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Author Topic: Which SWR meter reading to believe?  (Read 5276 times)
AE7VA
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Posts: 37




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« on: March 23, 2014, 09:30:02 AM »

I just picked up a brand new Palstar PM2000A meter for its peak reading function.  The peak reading function is light years better than the passive Daiwa meter I was using before.  (My Kenwood TS590S would not read a peak 100 watts on the Daiwa but certainly does on the new Palstar.)  That aside...

All four of my SWR meters give different results.  The meters are:

Palstar PM2000A
MFJ Antenna Analyzer
Daiwa CN101
Kenwood TS590S internal meter.

I checked the SWR swapping out external meters.  To test the Kenwood’s meter, the Palstar and Daiwa were not connected.

Here is the SWR chart:
             Palstar      Daiwa     TS590S  MFJ
10M        2.5             1.2          1.1      1.5
15M        1.9             1.2          1.2      1.5
20M        1.5             1.1          1.1      1.3

The Palstar obviously seems like the odd duck here.

The meter reading for forward RF are: Palstar is 90 watts, the Daiwa is 70 to 80 watts and the TS590 is 100 watts.  

When connected to a dummy load, all four meters were the same.  Could I assume that my Palstar did not come correctly adjusted from the factory or is it simply more accurate than the others???  
« Last Edit: March 23, 2014, 09:38:24 AM by AE7VA » Logged
KE6EE
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Posts: 399




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« Reply #1 on: March 23, 2014, 02:05:42 PM »

Look at the specs for high end wattmeters like the LP100 or one from Array Solutions. Or a Bird. In the $500 range. I think they claim, and can show, accuracies of 5% or better.

The purposes of less-accurate meters like you and I have is to give sufficiently-accurate relative readings for the purposes of adjusting our antennas and antenna tuners and so forth. In these cases absolute accuracy is maybe 10% or maybe 20% which seems to be what the various readings on your meters are showing. So long as we are getting a means to tune to the lowest SWR or maximum peak power, these cheaper meters are doing what they need to do. They're not lab-grade gear, but plenty good enough.
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NO2A
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Posts: 801




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« Reply #2 on: March 23, 2014, 02:08:21 PM »

This is why I always go by the built in swr meter on my FT-857D. It will often show unity when the MFJ-989C might show 1:1,1:2,1.3,etc. It`s only important the swr the radio sees,not what the tuner sees,cause it will always vary somewhat. Of course if your rig doesn`t have a built in meter,you`ll have to use the external one. My rig just has the bargraph swr meter,so I never know the exact swr. I simply tune for no swr reading.
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AA4PB
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Posts: 12899




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« Reply #3 on: March 23, 2014, 05:12:19 PM »

Yes, even with an expensive meter one meter could be reading 5% high and another of the same type reading 5% low so you see a 10% difference. With watt meters the spec is 5% of full scale reading. If your full scale is 100W then the possible error is 5% of 100W or 5W, even if you are trying to read a power of say 10W. So, a 10W transmitter can read between 5W and 15W on a meter that is working within factory spec.

That's why it is best to set your meter (if you can) to a scale low enough to make the anticipated power read close to full scale.

« Last Edit: March 23, 2014, 05:14:39 PM by AA4PB » Logged
W3HKK
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Posts: 596




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« Reply #4 on: March 23, 2014, 05:42:41 PM »

This is a classic story.  Ive done it many times. 

If you can  borrow an LP100 or a Bird, you can get to less than 5% error, and use the variable pots inside  most swr meters to adjust the low power, medium power and high power ranges.

I daisy chain  a half dozen meters to a dummy load, then  record the Po and SWR.  After that,  if you cant get a LP100 or Bird, take the one you think is most accurate and adjust the others to it.

20-30% error is not uncommon on meters that have never been recalibrated, or were recalibrated 5-10 years ago.   By periodically recalibrating them, you can get the error down to  5-10%.    Over time mine  got to be way off, but  after  plotting the  numbers ON EVERY BAND, then  tweaking the internal pots,  they become surprisingly close.

Its a nice way to spend a cold winters night.
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AE7VA
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Posts: 37




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« Reply #5 on: March 23, 2014, 08:30:33 PM »

I've never had a meter so far off brand new out of the box.
Quote
Its a nice way to spend a cold winters night.
or a hot Arizona afternoon as I did today.  Smiley

First, I noticed that both needles were a little off zero without RF applied, so slight adjustment to zero them both.

Second, I checked all my other meters/analyzers into a dummy load running low power and high power; all showed 1.1:1.  So I adjusted the low power (<300W) and high power (>300) pot for the reflected needle so that I got the same 1.1:1 reading across all bands.  

Third, reconnected to the antennas and checked each band on each antenna running the low and high power settings.  The Palstar is now within the margin of error of the other meters.  

The only two odd readings are into my vertical where 17M and 20M are showing 1.1 on the Palstar but are closer to 2.0 on all the other meters.....can't explain this one.
« Last Edit: March 23, 2014, 08:37:03 PM by AE7VA » Logged
WB2WIK
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Posts: 20612




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« Reply #6 on: March 24, 2014, 10:00:45 AM »

Man with one clock always knows the time.

Man with two clocks never sure.
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AA4PB
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Posts: 12899




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« Reply #7 on: March 24, 2014, 10:55:28 AM »

Aren't you glad that in the old days we never had to worry about SWR meters or SWR? If you could set the tune and load controls to get the proper plate current you just made contacts and didn't worry about anything else.  Shocked

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WD8KNI
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Posts: 149




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« Reply #8 on: March 24, 2014, 12:00:37 PM »

Aren't you glad that in the old days we never had to worry about SWR meters or SWR? If you could set the tune and load controls to get the proper plate current you just made contacts and didn't worry about anything else.  Shocked



He has already spent more time worry about this than the whole SWR subject is worth... I am still trying to understand why anyone would want more than one SWR meter.   Like you said load the thing up and make contacts.  who cares as long as the transmitter is not folded back.  I wish I was the one who convinced people they needed PEP reading meters.. I could have made a fortune...  Fred
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KE6EE
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Posts: 399




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« Reply #9 on: March 24, 2014, 02:21:43 PM »

Man with one clock always knows the time.

Man with two clocks never sure.

Many clocks in my house, all with slightly different time. I refer to the nearest, easiest-to-read clock. Whatever it tells me is plenty close enough for what's happening in that room. Wink

I have two SWR meters in line (an analogue cross-needle meter on my antenna tuner and a bar graph Elecraft SWR/power meter). I also have a power and SWR indicators on my transceiver. The bar graph meter is much quicker and easier to see for touching up my antenna tuning, so I mostly use it. In a pinch, as when I haven't powered up the bar graph meter, I use the meter on the antenna tuner--it works fine and is sufficiently readable. Every now and then I check the meter functions on my transceiver--just for the hell of it.

It's very entertaining to have choices but not to worry about perfection.  Cheesy

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KI6LZ
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Posts: 587




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« Reply #10 on: March 24, 2014, 03:12:48 PM »

Quote from LP-100 manual: "This is a subject that’s almost as controversial as antenna gain measurement. It seems like it should be much simpler, since it’s a measurement that can be done in a controlled laboratory environment, but it is an insidiously complicated measurement to make."
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KB4QAA
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Posts: 2407




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« Reply #11 on: March 25, 2014, 10:14:14 AM »

'Peak Reading' is a feature of no value when measuring SWR!

Sanity Check:  You aren't using SSB when checking SWR are you?  A carrier mode like CW, AM, RTTY, FM should be used.

SWR should be measured with a steady carrier and no modulation.  Attempting to read SWR in SSB or AM while talking is a pointless excercise.

Meters can have  significant errors at levels near the bottom of the power scale.  Even the best Bird wattmeters have only a 5% of scale accuracy.  I recommend using a power level that is near the middle of scale, e.g. 50 watts on a 100 watt scale, rather than cranking all knobs to the right for maximum power.

-Antenna analyzers can give false readings when exposed to high RF levels such as with a nearby high power broadcast station, particularly when measuring the lower HF bands.

-Common mode current on the feed line can affect both radio and external SWR meters.  Do you have quality ferrite BalUn's or Chokes on your feedline at the antenna and ideally at the shack entrance point?  Air coiled coax chokes are seldom effective over more than one or two bands, especially if they weren't made to specification or formula.

-Maximum power ratings on transmitters are a generalization which can vary with band, mode and other conditions.  They aren't 'guarantees'.

-It unrealistic to expect a perfect 1:1 SWR match on every antenna system, on every band, regardless of how well the antenna is tweeked, and regardless of how expensive the antenna tuner (match).
« Last Edit: March 25, 2014, 10:29:59 AM by KB4QAA » Logged
G3RZP
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« Reply #12 on: March 25, 2014, 03:30:50 PM »

usually, the S means 'Suspicious'.
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K7KBN
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« Reply #13 on: March 26, 2014, 09:39:00 PM »

Again with the "5% accuracy" stuff!  If I had a meter that was only ±5% accurate I'd certainly replace it with something better.

Now, if you're talking about a ±5% FSV maximum error, that makes sense.  This corresponds pretty closely to a ±95% FSV accuracy.
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73
Pat K7KBN
CWO4 USNR Ret.
G3RZP
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Posts: 4727




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« Reply #14 on: March 27, 2014, 12:28:19 AM »

The  standards for approved test houses require a 95% certainty on conducted power measurement of no worse than +/- 0.75dB. That's 18.9%. 5% is +/- 0.212dB. You also have to take into account the total mismatch loss from the load, connectors and cables.

The best claim I've seen from a Test house (as opposed to a national standards organisation) is +/-0.1 dB for frequencies below 1 GHz. That's 2%.

I fall into the same camp as AA4PB on this one....

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