OBSERVATIONS FROM NORWAY. 010620101730Z.
A SATELLITE PREAMP.
Sometimes, a preamp can be useful - though if you have an effective aerial for
LEO satellites and a half-decent receiver, you don't really need one.
For *REALLY* weak signals like EME they may be a "necessary" part of the station
but for LEO satellite operating ... unless your aerial is about as effective as
a rusty six inch nail with a 50 ohm resistor loading it ... you don't "NEED" a
preamp.
However ... for those that are convinced they *DO* need one - but balk at the
price of an ultra low noise GaAsFET ... which you only need above 2Ghz ...
here's a design just for you!.
It uses a common or garden variety UHF transistor, the veritable BFR91. This
transistor is *NOT* of the ant excrement variety but has legs even a novice can
sizzle without frying the junction. This transistor is good up to the gigahertz
range - or, as GM1SXX is wont to say ... gigahercles. As we all know - or
should! know, a preamp is most effective 'sitting' right behind the aerial (so
as not to amplify excessive cable losses). To that end, this one is built into
an aluminium lunchbox - to keep the rain and insects out.
NB.
Insects are the only things with antennae ... not! radio amateurs ... unless
they're Martians.
The input is a tuned circuit with a prallel tuned air variable capacitor to
allow for tuning for minimum noise.
* Be aware that minimun noise and maximum gain very rarely occur at the same
point.
The output is *NOT* tuned ... to aid stability.
The DC portion is isolated from the RF portion and is bypassed with feedthrough
capacitors.
An RF choke is provided for feeding DC up the coax for relay switched preamps -
but in this one - which used for FO-29 only, there are no relays so the preamp
is attached to the 70cm receiver permanently ... however, a 3 pole two way
switch allows for switching the preamp off and allowing the RF to pass from the
aerial to the receiver without amplification.
The input inductor is a strip of copper 5*53mm in size.
I do this because I find it interesting to see how often a preamp is *not*
needed. Just like power ... (or lack of) ... it's of great interest to me to
see how much *LESS* power is required as the satellite approached TCA - (time of
closest approach).
Most satellite operators don't even bother to reduce power as the satellite
approaches. At TCA, a 3Db reduction of power isn't even noticable ...
THINK ABOUT IT.
It's a complete fallacy to think you need large aerials and a lot of power to
work LEO's. You don't !!! ... however, good operating practice and a little bit
of skill often make the difference between success or failure. Try some
QRP, you might find the challenge absorbing. You *KNOW* it makes sense.
73 John. <la2qaa@amsat.org>
This preamp first appeared in 'The Beginner's Guide To Amateur Radio satellites'
by LA2QAA & GM1SXX