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Author Topic: Why SWR Reading Discrepancies?  (Read 983 times)
KW4CQ
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Posts: 129




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« on: May 23, 2003, 09:10:19 AM »

Getting reliable SWR readings seems to be a bit of a mystery.  I make my SWR readings here relying on my Model VFD SWR/Wattmeter (mfg. by RF Applications, Inc.) which sits betwen my Linear Amplifier and my Palstar AT4K Antenna Tuner, just where it is suppsed to go.  I notice that I get significantly different SWR readings at relatively low TX power levels (say 25 to 50 watts) than I do at 1,000 watts (power in watts as indicated on the VFD display). Apparantly the power relationship is not a linear one.  But why?  Is the SWR or the indicated Power in watts wrong?  The folks at RF Applications, Inc. could not provide me with an an answer.

Oddly enough, the SWR/Power readings displayed on my Palstar antenna tuner meter do not track those of the RF Applications VFD SWR/Wattmeter.  Shouldn't they?

Can anyone shed any light on this mystery?  
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WB2WIK
Member

Posts: 20611




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« Reply #1 on: May 23, 2003, 01:16:09 PM »

SWR is completely unrelated to power level, so it sounds to me like your RF Applications instrument is not working correctly.  That it doesn't track the Palstar meter is no big surprise, since all these instruments are relatively crude with regard to absolute accuracy and are mostly intended to provide relative indications, like the dip in reflected power indicated when a match is achieved.

The ubiquitous and ever-trusted Bird Thruline only has claimed best accuracy of "5% of full scale," meaning that when reading 50W on a 1000W scale, its reading can be +/- 100% (+/- 50W, since 5% of 1000W is 50W)...not very good, is it?  And it's about the best there is.

WB2WIK/6
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WA4PTZ
Member

Posts: 528




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« Reply #2 on: May 27, 2003, 06:16:20 AM »

Sounds to me like a little RF feedback. I seriously
doubt that its anything to worry about. I do have a
question....why do you need a SWR meter inline with
the amp ? Once you have checked the jumper to amp
swr there should not be any reason to continually
monitor it. You are only creating another RF radiator
and another possible trouble source. Follow the
KISS principle ( Keep It Simple Stupid )
Nothing personal...that's just the way it reads.
73 - Tim
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WA9SVD
Member

Posts: 2198




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« Reply #3 on: May 31, 2003, 09:08:15 PM »

Steve is correct; SWR is completely independent of power level.   BUT... there is one factor to consider (and since you are using moderate power levels, it probably doesn't apply here.)  If you measure SWR at very low power levels, line loss may significantly attenuate your reflected power indication, giving you an INDICATED SWR that seems better than it actually is.  So it's always prudent to re-check SWR at some significant power level (e.g. 5W, or your actual operating level) rather than just relying on the attenna analyzers.

    In such a situation, the SWR itself doesn't change; it's only the indicated SWR that varies, and it's a function of the equipment used.
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Guest

« Reply #4 on: June 03, 2003, 08:43:13 AM »

You might want to try using one of those cheapy swr only meters.  Even though they are crude compared to a watt meter, they tend to be consistent.  You may have something loose in your antenna system or corrosion somewhere that will cause weird symtoms.
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AA4PB
Member

Posts: 12890




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« Reply #5 on: June 14, 2003, 12:52:42 PM »

Line loss is the same regardless of power level. The line will attenuate your reflected power by the same number of dBs at 5 watts as at 5000 watts. Unless your "low power" is so low that the meter is unable to read the reflected power, the SWR reading should be the same at all power levels.

Now I have seen instances where the SWR actually did change with high power levels due to heating of traps in the antenna at high power.
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WA9SVD
Member

Posts: 2198




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« Reply #6 on: June 15, 2003, 04:47:47 AM »

To AA4PB:

     That was precisely my point!  If you are using a QRP- rig, or one of the antenna analyzers (milliwatt range,) the line loss may be enough in some cases to mask the true SWR.  So the SWR would appear to change when you applied any real power.  And if you are in a situation where you have varying SWR because of trap heating, etc. Then (IMHO) something's wrong!  Is this a cae of runnin too much power for the design, or poor design.  (I don't think this should happen under normal circumstances.)
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