"Joel Koltner" wrote in message
...
I'm trying to figure something out here...

Assume that I have a 2m antenna that (through whatever means) happens to
have an impedance in the ballpark of 50ohms. Also assume that this
antenna will generate 1uV open-circuit.

Assume that I have a MMIC amplifier that has FET inputs and therefore has
a high input impedance... let's say it's 500ohms.

If I directly connect the antenna to the amplifier, the amplifier input
effectively sees ~0.909uV and then amplifies it.

If I build a matching network to make the amplifier "appear" as 50ohms, at
the input of the matcing network I'll have 0.5uV, but since energy has to
be conserved (my matching network is assumed to be lossless),
(0.5uV)^2/50=Vamp^2/500 -- Vamp=1.58uV. This is a gain of
20log(1.56/0.909)=4.8dB over a direct connection.

Is that all correct?

Now let's go down to HF. My antenna is still 50ohms, but at such low
frequencies parasitics aren't that bad and I just use a JFET or MOSFET and
have a 5kohm input impedance. In this case, without the matching network,
the amplifier sees 0.99uV. With the matching network, it sees 5uV (!), a
gain of 14.1dB over a direction connection.

Yet many designs for HF/shortwave antenna amplifiers don't bother with a
matching network, just feeding the antenna directly into a FET. So why is
this? Is the extra gain just not needed on HF? Or too unwiedly to build
when you're trying to cover everything from 1MHz-30MHz?

Thanks for the help,
---Joel Koltner


Receiving antenna matching is the same as a transmitter loss, except in
reverse. As you said if you start with a 50 ohm antenna and transmission
line and feed it into a 500 ohm receiver you will have a 10:1 swr and the
additional loss of the transmission line will come into effect. If it is
very short , it will not mater too much . If the line is long it will.
Above around 140 mhz the noise is low enough you need to use a low loss line
of a very short length. The noise figure is most important. Usually there
is a mismatch when tuned for lowest noise figure. Short line , no problem,
long or lossey line will cause much signal loss.

On the lower frequencies it usually does not mater too much what you do as
the noise comming in from the antenna will usually be the major factor.