"David Harper" wrote in message
om...

an amp between it and the antenna. Is the preamp basically everything
before the power amplifier? If not, where am I mistaken?

Dave,

Both Roy and Jonesy are right. As Jonesy says, there are all sorts of
preamps. But generally, in a radio context, with no other qualifiers, we
are talking about recieve preamps, which Roy assumed.

Roy (who by the way happens to be one of the giants in this business)
mentioned that a receive preamp is rarely a good thing. It's easy to get
talked out of believing this without some explanation.

What happens is this. There is atmospheric noise. This noise decreases
with increasing frequency. If a signal is below the noise, it's below the
noise. No amount of amplification is going to help, because you will
amplify the noise just as much as the signal. Now, gain is easy to come by
at low frequencies, but it gets harder to get at higher frequencies. At HF
(typically we mean below 30 MHz), even the cheapest receiver has enough gain
to hear the atmospheric noise, and won't be helped with a preamp.

You can run a simple test. Unplug your antenna and turn the volume up until
you hear the whooshing sound. (FM doesn't count here - only CW or SSB - FM
is a slightly different problem). Now, plug the antenna in. Did the noise
go up? If so, then a preamp won't help. Your receiver can already hear the
noise, so amplifying it more will do no good. While the need, or lack
thereof, holds true for FM as well, the test doesn't work for FM because of
the nature of the FM detector.

But wait, there's more. As Roy mentioned, they can do more harm than good.
What happens is that in your receiver there are all sorts of nonlinear
stages, some intentionally, some because nothing is perfect. When a strong
signal gets in, these stages are driven to their nonlinear ranges, and
generate other signals, or often, simply more noise. Well, there are always
some other signals getting in, by making them stronger, the preamp makes
this problem worse. And since the preamp amplified the noise as much as the
signal, and now added noise, the volume out of the speaker may have gotten
louder, but the signal you want to listen to has gotten harder to hear.

Now, as you go up in frequency, a number of things change. First, the
atmospheric noise goes down, so you can make use of more gain. Meanwhile,
transistors don't work as well, so that gain is harder to come by. To top
it off, the internal noise of the amplifiers and so forth increases. In
addition to all this, all feedlines have loss, and those losses increase as
you go up in frequency. At UHF you need to move to very expensive feedline
to avoid loosing most of your signal. So at the higher frequencies, a
preamp can be useful, especially if it can be placed up on the tower, ahead
of the feedline. This way the signal can be amplified above the noise level
of the circuitry before it has been attenuated down into the mud by the
feedline.

So basically, for HF a preamp is almost never a good idea. For VHF, it can
be useful for some special situations, especially if you have an older
receiver. For UHF, it can be handy if you have a need for a longer feedline
and can't afford the gold plated stuff. For microwave, you almost always
need a preamp.

Hope this helps

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