Pre amps
 

I'm still new in regards to radio, but had a couple questions
regarding equipment. I've seen some info about pre-amps, and was
wondering if this is just a low-power transmitter that "should" have
an amp between it and the antenna. Is the preamp basically everything
before the power amplifier? If not, where am I mistaken?

Thanks in advance!
Dave

The term "preamp" (short for preamplifier) is used to describe an
amplifier put at the input to a receiver. It's not used for transmitting.

When used with a modern receiver, they often do more harm than good, by
generating spurious signals while not improving the signal to noise
ratio. One of the few places where they are useful is when mounted at
the antenna in a VHF or UHF system when there's significant loss in the
feedline between the antenna and receiver.

Roy Lewallen, W7EL

David Harper wrote:
I'm still new in regards to radio, but had a couple questions
regarding equipment. I've seen some info about pre-amps, and was
wondering if this is just a low-power transmitter that "should" have
an amp between it and the antenna. Is the preamp basically everything
before the power amplifier? If not, where am I mistaken?

Thanks in advance!
Dave

On 20 Sep 2004 11:00:44 -0700, David Harper hath writ:
I'm still new in regards to radio, but had a couple questions
regarding equipment. I've seen some info about pre-amps, and was
wondering if this is just a low-power transmitter that "should" have
an amp between it and the antenna. Is the preamp basically everything
before the power amplifier? If not, where am I mistaken?

Thanks in advance!
Dave

There are preamps, and there are preamps.
In general, a preamp is used to amplify a Very Low Level signal
up to "general, utility" levels.

E.g.: A mic preamp is used to amplify the Very Low Level signal
of a microphone up to a level where the audio circuits can
better modify and/or filter it. Or, a rcvr preamp is used at the
front of a receiver's antenna input to get the rf input up to a level
where the signal(s) can better be mixed, filtered, or modified.
There are many considerations in the design of a preamp -- but
foremost, maybe, is the design for low noise due to the fact
that these signals do not stand very tall over whatever noise
exist in the system.

Preamps may be external or internal. I.e., a mic preamp could
be found in the base of the microphone, on the cable, or in the
device (transmitter, P.A., etc.) to which the mic is connected.
Same-o for a rcvr preamp: It could be mounted up at the antenna,
somewhere in-line in the transmission line, or inside the receiver.

HTH es 73
Jonesy
--
| Marvin L Jones | jonz | W3DHJ | linux
| Gunnison, Colorado | @ | Jonesy | OS/2 __
| 7,703' -- 2,345m | config.com | DM68mn SK

"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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