> If you really want to isolate the preamp from the duplexer,
> you need to put
> an isolator ahead of it.
>
> Bob NO6B
>From a matching standpoint, yeah, but now the NF goes up due to the added
losses of the isolator. You might come out ahead by modifying/retuning the
preamp's input network for a better input match even if means sacrificing a
few tenths of a dB of NF.
Ponder this Bob (and anyone else). Assume the preamp input is connected
directly to the output port of the isolator. For the sake of argument, say
that the preamp has a NF of 0.5 dB, and 6 dB input return loss. The power
transfer from isolator output to preamp input would be 75% for 6 dB RL; the
other 25% of reflected power was lost/dissipated in the reject load.
Wouldn't it be mathematically correct to say that there was effectively 1.25
dB of additional loss ahead of the gain stage, and therefore the noise
figure would likewise then be 1.25 dB worse (i.e. 1.75 dB)? Yes? No?
Maybe?
If the answer is a yes, couldn't the same argument be made when using a
preamp connected directly to an antenna+feedline as in a weak-signal
installation? Yeah, I know, the reflected power wouldn't be dissipated the
same way as it would be in the isolator case above, but you would still have
VSWR on the feedline, imperfect power transfer from antenna to feedline, and
imperfect power transfer from feedline to preamp. So, is it really right to
say a preamp has 0.4 dB of noise figure when it's input Z is so far out of
whack that you could never realize that kind of performance in a 50 ohm
world? Or are we supposed to tune our antenna feedpoints to match our
preamps? :-)
--- Jeff
