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R: SWR on a receive-only antenna




----- Original Message -----
From: Edward R. Cole <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, March 06, 2002 10:41 AM
Subject: Re: [amsat-bb] 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
>

Hi Ed,

Any active device to be used in a LNA input stage has an impedance for the
lowest noise figure NF

Noise parameters for low noise devices  show the GAMMAopt in magnitude and
angle i.e. the reflection coefficient for the lowest NF

For example the HEMT CFY65 has the following Noise Parameters at 10 GHz

F (GHz)  = 10
NF dB    = 0,86
GAMMAopt= 0,53 /_ 160
Rn = 0,06

0,53 is the magnitude of the reflection coefficient and 160 is the angle in
degrees of the reflection coefficient that the device want to see to supply
the lovest NF of  0,86 dB

If you put 0,53 /_ 160 in a Smith chart for 50 ohm system you will get an
input impedance of  15,79 + j 7,96 ohm

And this is the impedance that the active device will show when it is
matched for the lovest possible NF in a 50 ohm system.

Now it is necessary to built a matching network between the LNA input
connector and the device gate in order to let the 50 ohm  antenna system
to see the  gate coniugate match or 15,79 - j 7,96 ohm of the input
impedance.

As soon the matching network has been designed the LNA or preamplifier or
receiver input  must be tuned for the lowest NF using a Noise Generator Head
wich impedance is purely resistive or 50 +j 0 ohm

After the adjustement for the lowest NF has been made if you now measure
the input return loss of  your  LNA or preamplifier using an impedance
bridge you will find a very low return loss or a very high VSWR for the
receiver input.

Using GaAsFET it is possible to get input return loss in the order of only 5
or 6 dB for the lowest NF

In any case the low input return loss or high input VSWR do not represent a
problem since the LNA is directly connected to the antenna input without
long runs of transmission line.

In conclusion a low input return loss or hygh input VSWR is a condition
dictated by the device itself to supply the lowest NF and that is wath
should be payed for.

At this point having  tuned the LNA input for the lowest NF  on a 50 ohm
system if you connect to its input an antenna system wich is not purely
resistive or 50 + j0 ohm  at that frequency it happens that the NF of your
receiver degrades and increases in any case because its  NF has been
adjustet for the lovest possible value using a Noise Generator purely
resistive or 50 + j0 ohm

I hope this help a bit

73" de i8CVS Domenico


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