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Re: Noise, Bandwidth and Gain



>Any attempt to monitor AO-40 or any other signal at S-Band has been
>fruitless.  However, I recently added a cavity filter cut for 2.4 MHz
>between the Conifer amp and the antenna and WOW!  Signals have appeared from
>nowhere.

Better still would be to have the signal go through a narrowband 2.4 GHz
cavity, *then* a GaAsFET preamp, *then* a downconverter with an IF output
that's easier for humanly affordable feedline to carry, at the antenna feed
or very close to it.  The preamp will be saturated to some extent by strong
signals in or near its passband, so if you put the cavity upstream of the
preamp, you will notice a dramatic drop in the noise floor.  2.4 GHz
signals are attenuated *severely* by any significant length of normal
coaxial cable, too, so if you convert the signal down to 144 MHz or
something similar, you will notice a major increase in efficiency.

>Second, the term "Preamplifier" is a misnomer.  While one might think that a
>PreAmp is an outboard stage of added amplification, from my perspective
>PreAmplification is the sum total of all active stages and associated gain
>prior to signal detection.  The amount of gain or the link budget in any one
>situation or at any one time with a fixed station changes dramatically.
>Were it not for receiver dynamic range, this whole process of Ham Radio
>would be nearly impossible.

Absolutely, and adjusting the signal to a level you can use and filtering
out as much of what you don't want as possible is part of what makes the
difference between a good receiving system and a fantastic one.  Wide
dynamic range helps, but ultimately you need to control the signals you
feed into every stage, and for 2.4 GHz and higher, this starts right at the
antenna.

>Third, an attenuator is just as valuable as an active amplifier.  Both have
>their place and both can be used together to achieve the ultimate goal of
>signal reception and the act of decoding information.

Again, absolutely right.  Sometimes a signal will be too strong to decode
due to harmonic distortion, even with the preamplifier turned off, and
reducing the amplitude so the signal is linear and not clipped will make it
possible to decode it.

Sounds like you're enjoying the learning process!  ;-)


"Orthodoxy is orthodoxy because it won, not because it is true." -- Bp.
John Spong
 "Power takes as ingratitude the writhing of its victims." -- Rabindranath
Tagore
               --... ...-- -.. . -. ..... ...- -...
         Bruce Bostwick  mailto:lihan@ccwf.cc.utexas.edu
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