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 Different SWR when Changing Radios 
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Post Different SWR when Changing Radios
Pal of mine (non member) asked if I had any idea on the following;

On the same antenna, SWR meter, patch lead and on the same band (15m) with the same power output etc:

With two of his HF radios the meter shows 1.2/1 VSWR but with a third radio it shows 1.8/1 VSWR.

This is on an external meter with the only changing factor being the transceiver.

While not a crucial difference, he's wondering why changing to the third radio is causing this slight change in the reading?


Edit: just had a thought; could the output impedance of radio three be slightly above / below 50 ohms compared to the other two?

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Sat Jul 18, 2015 6:09 pm
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
Solved!

Turns out there was a very small amount of RF on the coax.
Can only assume one set was more sensitive to it than the other two!
Tells me he's put on some ferrites and now gets the same readings with all three radios.

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Sat Jul 18, 2015 11:55 pm
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
Those sorts of puzzles often turn out to be common-mode issues; still it's interesting that one radio should have a different effect from the others.

The output impedance of the radio shouldn't alter the SWR - it's determined solely by the load impedance. I once measured the output impedance of all my radios - not one of them was 50 Ohms!

Steve G3TXQ


Sun Jul 19, 2015 8:07 am
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
Glad you found the answer as it can be a nightmare when things like this happen. I have also had similar issues with a few differing transceivers. Also noted what I think was common mode causing an issue with a basic SWR meter, as I approached the meters body with my hand the results on the meter would change depending upon the distance of my hand! Only item that I have used that was unaffected by the common mode or stray RF was my YouKits FG-01 antenna analyser which offered an exact reading each time with great accuracy, then again it was only transmitting in the mW range of power.

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Sun Jul 19, 2015 8:16 am
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
Thanks for the replies guys.

I had suggested it could be common mode RF, but odd it only showed up on the one radio out of three!

Ironically, if the VSWR metre had shown the same readings with all three radios he might never had known there was a problem!

As for radio output impedance; that was me trying to think outside the box for other possibilities.

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Sun Jul 19, 2015 11:15 am
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
Are all three radios grounded to the same source....or if none are grounded perhaps a temporary earth to the one showing the higher reading may alter it's reading more in keeping with the other two.

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Sun Jul 19, 2015 11:33 am
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 Different SWR when Changing Radios 
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Post Re: Different SWR when Changing Radios
I'll ask him Gerry.

Though he did text me: he thinks he's resolved it by fitting a choke on the feed line, added some ferrite beads I think.

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Sun Jul 19, 2015 12:38 pm
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