>
> Not trying to be a smart A$$,
> but why would you put a preamp in line and then an attenuator?
To prevent receiver overload.
If the noise level received by the antenna is sufficiently high (i.e. higher
than the natural thermal noise floor of the receiver/preamp), an attenuator
ahead of the preamp will lower the risk of overload from strong off-channel
signals *without* degrading the S/N performance of the system. Any
attenuation ahead of the preamp adds directly to the noise figure of the
system, so you want to keep the combined noise figure of the
attenuator+preamp lower than that of the receiver would otherwise have
without them if any improvement is to be realized. Or in other words, if
the amount of attenuation inserted results in a noise figure that is too
high, the received signal will end up having a lower S/N than it would at
lower attenuation values.
An attenuator after the preamp has its place too, and is often a better
place to put it if the preamplifier has excessive gain, the background noise
(as received by the antenna) is naturally low, and/or if there aren't any
strong off-channel signals to contend with.
In some cases, the best scenario is attenuation both before and after the
preamp. The value of the attenuator before the preamp is chosen based on
the ambient noise floor, and the one after based on how much gain is really
necessary to realize any S/N improvement (i.e. to negate excessive preamp
gain). Maximizing both of those to point where S/N just starts to degrade
would give you the best overload protection.
Noise levels will vary at a given site depending on what other emitters are
keyed up, weather-related effects, "unintentional radiators" generating RFI
perodically, etc., will all affect the noise floor over time. Bench tests
for receiver performance with a high-gain, low-NF preamp don't give a good
indication of how the system will perform when hooked up to an antenna.
Selectivity (e.g. pass cavities) ahead of the preamp is almost always
preferable to after it unless you're blessed with being at a site with a
very low noise floor, no other strong off-channel signals to contend with,
and sufficient Tx/Rx isolation to prevent overloading the preamp. Also,
some preamps that aren't unconditionally stable may oscillate or act
squirrelly with a high-Q filter after them.
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
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