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Re: SWR on a receive-only antenna



What is the input impedance of most of our receiver / preamps?  I am under
the impression it is often quite high.

73,
Joe
ka0yos@amsat.org

----- Original Message -----
From: "Franklin Antonio" <antonio@qualcomm.com>
To: <gjmouse@neo.rr.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Tuesday, March 05, 2002 11:09 PM
Subject: Re: [amsat-bb] SWR on a receive-only antenna


> At 07:00 PM 3/5/2002, Gunther Meisse wrote:
> >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?
>
> Less consequence than for a transmitter, but still some consequence.  VSWR
> is a measure of mismatch.  The issues that drive concern over mismatch are
> different for a transmitter than a receiver, but mismatch has consequences
> in both cases.
>
> When energy comes rolling in from a receive antenna, and hits an impedance
> discontinuity, some of the energy is reflected.  That's bad.  You want all
> the energy to go to into the receiver.  The energy reflected is lost.  It
> goes flying back out the antenna!
>
> The appropriate measure is "mismatch loss", which tells you the ratio of
> signal out to signal in.  (Don't confuse this with "return loss", which is
> something else.)  In other words, a mismatch loss of 1 dB does the same
> thing to the signal entering your receiver as throwing an attenuator of 1
> dB in a perfectly matched line.
>
> Example: A VSWR of 3:1 corresponds to a mismatch loss of 1.24 dB.  Whether
> that's ok or bad depends on your situation.  If losing 1.24 dB offends
you,
> then you would like a VSWR somewhat better than 3:1, etc.
>
> Here's a web page with the formulas, and a table relating VSWR to mismatch
> loss:  <http://ewhdbks.mugu.navy.mil/VSWR.htm>
>
> If you have two or more impedance discontinuities things get more
> complicated.  (You can play tricks with combinations of
> discontinuities.  That's how matching sections and some kinds of filters
work.)
>
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