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Re: SWR on a receive-only antenna



>From: "Gunther Meisse" <gjmouse@neo.rr.com>
>Gentlemen & Ladies:
>I have read my VHF antenna book until I am blue in the face.
>I seem to remember that in a receive-only antenna the SWR is of little
>consequence. IS THAT CORRECT?
>
>I am building a two element antenna (driven and a reflector) spaced at 0.18
>wavelength. I am using a strip line element technique using 3M 1/4" copper
>tape on the face of a Plexiglas structure. I am strip lining back from the
>driven element about 4" to the BNC connector. The inherent impedance of the
>antenna is in the neighborhood of 20 ohms and will be fed with RG6. The
>field tests show a gain within .25DB  of a commercially available antenna.
>
>Any thoughts?
>
>Thanks,
>Gunther Meisse
>W8GSM

I read the replies that address power transfer issues of impedance
mismatch, but generally it is of less significance than for transmitters
which commonly use power control circuits to protect the final Tx stage in
the face of high VSWR, and the  power loss in high SWR transmission lines.
(e.g. open wire feed line has historically been used with high SWR
successfully because of its very low line loss).

There is another issue that is probably more significant for LNA's and that
is that proper impedance match affects noise figure performance.  Typically
a low noise device wants a slight mismatch for best NF.  The experts say
that is around 200 ohms (and most preamps are designed with this built-in).
 But that is prefaced on the antenna port being 50 ohms, so if it is
significantly far from 50 ohm then the source impedance for the low noise
device is shifted degrading the NF performance.

I am not an expert on this topic so cannot tell you how much NF degradation
take place if a high SWR exists.  If you have sufficient skill and test
equipment then the input impedance for the LNA might be tweeked to offset
the SWR effect.  Likewise, you can just match the antenna to 50 ohms using
a tuner.

A simpler test might be comparing your stripline antenna vs. a matched
antenna on a known noise source (e.g. the sun).

Ed 

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