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Author Topic: SWR Follies  (Read 3282 times)
G0UIH
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« on: December 10, 2012, 12:11:28 PM »

Here’s an interesting SWR problem that a friend is experiencing.
His antenna is a 10m multiple loop being fed with good low loss coax. The loop is gamma matched.
He says that the SWR changes wildly when going from a carrier to SSB. At a 100w carrier keydown the SWR reading is 1 to 1.2, but goes up to 2 to 1 and more when on SSB. The gamma is PTFE based and will handle QRO without any trouble, but the fact that the readings seem to change depending on ‘Mode’ is an odd one.
The original setup had no balun coil but now has a homebrew ‘ Ugly Balun’ .This has not made any change in the readings. We thought at least the ugly balun would isolate possible feeder radiation issues. My own thought is that something could be saturating or heating which is causing impedance changes. I’m assured the gamma match internals are dry and that the gamma is not shorting to earth.
Any thoughts appreciated
73 Steve – G0UIH
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KA4POL
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« Reply #1 on: December 10, 2012, 12:28:49 PM »

Could this be an adjustment problem? What meter is he using and how does he measure (single tone)?
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N6AJR
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« Reply #2 on: December 10, 2012, 01:38:25 PM »

well the swr being better at the low end, ( cw part) and higher at the ssp part( higher in frequency, usually means the antenna is too long.  have him try shortening it up a couple of inches and try it again. or borrow a mFJ 259b and see where it is resonant, and adjust from there.
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G8HQP
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« Reply #3 on: December 10, 2012, 02:29:29 PM »

Funny SWR readings usually mean a fault in the SWR meter, or user error. Some SWR meters have a switch setting for SSB, I believe. SWR is a ratio, so the meter has to divide one voltage by another (or use a twin needle meter). In either case if one of the voltages is based on a different smoothing time constant from the other then errors will arise if the RF is modulated. Sorry, that is a long-winded way to say that a capacitor in the meter could be faulty.
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AC5UP
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« Reply #4 on: December 10, 2012, 03:14:52 PM »

Generally an SWR reading is taken with a constant carrier mode like CW, FM, FSK or AM. The meter sensitivity is calibrated to full-scale in the FORWARD power position then the meter is flipped to REVERSE power and the operator observes the SWR reading.

Which brings us to trying to read the SWR of an SSB signal. A signal where the TX power level can be anywhere from milliwatts to QRO. So how do we calibrate the forward power when it's constantly changing? If the meter does auto-set, an SSB signal could easily give it a false calibration point.

If it were mine I'd manually set the meter against a full-power CW signal. SWR should read lower (on average) compared to CW because the TX output in SSB is lower (on average) than CW.

If none of this works as expected, test the meter against a dummy load. If all OK, my next suspect would be the ugly balun. Could have hit an odd reactance in the coil.......... ( ? )
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N4NYY
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« Reply #5 on: December 10, 2012, 03:20:01 PM »


Which brings us to trying to read the SWR of an SSB signal. A signal where the TX power level can be anywhere from milliwatts to QRO. So how do we calibrate the forward power when it's constantly changing? If the meter does auto-set, an SSB signal could easily give it a false calibration point.


I think I would test it with a tone, from a function generator or so. But that is a pain in the ass. It is easier on to just do it on AM. But his SWR is not that bad on SSB. If he is using a tuner, does it matter?
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WA8UEG
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« Reply #6 on: December 10, 2012, 03:47:29 PM »

Sounds normal to me. SWR is measured with a carrier, bouncing up to 2:1 when looking at the meter peaks while operating SSB would indicate higher readings then your real swr measurement. If measuring output power on SSB you'll need a peak reading meter.
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STAYVERTICAL
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« Reply #7 on: December 11, 2012, 02:47:34 AM »

Sounds normal to me. SWR is measured with a carrier, bouncing up to 2:1 when looking at the meter peaks while operating SSB would indicate higher readings then your real swr measurement. If measuring output power on SSB you'll need a peak reading meter.

I agree, if your SSB signal is more than 100Watts on peaks, and your SWR meter can respond quickly enough, it will indicate higher reverse readings.

Remember how SWR meters work.
The full scale calibration is just a way of making sure that the reverse reading is a fraction of full scale deflection.
This is really just a simple analog computer way of representing the division of forward by reflected power and the scale is calibrated accordingly.
So, unless your forward power is adjusted to the CAL point (FSD), your reverse readings, and scale are essentially useless, except as a relative indication.

Measure your SWR on a 100W carrier and forget it on SSB.

73 - Rob
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KB3HG
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« Reply #8 on: December 11, 2012, 08:55:07 AM »

You might want to check out K4DPK his website: http://home.comcast.net/~k4dpk the link came from his call search here. His PEP meter circuit and discussion. Yeah he sells them but I'll bet it works on SWR as well, ask him.
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KB4QAA
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« Reply #9 on: December 11, 2012, 12:14:58 PM »

Just to hammer the point:  You don't measure SWR in SSB mode.  Use a carrier mode, CW, FM, AM.  Period.   

This is a non-problem.
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WB6BYU
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« Reply #10 on: December 11, 2012, 12:49:59 PM »

Quote from: KB4QAA
Just to hammer the point:  You don't measure SWR in SSB mode.  Use a carrier mode, CW, FM, AM.  Period.   


This is not entirely correct.  You can monitor SWR in any mode:  I've certainly trimmed
my tuner while talking on SSB, even with a conventional SWR meter, by adjusting for
minimum reflected power even when it is jumping around.

With proper equipment that can compare forward and reflected power instantaneously
you can monitor the actual SWR on any mode.  There was a simple automatic meter
in QST some years ago using an LM3914 LED bargraph driver that worked with SSB
signals.  (The reading was pretty consistent from 5 to 100 watts output.)  And any
power meter with a peak reading hold circuit for each channel can compare them
and generate an SWR indication that is pretty consistent.

The point being, of course, that you can measure SWR on an SSB signal as long as
you have equipment that is designed for that purpose.
  If you set the forward
sensitivity manually, as with many inexpensive meters, then the varying output in SSB
mode will cause the reflected power, and therefore the indicated SWR, to jump around.
Without knowing what type of meter is being used, we can't really say whether the
observed symptoms are to be expected or not.


But I have seen a few things that can lead to a difference in SWR with changes in the
mode or similar sorts of things.  They aren't common, certainly, but they can (and have)
happened.

Probably the most common one is harmonic or other spurious outputs.  These don't show
up on a dummy load, but will to various degrees with any frequency-specific load like an
antenna.  If the transmitter is being overdriven in SSB mode such that the peaks of the
waveform are being clipped, that will generate harmonic output that may not be present
in the CW waveform (especially if the rig is driven to the same output power on an average
reading wattmeter, in which case the finals may be well over driven on SSB peaks.)
The SWR meter will read higher if the antenna has a higher SWR at the harmonics than
at the fundamental.

I had one case where my SWR read higher when I wasn't talking than when I was:  that
turned out to be an oscillation in the IF stage about 100kHz away from my operating
frequency.  After I adjusted my tuner for a 1 : 1 SWR at the operating frequency, the
spurious read about 2.2 : 1 when I wasn't talking.  The amplitude of the spurious signal
decreased as my modulated signal increased, so the SWR really did vary as I talked.
(The fact that the meter indicated output when I wasn't talking was a clue that
something wasn't right.)

You can even have the same problem with a very narrow bandwidth antenna.  This most
often appears as an issue limiting the keying rate for VLF stations, but a very high Q
antenna on 80m or 160m might show the same symptoms (or if the tuner is adjusted to
a very high Q setting).  Basically the CW signal is at only a single frequency (+/- a few
tens of Hz for keying bandwidth) while the SSB signal is spread out over a 2 kHz range.
With a very high Q antenna / tuner combination the SWR may vary over that 2 kHz
bandwidth, so the SWR will be higher in SSB.  Most of us won't encounter such a problem,
however, unless we are experimenting with very small, efficient tuned loops on 80m or 160m.
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KA4POL
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Posts: 1961




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« Reply #11 on: December 11, 2012, 09:50:49 PM »

Could this be an adjustment problem? What meter is he using and how does he measure (single tone)?
We still do not have any feedback. I would rather like to continue the trouble shooting based on hard facts.
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WA8UEG
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Posts: 348




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« Reply #12 on: December 12, 2012, 04:44:02 AM »

Nobody needs to troubleshoot anything, there is no problem. He is seeing an increase in the swr reading between constant carrier mode and ssb, thats normal with most every swr bridge. The swr he has in constant carrier mode is the correct swr. What he sees when switching to ssb is just meter movement thats of no value. If all else fails read the directions for the swr meter.
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K6UJ
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Posts: 307




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« Reply #13 on: December 13, 2012, 08:14:24 AM »

I think WA8UEG is probably right.  There will be a difference when reading constant carrier
and SSB if the meter isnt designed to show peak ssb.    The first thing I would do is disconnect the antenna feedline and connect a dummy load to the output of the SWR meter and see if it
behaves the same.  Then you are eliminating the antenna feedline and antenna.  Let us know what happens.

73,
Bob
K6UJ
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WA8UEG
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« Reply #14 on: December 13, 2012, 05:48:19 PM »

Just need to know the model number of a peak reading SWR meter designed to be used to meassure SWR operating SSB. Not saying there isn't one, just saying I have never seen one. Of course a Peak reading watt/swr meter will not measure swr accurately in the peak mode just wattage.
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