On Mar 17, 10:16*am, "Joel Koltner"
wrote:
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

You didn't mention noise figure, and that's important. At UHF and
above, atmospheric noise is typically very low, and you benefit from a
really low noise figure. In that case, you want to match the source
to the receiver input in such a way as to achieve the best noise
figure, which in general is NOT the same as getting maximum power
transfer into the receiver. Data sheets for parts intended as input
receiver amplifiers generally have a graph and/or table of optimal
source impedance for lowest noise figure versus frequency. Some will
have noise contours on a graph for some critical frequency or
frequencies.

At HF, atmospheric noise is generally very high, and you also have to
put up typically with some very large signals. So the design of the
receiver input is driven by different needs. You want to keep
distortion low, to avoid problems from those large signals, and you
want to achieve a "reasonable" noise figure -- but reasonable might
well be 15dB, an absolutely terrible noise figure for a 440MHz
receiver front end. And you might well put in a pad you can switch in
to lower distortion, and out for better noise figure.

Also, in some cases such as a car radio, the antenna impedance isn't
anything close to 50 ohms (for the AM broadcast band). The design
there would of course be different than if you were dealing with a 50
ohm source.

Cheers,
Tom