On 7/30/2014 1:16 AM, Lostgallifreyan wrote:
Jerry Stuckle wrote in
:

The problem with what you're trying to do is you can easily get RF
feedback between the output antenna and the input antenna, causing
oscillations. And those oscillations may not be in the FM band - they
may be on an adjacent band (like aircraft, which is just above the FM
band).


That's my main concern, I want it to be well behaved, or it won't happen. It
has simple 1-pole LC filters to limit it to the FM broadcast band, but I will
leave it intact and not cobble it into another purpose because it's very good
at its original intent.

If I do experiment further, it looks like the best thing to watch out for is
any unexpected oscillation frequencies despite its filters (LPF on input, HPF
on output), and deliberately orienting a radiating dipole to minimise
feedback. Is it easy (or possible) to catch clues of such bad behaviour while
listeing using somethign like a Tecsun PL-390 or other radio using those new
DSP IC's? I ask that because they're cheap, easy to get, and fairly
consistent, and include a usefully specified signal strength meter. If so,
what should I consider to be a warnign sign?

But the amplifier you're trying to use is meant to feed a receiver
directly, not another antenna. So output is going to be very low (on
the order of microwatts) - much lower than any amplifier which feeds an
antenna.


Agreed.
I only tried because the last thing I want to do is pollute the spectrum.
Start small...


1-pole LC filters won't be narrow enough to limit potential oscillations
to the FM band. You'll need much more than that.

As for determining whether it is oscillating or not - I wouldn't trust
anything short of a good spectrum analyzer. The signal could be
anywhere (and changing frequency). A spectrum analyzer will still show
it; a receiver won't necessarily.

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Jerry, AI0K

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