Re: [amsat-bb] SWR on a receive-only antenna

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

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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