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Re: SWR on a receive-only antenna



All the replies to this subject are correct, but the full story is a little
more complex than might at first be thought :-

Both the source and the load have an effect on the overall SWR - if the
antenna has an impedance of (for example) 20 ohms and is connected to a 50
ohm receiver via 50 ohm  coax then there will be a mismatch and hence a
reflection.  (What happens to the reflected signal has been the subject much
debate, and is actually not really important).

If a 'perfect' 50 ohm antenna (i.e. SWR = 1:1) was connected to a receiver
that did not have exactly 50 ohms impedance (i.e. SWR > 1:1), then there
would also be a reflection.  HOWEVER, in this case, although it may appear
that some of the received signal has been lost, it may actually lead to a
better received signal/noise ratio - because :-

The first stage of most receivers operating at VHF frequencies and above is
a low noise amplifier.  In order to achieve the best oveall signal/noise
ratio at teh output, it is (nearly always) necessary to have the lowest
possible noise figure (NF) for the low noise amplifier.  The NF of the
amplifier is dependent on many variables, and one of those variables is the
impedance presented to the actual transistor or FET in the LNA.  The vast
majority of low noise transistors and FETs have the lowest noise figure when
the impedance presented to them is not 50 ohms - the actual figure varies
enormously from one type of transistor/FET to another, and it is necessary
to read the manufacturer's data sheets to determine this.

Therefore, in order to achieve the best NF, an impedance matching circuit is
used to transform the input impedance of the receiver (i.e. 50 ohms) to the
impedance that the first transistor/FET requires.  Now, if one were to
'look' into the receiver, i.e. if one were to measure the input SWR (which
is possible, but meaningless), one would see the reverse effect of the
impedance transform.  Due to the fact that the impedance matching will
(hopefully!) have been designed for best overall noise performance, the
input of the receiver will not look like 50 ohms - it will have an SWR of
greater than 1:1.

It is for this reason that most manufacturers of LNAs and recievers do not
specify the input SWR - they may specify that the impedance is 50 ohms, but
this means that for best performance, the receiver expects to see a source
impedance of 50 ohms (from the antenna and feeder).

So in summary, the receive antenna should have an SWR as close to 1:1 as
possible - not because of mismatches and reflections, but becuase this will
result in the overall best signal/noise performance.  The SWR of the
reciever is of no concern if it is connected to an antenna - the receiver
(or LNA) will have been optimised by the manufacturer for lowest noise
figure, not for best SWR.

I hope that this helps to clear up a possible mis-understanding.  However,
it is worth making the point that the SWR of a receive antenna is less
important than for a transmit antenna, but the overall system N/F will be
reduced if the antenna has a fairly high SWR.  Maybe not too important for
Gunther's 2 element strip line antenna, but when working AO-40 near perigee,
every little helps.

Grant Hodgson
g8ubn@amsat.org

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