[Drake] Preamplfiers and the Non-Need Thereof

[Barry L. Ornitz]

Thomas Beltran, W6EIJ, tbeltran@earthlink.net asked about
the need for a preamplifier for the R-4c receiver.

> I saw a post on a list serv, about AR2 [Advanced Receiver
> Research]communications - someone gave the address, and I
> tuned in.  They make, among other things, low priced
> preamps.  Since it is so common to have a preamp on
> modern receivers, I was wondering a preamp would benefit
> the R-4C.  I know Sherwood sells a preamp.

There are several points to be made here.  First ARR
specializes in equipment for weak signal VHF/UHF/microwave
reception.  Their equipment is optimized for this use.

Secondly the addition of a preamplifier on modern receivers
is a method of separating your money from your wallet.
Actually an attenuator is a far better addition.

Tom then went on to post some specifications including
noise figures for various models.

Probably the best place to start is to explain that in
addition to gain, every amplifier adds additional noise to
a signal.  The full meaning of noise figure can be found in
the ARRL Handbook so I won't go into this here.  Suffice it
to say that the lower the noise figure of a receiver
system, the weaker signal it can detect.  So a low noise
figure is good.

But you also have to remember that electrical noise is a
part of our environment being generated by thunderstorms,
electrical sparks, modern switching power supplies, etc.
The amount of this noise is frequency-dependent.  I am not
sure if the Handbook prints the curves showing natural
background noise versus frequency, but they can be found in
most professional books on communications and antennas.
Essentially on the high frequency amateur bands, the
background noise is quite high.  It drops off with
frequency as you get into the VHF and UHF bands.  It is
higher in populated areas where man-made noise is stronger.

Even at the upper end of the 10 meter band, the natural
noise in most areas exceeds about 8 to 10 dB.  Even in the
quietest areas electrically, it is usually above 7 dB.  So
if your receiving system has a noise figure at this level
or below, you will be able to hear everything that you
can.  Your receiver sensitivity will no longer affect the
weakest signal you can receive.

So adding a preamplifier to your R-4c receiver will not
help it receive any weaker signals.  And it may hurt.  The
extra gain of the preamplifier can cause your receiver to
overload on strong signals.  If you are trying to listen to
a weak signal in the DX portions of a band, and you have a
nearby kilowatt higher up the band, its strong signal can
often ruin the reception of the weaker signal.  Adding a
preamplifier will only make this situation worse.  If the
weaker signal is not too weak, placing attenuation in the
receive path can help.  It lowers the desired signal but it
also lowers the undesired signal.  The processes that
produce the interference are nonlinear, however, and
lowering the strength of the stronger signal may eliminate
much of the interference.  Both fundamental overload and
intermodulation distortion, which cause this interference,
are reduced when both signals are reduced in amplitude.

Essentially ALL HF ham receivers made after 1960 or so have
adequate sensitivity.  Adding a preamplifier to these
receivers will NOT improve their sensitivity.  Many of the
earlier receivers, typically many surplus W.W.II receivers
like the BC-348, will benefit from a preamplifier as they
do not have adequate gain above 15 MHz.  Some of the
general coverage receivers made before 1960 fall in this
category too.

So where does a preamplifier really help?   When the
natural background noise is low.

At two meters, the background noise has typically dropped
to around 3 dB, and it gets even lower at higher
frequencies.  Thus you often see preamplifiers sold for the
VHF and higher bands.  This is where Advanced Receiver
Research products are quite useful.

Feedline loss, the attenuation of a signal between the
antenna and the receiver, adds directly to the system noise
figure.  At VHF and above, coaxial cable loss can be quite
high.  But placing a preamplifier at the receiver to make
up for this loss will not work.  The preamplifier must be
placed at the antenna where its low noise figure can be
taken full advantage of.  Its gain makes up for the loss in
the feedline.  Of course, switching the preamplifier in and
out during receive and transmit can be an interesting
problem.  Advanced Receiver Research specializes in such
systems.

There are some applications for exceptionally low noise
receiving systems on HF.  These are used when antenna-
mounted microwave converters are used to down-convert an
incoming signal to HF.  The applications are rather
specialized, however, and if you are working these
microwave bands, you should already know about receiver
noise and would not be asking here.

When I mentioned earlier about modern receivers having no
need for a preamplifier, you might have asked yourself why
some new rigs include them.  Actually these rigs already
have front-ends with low enough noise figures.  What the
preamp switch actually does is to short out an internal
attenuator.  There generally is no real preamp.

Now for some experimental evidence.  About 20 years ago I
built a simple Field Effect Transistor preamplifier for a
friend who had a Hammarlund general coverage receiver that
was not sensitive enough over 20 MHz.  I tried it on my R-
4a before giving it to him.  Yes, it did amplify the weak
signals, but it amplified the background noise too.  At the
time I had been playing with extended ground wave on 10
meters (working "local" signals when the band is dead).
The preamplifier showed no improvement in the distance I
could hear from.  But on that Hammarlund, my friend could
suddenly hear the same signals I could where they were not
detectable before.

So to conclude, I suggest you save your money.  Unless your
R-4c is in really bad shape and grossly out of tune, the
preamplifier will not help - and it may hurt.

        73,  Dr. Barry L. Ornitz     WA4VZQ          ornitz@tricon.net