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 Do IMD figures for a PA change with bad antenna matches? 
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Post Do IMD figures for a PA change with bad antenna matches?
Not sure if this is the right part of the forum to ask, sorry. I wish to know if a semiconductor linear amp, or semiconductor PA stage of a transmitter will show poorer IMD performance if mismatched into a poor antenna load? Or is the mismatch irrelevant to IMD figures? Thanks

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Sat Jun 08, 2013 11:24 am
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 Do IMD figures for a PA change with bad antenna matches? 
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Post Re: Do IMD figures for a PA change with bad antenna matches?
I would expect that changing the load impedance will change the IMD performance. That's not to say it will always get worse, but if you want to be sure the amp will meet the specified IMD figures it will need to see its specified load.

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Sat Jun 08, 2013 12:02 pm
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 Do IMD figures for a PA change with bad antenna matches? 
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Post Re: Do IMD figures for a PA change with bad antenna matches?
OK, thanks for that Steve. I'll tell you why I ask. I have built a couple of Softrock SDR Ensemble RXTX's. One for 160 meters only, one for 30, 20 and 17 meters. I decided to see how well it would load up on 40 meters, and was surprised to get a fairly decent SWR reading via my FC-902 matching unit into my horizontal quad loop aerial. I saw about 1.7 to 1. I decided to risk a quick transmission and found I could easily get into Europe and Russia with its devilish 1 Watt output (less actually as the power on 40 meters dropped a bit). I thought it wise to see if there were significant IMD issues, and to ask the question. It also receives OK on 40 meters.

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Sat Jun 08, 2013 1:03 pm
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 Do IMD figures for a PA change with bad antenna matches? 

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Post Re: Do IMD figures for a PA change with bad antenna matches?
SWR alone doesn't tell you the whole story. I'm sure Steve will confirm there are figures of R and X that would produce a low SWR but you'd not want to transmit into it.

I have a Panorama 5/8 wave VHF antenna that shows <2:1 SWR over about 30MHz but once you get more than 5MHz either side of the resonant point the performance nose dives.


Sun Jun 09, 2013 2:56 pm

 Do IMD figures for a PA change with bad antenna matches? 
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Post Re: Do IMD figures for a PA change with bad antenna matches?
M0GVZ wrote:
SWR alone doesn't tell you the whole story. I'm sure Steve will confirm there are figures of R and X that would produce a low SWR but you'd not want to transmit into it.

I have a Panorama 5/8 wave VHF antenna that shows <2:1 SWR over about 30MHz but once you get more than 5MHz either side of the resonant point the performance nose dives.


If the complex impedance is close enough to 50 ohm so as to give one a reasonable VSWR, why wouldn't you want to transmit into 'it', and by 'it' I assume you are talking about an antenna and not some devilish device.

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Sun Jun 09, 2013 9:09 pm
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