Here's a seldom-talked about issue: "effective sensitivity" . The receiver
sensitivity you get in the shop on a 50-ohm dummy load (say, 0.3 microvolts) is
the sensitivity as limited _only_ by the receiver's own "self noise" specified
by it's noise figure--that's mostly strongly dependent on the noise performance
of the first active device in the rx, the first RF amp or the mixer in the
absence of an RF amp.
Now you get to a repeater site that has, say, 2 microvolts of steady noise
coming in from the antenna (not unusual). Where's that 0.3 microvolts of
signal which quieted the rx in the shop? It's buried in the 2 uV of noise &
unreadable, nay, completely unnoticeable.
What then is the actual, effective sensitivity of the rx in that environment?
Certainly NOT the 0.3 uV found in the shop! It will be somewhat above 2 uV, as
this is noise level which sets the "starting" point of the quieting curve of an
FM rx, not the receiver's own self noise.
In this case it's antenna noise, not the rx's self noise, which sets the actual
sensitivity in use.
We need to remember that in a _system_ (antenna (galactic or other noise),
feedline (losses), RF selectivity (losses), & rx (self noise)) the receiver's
intrinsic noise level ("noise figure") is further degraded by feedline loss and
any further losses due to RF selectivity in front of the receiver, e.g.
duplexer/cavity filter, etc.
That's why I think it's important to be able to routinely monitor the noise
level at the site (see my previous note on S-meters).
--John
--John
