Topband: LF, MF Preamps, a background de K0FF

[K0FF]

When line loss or noise pickup on the feedline is a significant factor,
locating the preamp at the antenna is the preferred method. For 2M and up
for example where line losses are high, it is imperative to have the preamp
on the mast, even with good coax. Any losses encountered before the preamp
will add to the overall noise figure of the station by a like amount.
On lower frequencies, even 6 meters, there is not much gained in noise
figure because the atmospheric noise is already the limiting factor, so it
becomes less important to locate the preamp remotely.
In my own case, using 7 Beverages for 160 meters, I chose to put the preamp
at the switching hub, and also use a very good double shielded feedline,
even though the performance would probably be exactly the same with the
preamp in the shack.
Doing so does increase the complexity and therefore compromises the
reliability of the system however.
In the case of small antennas and all of those with negative gain ( and a
Beverage is about 20 dB down, SLOOPS even more),  these are used because of
favorable directional or noise pickup characteristics which impact the
signal to noise ratio in a positive manner. Bandpass, Lowpass, and Highpass
filters are also often used to tailor the signal before it reaches the
receiver,  and those too all introduce loss.
A good way to increase the signal without raising the noise floor ( the
ratio is basically already set by the antenna/filters) is a preamp.
I have been experimenting with several designs and testing them for
effectiveness. All of them are 50 (or 75) Ohms in/out and can be placed
anywhere in the feedline before the receiver. A caveat here is that you can
never transmit into any of these receive antennas, and you should make
arrangements that this cannot happen ( assuming you have a transceiver).
Today's Ham rigs are all transceivers now.the last receiver-transmitter pair
went with the Collins S-Line, and are sorely missed by some of us
old-timers. Many modern rigs (FT1000, IC756 etc) have external receive
antenna inputs which are ideal for this application. Others which don't have
this feature can be modified to provide same ( see my mods on the Yaesu
pages for the FT-847 for example).

Back to the preamps: it seems that multi section filters introduce 3 dB or a
little more loss at the desired frequency and of course much more on the
design reject frequencies. This must be made up, as well as increasing the
overall weak signal from the antenna. If too much gain is introduced in the
preamp, several undesirable features emerge, so a balance must be reached. A
compromise gain figure of 10 dB and no more that 13 dB seems in order.

Numerous solid state devices can be utilized to provide that much gain, and
the decision which to use is up to the individual builder, and the cost
constraints if any.

A) The standard arrangement which has been around for years is a J-FET input
followed by a bipolar transistor in as an  emitter follower. Many units
have used this 2 stage setup in conjunction with a simple variable parallel
tuned circuit input as a receiver preamplifier. Very effective and gain can
be varied by appropriate controls. Disadvantages: high parts count, cost,
complexity.

B) Mini-Circuits has a line of MMIC's which offer very broadband response
and fixed gain, and the parts. Typical bandwidth is dc-1000 MHz . The unit
with the ideal gain for our application appears to be the MAR-3 ( orange
dot) and will provide +10 to +13dB  across the range. Very simple
implementation, a bias resistor, and a blocking capacitor at the input and
output. Good overload characteristics too. Disadvantage is higher cost per
device ( but still less than $5.00 USD and this is equalized by the lack of
ancillary parts needed), and they are small in size, making them difficult
for some to work with.

C) Using a VHF transistor as a single stage preamp works well, but heavy
negative feedback needs to be applied to keep the gain down to the desired
levels. Otherwise, transformer coupling input and outputs makes for a very
reliable circuit, but of course ads to the parts count/ cost factor.

As soon as I finish the bench evaluations, I will publish the circuits for
everyone to see, with comments.

PS if anyone has a favorite circuit that he would like to recommend, I would
be glad to try it on the bench, time permitting, or better yet, a sample
could quickly be evaluated and returned. Also open discussion of any of the
above points pre or con are appreciated.....Geo  K0FF


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