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1-9 of 9 messages
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Should I expect a SWR meter problem?
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by K6IXB on November 3, 2009
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Recently, I have been having a problem getting a good SWR on 20-meters with my screwdriver antenna. 1 month ago, I was getting very good SWR reading on 80, 40 and 20-meters (<1.5:1). Now, I have and SWR reading of 3.0:1 now. Nothing has changed int he configuration of the station. I sent the antenna back to the maker who reported to me that he found nothing wrong (SWR-wise) anywhere on the 20-meter band.
I do have a very cheap SWR meter (Workman) and I'm wondering if I may need to point my attention to the meter now...
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RE: Should I expect a SWR meter problem?
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by WB6BYU on November 3, 2009
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What does the meter measure into a dummy load?
Actually, while that is a good test that it is calibrated
for the proper impedance, that doesn't guarantee that
the overall response is correct. A good check is to
connect two dummy loads in parallel to see if the meter
reads 2 : 1. (You can use various lengths of coax between
the load and the meter if you want to verify performance
over a range of impedances.)
A couple places I'd check; first, do you have a shunt
matching coil between the bottom of the screwdriver and
ground? If that is disconnected (or if you adjusted it
to improve the match on 80m) that will change the SWR on
20. Also, any change in the ground bonding can make a
difference. Did one of your ground straps come loose?
Does the antenna tune properly on the other bands?
If you add a capacity hat (such as a couple pieces of
bare wire twisted around the top of the whip and sticking
out several inches on each side) does it allow you to
get a match or change the SWR? (While this would be
expected to vary the SWR a little, if there is a significant
change it might indicate a poor contact point on the
coil at the normal 20m mark.)
It certainly is possible to have an SWR meter problem:
I replaced a diode in one of mine and now the reflected
power reads high on 80 and 160m, though it seems to work
well enough on 40m and up. More common with an inexpensive
meter might be a dirty spot on one of the pots that
causes the SWR to be quirky at certain settings. It's
worth checking, if for not other reason than to rule it
out as the source of the problem.
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RE: Should I expect a SWR meter problem?
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by K7BAB on November 3, 2009
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I would test the SWR bridge in the following manner.
(This used be a procedure for calibrating kit SWR bridges.)
First, get a good dummy load.
Connect the transmitter to the “transmitter” input of the SWR bridge and the dummy load to the “antenna side.” When the meter in the forward position reads full scale and you switch to reflected power observe there is no reflected power.
Reverse the connections (that is, the transmitter is now on the “antenna” connection and the dummy load is on the “transmitter” connection). Observe the reflected power is now full scale (in the reflected position) and there is no forward power indicated when you switch to the forward position.
Use only enough power to get a full scale deflection using each test scenario.
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RE: Should I expect a SWR meter problem?
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by N5LRZ on November 4, 2009
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Attach a test, temp, transmission line to the antenna and retake the SWR. IF the SWR goes back down to reasonable levels then you know its either your transmission line (water in the line) or the connectors themselves (a connection broken loose).
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RE: Should I expect a SWR meter problem?
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by WA3SKN on November 4, 2009
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Well, most SWR meters are cheap... or at least inexpensive. But for trouble shooting you need to start from a known good point, and that is the dummy load.
Connect radio to dummy load... how does it respond?
Connect dummy load to other end of coax... what do you measure?
Connect dummy load through SWR meter... what does it measure?
And, (at HF frequencies) wire two dummy loads in either series or parallel... and use this to calibrate your SWR meter for 2:1 SWR!
But you probably either have a bad connection or water has gotten into the coax, because something has changed.
Good luck, and 73s.
-Mike.
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RE: Should I expect a SWR meter problem?
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by K6IXB on November 4, 2009
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I forgot to add in my haste to post this that I am getting very good SWR readings on 40 and 80-meters. I suspected a connection/water problem (although I wrapped all of the connections in rescue tape), and cut the coax and made a new connection (the coax does not have a PL-259 on the end). I have wrapped the connections again and it has been dry for the past two weeks. All connections are soldered (including the ground straps).
After all this, I still see 3.0+:1 on 20 meters, but flat nor near flat on the other bands. I thought that it might be the length of the coax (75 feet), but it is the same length as when I was getting 1.1:1 on all bands.
This has been a real learning experience and I truly appreciate all of your suggestions! I will start employing your suggestions and will report back!
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RE: Should I expect a SWR meter problem?
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by VK1OD on November 4, 2009
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K7BAB suggested "Use only enough power to get a full scale deflection using each test scenario" when calibrating a VSWR meter.
If the VSWR meter is of the type with a stepless variable sensitivity, they almost invariably are not accurate at maximum sensitivity as the meter scale markings are made for lesser sensitivity. (The cause of this is the non-linearity of the meter circuit due to the diode forward voltage drop.)
Users of these things often observe that VSWR is apparently lower at lower power levels. I hear them on air from time to time for example, explaining how their antenna achieves 1.1:1 at low power, though there is something wrong with the meter, it indicates 1.5:1 on full power.
This is almost always attributable to the above design failing of many of these VSWR meters, and in the example case above, it is the 1.5:1 figure that is believable.
Unless you have proven that the scale markings are valid at maximum sensitivity, then calibrate or test it at low sensitivity (ie high power).
Owen
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RE: Should I expect a SWR meter problem?
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by K7BAB on November 6, 2009
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The test protocol I proposed for checking the SWR bridge is only to see if the sense circuits for are equally functional and calibrated.
For example: The MFJ, and many “low end” SWR bridges use two diodes (one for forward and one for reverse power). See this diagram to see an example of this circuit in use: http://www.mfjenterprises.com/man/pdf/MFJ-812B.pdf
The reversing of the in and out connections will indicate any mismatch between the detectors.
The referenced SWR bridge uses a “strip line” but “current transformer” bridges (with one meter) may also use a dual detector switched meter scheme.
VK1OD: There is an excellent article in the November 2009 QST that touches on the subject of non-linearity of detector diodes in SWR bridges.
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RE: Should I expect a SWR meter problem?
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by VK1OD on November 6, 2009
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Barbara,
I have not seen the QST article to which you refer. I don't subscribe, due mainly to a question of quality.
I am somewhat of a loss to understand the value of any tests of an instrument outside of the range where it provides sufficiently accurate / valid results. Nevertheless, the non-linearity of the detector does not affect the 'accuracy' of the null, just the magnitude of a less than perfect null.
Having seen three postings about low power effects on VSWR meters in as many days, I wrote a short note entitled "Accuracy of VSWR meters at high sensitivity" at http://vk1od.net/blog/?p=774.
Testing a VSWR meter is more than checking detector symmetry, my article at http://www.vk1od.net/transmissionline/VSWR/VSWRMeter.htm list six tests that a good meter should pass.
Owen
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