuring -incident-, but reflected signal. True. I was assuming a double directional coupler, and asserting that the "forward" output on the coupler will produce a signal equivalent (except for scaling and perhaps a tad of phase shift) to a signal taken from the far side of the splitter-and-pads "T". In any event, Dan has stated that he doesn't have all of this stuff and is stuck using the DC only. My suggestion holds, put a pad at the DC input, measure the incident at the DC input and of course, the reflected at the coupled port. Yes, that should work quite well, and I think it'd give results pretty much equivalent to [1] a dual directional coupler or [2] the splitter-and-two-pads isolation arrangement. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
using an HP 8405A to measure SWR ?
|
using an HP 8405A to measure SWR ?
On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror
wrote: Owen, Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Dan, it seems to me that you should be able to make measurements with a single directional coupler (DC) You could connect your signal generator to the directional coupler via a 40dB attenuator, and put the chan A probe T on the sig gen end of the attenuator. This sample should be fairly independent of the reflection from the unknown load (to the extent of the 40dB attenuator), and so approximately proportional to the incident wave alone. The sample from the DC "reflected" port (properly terminated) is fairly independent of the incident wave (depending on the F/B ratio of the coupler) and so is approximately proportional to the reflected wave alone. Calibration of the B channel magnitude with a s/c and o/c taken as rho=1 provides the basis for measurement of Gamma. The angle of Gamma should be calibrated to 180 and o deg respectively. BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho is very small. The angle of Gamma is real important for s/c and o/c and ought be almost exactly 180 deg difference (if not, you have a instrument problem). Following this procedure, if the magnitude of the B channel on the unknown load measures for example 9.5dB below the B chan magnitude on a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the magnitude of Gamma) is 0.333 and you could measure the phase offset from the o/c angle to determine the angle of Gamma. Why won't this work? Owen -- |
using an HP 8405A to measure SWR ?
Owen,
That idea seems to work. I set it up and then looked and the 'interference', the change in phase based on changing the pad. Zero pad showed several degrees phase shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about equal. I decided on 20 dbm as a practical base. Thanks - Dan Owen Duffy wrote: On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror wrote: Owen, Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Dan, it seems to me that you should be able to make measurements with a single directional coupler (DC) You could connect your signal generator to the directional coupler via a 40dB attenuator, and put the chan A probe T on the sig gen end of the attenuator. This sample should be fairly independent of the reflection from the unknown load (to the extent of the 40dB attenuator), and so approximately proportional to the incident wave alone. The sample from the DC "reflected" port (properly terminated) is fairly independent of the incident wave (depending on the F/B ratio of the coupler) and so is approximately proportional to the reflected wave alone. Calibration of the B channel magnitude with a s/c and o/c taken as rho=1 provides the basis for measurement of Gamma. The angle of Gamma should be calibrated to 180 and o deg respectively. BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho is very small. The angle of Gamma is real important for s/c and o/c and ought be almost exactly 180 deg difference (if not, you have a instrument problem). Following this procedure, if the magnitude of the B channel on the unknown load measures for example 9.5dB below the B chan magnitude on a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the magnitude of Gamma) is 0.333 and you could measure the phase offset from the o/c angle to determine the angle of Gamma. Why won't this work? Owen -- |
using an HP 8405A to measure SWR ?
Now that the setup is reading consistently I will 'test' a loaded 2m monopole
over a 1 m**2 ground plane. Dan Owen Duffy wrote: Perhaps this lab document might help you: http://emclab.concordia.ca/~trueman/...ent_2_2005.pdf -- |
using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 13:34:54 -0800, dansawyeror
wrote: Owen, That idea seems to work. I set it up and then looked and the 'interference', the change in phase based on changing the pad. Zero pad showed several degrees phase shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about equal. I decided on 20 dbm as a practical base. This doesn't make sense... are you using "dbm" to mean decibels of attenuation, usually written "dB". The units "dBm" are usually written to qualify a power level with respect to one milliwatt. The attenuator on your sig gen might be marked in dBm, but that applies to the combination of the oscillator, possibly its level meter, and the attenuator as a system. Using the wrong terms for things is often a result of a concept gap! A 20dB attenuator will reduce the effect of the reflected component to about the same level as you would expect from a practical directional coupler, more attenuation is better if you have the power from the sig gen and the VVM probe chan can operate at the higher input level. Owen -- |
using an HP 8405A to measure SWR ?
dansawyeror wrote:
Now that the setup is reading consistently I will 'test' a loaded 2m monopole over a 1 m**2 ground plane. I suggest that you start with an unloaded monopole or some very simple antenna with a well known impedance. (You will of course have to know and allow for the effect of the finite ground plane.) You also need to take measures to prevent coupling between the antenna and the outside of the feedline. The ground plane you mention will help, but there can still be substantial coupling. Some high impedance ferrite beads at the feedpoint and another set about a quarter wavelength down should reduce the coupling to a small value. Roy Lewallen, W7EL |
using an HP 8405A to measure SWR ?
I proceeded before reading this note. The procedure was to zero the phase meter
on an short and then to test the loaded 2m vertical. The result was +10 dbm forward (before the 20 dbm pad) and -50 dbm reflected. The coupler measures about -14 dbm. The total was about -60 dbm, with 34 db of that due to the pad and coupler. The net is -26 db forward - reflected. (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. How can it measure very close to 50 Ohms? Is there something wrong with this analysis? Thanks - Dan Roy Lewallen wrote: dansawyeror wrote: Now that the setup is reading consistently I will 'test' a loaded 2m monopole over a 1 m**2 ground plane. I suggest that you start with an unloaded monopole or some very simple antenna with a well known impedance. (You will of course have to know and allow for the effect of the finite ground plane.) You also need to take measures to prevent coupling between the antenna and the outside of the feedline. The ground plane you mention will help, but there can still be substantial coupling. Some high impedance ferrite beads at the feedpoint and another set about a quarter wavelength down should reduce the coupling to a small value. Roy Lewallen, W7EL |
using an HP 8405A to measure SWR ?
dansawyeror wrote:
I proceeded before reading this note. The procedure was to zero the phase meter on an short and then to test the loaded 2m vertical. The result was +10 dbm forward (before the 20 dbm pad) and -50 dbm reflected. The coupler measures about -14 dbm. The total was about -60 dbm, with 34 db of that due to the pad and coupler. The net is -26 db forward - reflected. (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. How did you arrive at this figure? I wouldn't hazard a guess without modeling it. How can it measure very close to 50 Ohms? 1. Inductor loss. 2. Effect of finite size ground plane. 3. Coupling to feedline. 4. Measurement error. Is there something wrong with this analysis? I don't know. What should the impedance really be? Roy Lewallen, W7EL |
using an HP 8405A to measure SWR ?
"Owen Duffy" wrote Using the wrong terms for things is often a result of a concept gap! =========================================== .. . . . . and using the wrong name for an SWR meter often results in a concept gap. ---- Reg. |
using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
wrote: The coupler measures about -14 dbm. What does this mean? Are you trying to tell us that the power on the coupler port is 14dB less than the through power? What has dBm got to do with it? You didn't report the power in the coupler port with a s/c and / or o/c at the measurement plane. Did you perform this cal? (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) That is understandable. How much coax between the A probe and the load, and the B probe and the load... how many degress does this total electrical length change for a 1% change in frequency? Does that explain some of the phase sensitivity? The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... How can it measure very close to 50 Ohms? Is there something wrong with this analysis? See if you can trust your measurements on known loads before wondering why the unknown load isn't what you expect when using unknown measurement technology... too many unknowns. Owen -- |
using an HP 8405A to measure SWR ?
On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote:
(The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) ...Does that explain some of the phase sensitivity? Hi Owen, Being very touchy, especially to the specifics of sitting very still, sounds like classic common mode problems. 73's Richard Clark, KB7QHC |
using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 23:06:43 -0800, Richard Clark
wrote: On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote: (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) ...Does that explain some of the phase sensitivity? Hi Owen, Being very touchy, especially to the specifics of sitting very still, sounds like classic common mode problems. Yes it does Richard. I saw Roy's response regarding isolation of the feedline, and it is a valid comment. My comment was towards the reported frequency sensitivity... until the effect of the propagation delay is removed from the results, the underlying impedance is obsured. Owen -- |
using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 21:56:37 GMT, Owen Duffy wrote:
On Mon, 02 Jan 2006 13:34:54 -0800, dansawyeror wrote: Owen, That idea seems to work. I set it up and then looked and the 'interference', the change in phase based on changing the pad. Zero pad showed several degrees phase shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about equal. I decided on 20 dbm as a practical base. This doesn't make sense... are you using "dbm" to mean decibels of attenuation, usually written "dB". The units "dBm" are usually written to qualify a power level with respect to one milliwatt. The attenuator on your sig gen might be marked in dBm, but that applies to the combination of the oscillator, possibly its level meter, and the attenuator as a system. Using the wrong terms for things is often a result of a concept gap! A 20dB attenuator will reduce the effect of the reflected component to about the same level as you would expect from a practical directional coupler, more attenuation is better if you have the power from the sig gen and the VVM probe chan can operate at the higher input level. Owen Let me pick a nit or two. More attenuation is not necessarily better. In theory the improvement in source match is two times the attenuation, so a 10 dB pad improves the return loss to no less than 20 dB, even with a zero ohm source, and with a decent source match of RL = 10 dB or so, is as good as you need. (I know you know this already) I say this because it's very likely that the return loss of the attenuator isn't any better than 25-30 dB, regardless of its attenuation. For example Narda makes a "precision" Type N attenuator: http://www.nardamicrowave.com/east/P...dPrecision.pdf Note the VSWR spec, 1.15 at low frequency. That's a 23 dB RL. So although a 20 dB pad in theory provides a minimum 40 dB RL, the actual RL can be as little as 23 dB. Manufacturers have to work really hard and typically use a precision connector like 3.5mm or 7mm to build a 40 dB RL termination although Anritsu will sell you a 40 dB RL type N termination for -only- $650 USD. Also, and this goes back a post or two, where you suggested that if a pad is used between the generator and the input to the coupler, the "A" probe (reference/incident) should be between the generator and the pad. This is contrary to what I tried to recommend earlier when I said: "It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there." Let me offer this thought experiment: If you had two directional couplers, such that one could be used to sample the forward signal and the other the reflected, would you place a pad between them to isolate the generator from the effects of the load? |
using an HP 8405A to measure SWR ?
On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart
wrote: .... Owen Let me pick a nit or two. More attenuation is not necessarily better. In theory the improvement in source match is two times the attenuation, so a 10 dB pad improves the return loss to no less than 20 dB, even with a zero ohm source, and with a decent source match of RL = 10 dB or so, is as good as you need. (I know you know this already) I say this because it's very likely that the return loss of the attenuator isn't any better than 25-30 dB, regardless of its attenuation. For example Narda makes a "precision" Type N attenuator: http://www.nardamicrowave.com/east/P...dPrecision.pdf Note the VSWR spec, 1.15 at low frequency. That's a 23 dB RL. So although a 20 dB pad in theory provides a minimum 40 dB RL, the actual RL can be as little as 23 dB. Manufacturers have to work really hard and typically use a precision connector like 3.5mm or 7mm to build a 40 dB RL termination although Anritsu will sell you a 40 dB RL type N termination for -only- $650 USD. Also, and this goes back a post or two, where you suggested that if a pad is used between the generator and the input to the coupler, the "A" probe (reference/incident) should be between the generator and the pad. This is contrary to what I tried to recommend earlier when I said: "It would also be nice it you had a 6 - 10 dB pad between the generator and the directional coupler (DC); located right at the DC. You want the source match to be set right there and the A probe to sample right there." Let me offer this thought experiment: If you had two directional couplers, such that one could be used to sample the forward signal and the other the reflected, would you place a pad between them to isolate the generator from the effects of the load? No, of course not... it just adds another source of error, and increases the gap between the measurements being made on both probes. But Dan does not have two directional couplers. To my mind, if the A probe is sampling the main transmission line, the sample is of the resultant of the algebraic sum of the forward and reflected waves rather than a sample of the forward wave alone (well, nearly alone) as you would get with a directional coupler. My suggestion of placing the A probe on the source side of the attenuator is to reduce the contribution of the reflected wave to the A probe measurement. The attenuator was proposed mainly for isolation of the forward wave component for measurement, rather than a source matching issue... which also exists. Have I got this wrong? All comments on practical limits of RL from pads / attenuators noted, and understood. Owen -- |
using an HP 8405A to measure SWR - update
Thanks,
I went back through the calibration procedure. 1. The meter appears to be performing as it is supposed to. The 'static readings work repeatably and predictably. With a balanced configuration shorts and opens read correctly. 2. The pad I was using appears to have a frequency shift. With the 'legs balanced for length' the measurement would change with frequency. This really confused the measurement process. I put it aside. 3. I have some 23 Ohm couplers. These of course confuse readings when used as normal couplers. However if they are used in a truly balanced configuration - that is the legs are configured as close to identical as possible then they appear to work. I am going to shelve this until I find or make a 50 Ohm bi-directional coupler. Thanks again - Dan Owen Duffy wrote: On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror wrote: The coupler measures about -14 dbm. What does this mean? Are you trying to tell us that the power on the coupler port is 14dB less than the through power? What has dBm got to do with it? You didn't report the power in the coupler port with a s/c and / or o/c at the measurement plane. Did you perform this cal? (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) That is understandable. How much coax between the A probe and the load, and the B probe and the load... how many degress does this total electrical length change for a 1% change in frequency? Does that explain some of the phase sensitivity? The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... How can it measure very close to 50 Ohms? Is there something wrong with this analysis? See if you can trust your measurements on known loads before wondering why the unknown load isn't what you expect when using unknown measurement technology... too many unknowns. Owen -- |
using an HP 8405A to measure SWR ?
On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart
wrote: Owen Let me pick a nit or two. .... Firstly, thanks for posting AN 77-3, it is a long time since I read it, and have little recollection of the recommended low frequency test setup. The HP 11549A is described as a power splitter, so I am guessing that it is some kind of hybrid (ie as in hybrid transformer) that in that role, whilst splitting the power to the "output" ports, will prevent power flow between "output" ports, so isolating the A probe to some extent from the reflected wave on the unknown load side. Additionally, the 8491 attenuator in the load path will improve the return loss at the B side 11549A port, so that combination seems to be stabilising the loads presented to the splitter (which if it is a hybrid, improves is cross port isolation), and improving the RL by the action of the attenuator and splitter. Without knowing the loss in the 8491 (I know they were available in 10dB, but I think there were -3, -6, -10 and -20s), or the isolation across the splitter, it is hard to quantify the total isolation of reflected wave from the A probe. It may be that Dan should consider constructing a hybrid or Return Loss Bridge, whatever you want to call it, it will be cheaper and have less loss that a dual directional coupler for HF measurements. IIRC, the ARRL has some simple designs. Owen -- |
using an HP 8405A to measure SWR ?
On Wed, 04 Jan 2006 21:35:40 GMT, Owen Duffy wrote:
On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart wrote: Owen Let me pick a nit or two. ... Firstly, thanks for posting AN 77-3, it is a long time since I read it, and have little recollection of the recommended low frequency test setup. The HP 11549A is described as a power splitter, so I am guessing that it is some kind of hybrid (ie as in hybrid transformer) that in that role, whilst splitting the power to the "output" ports, will prevent power flow between "output" ports, so isolating the A probe to some extent from the reflected wave on the unknown load side. Nothing so exotic www.k6mhe.com/n7ws/HP-11549.pdf Additionally, the 8491 attenuator in the load path will improve the return loss at the B side 11549A port, so that combination seems to be stabilising the loads presented to the splitter (which if it is a hybrid, improves is cross port isolation), and improving the RL by the action of the attenuator and splitter. Without knowing the loss in the 8491 (I know they were available in 10dB, but I think there were -3, -6, -10 and -20s), or the isolation across the splitter, it is hard to quantify the total isolation of reflected wave from the A probe. It may be that Dan should consider constructing a hybrid or Return Loss Bridge, whatever you want to call it, it will be cheaper and have less loss that a dual directional coupler for HF measurements. IIRC, the ARRL has some simple designs. Maybe, but I think Dan is trying some VHF measurements and the coupler approach is really the way to go. Wes |
using an HP 8405A to measure SWR
I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a different way. I don't understand "two 50 Ohm loads in parallel with the stub". Tests of stubs are now clearly reading correct for various lengths across frequencies. That is once the 1/4 wave is determined the next odd and even is very predicable. Thanks, Dan Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... |
using an HP 8405A to measure SWR
On Thu, 05 Jan 2006 22:24:54 -0800, dansawyeror
wrote: I now have use of a b-directional coupler for HF and would like to perform the following 'tests' however I do not understand them. Can you explain the last a different way. I don't understand "two 50 Ohm loads in parallel with the stub". Parallel two 50 ohm loads with a tee piece, then use another tee to put the stub in parallel. The tees will be imperfect, but at HF, the impact should not be major. Tests of stubs are now clearly reading correct for various lengths across frequencies. That is once the 1/4 wave is determined the next odd and even is very predicable. Do you get the correct answers for 1/8, 3/8 wave stubs in parallel with a 50 ohm load? (1/8 wave s/c stub has an impedance close to 0+jRo, and you need to put it in parallel with the dummy load, so series equivalent is 25+j25. 25 ohms in parallel with 1.8 wave s/c stub should be around 20+j10. Put a 25 ohm load at the end of a metre of coax and check its transformation at different frequencies (equivalent to quarter wave, eight wave etc) against your Smith Chart prog. They are some examples, work some out for what you have at hand. Prove that your measurement system works on predictable loads. Owen Thanks, Dan Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... -- |
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