On Sun, 18 Feb 2007 09:21:02 -0600, "Pete KE9OA"
wrote in
:

I understand what you are saying, but the RF amplifier should be conjugate
matched to 50 ohms anyway, in order to have uncondisional stability.


I don't have the schematic for your radio in front of me, but if that
1st RF stage is like most CB radios it's common emitter. So the input
impedance is a lot higher than 50 ohms, and is matched to the antenna
with a transformer or LC network. Not exactly ideal.


I am
not sure what the noise figure of this system is, but it seems that the gain
distribution is such that most of the gain is in the 2nd I.F. strip anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in the
15dB region.......


Are we talking 11m here?


so a receiver with a 12dB noise figure does just fine.
I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
RF gain and relatively low I.F. gain. As soon as you connected an antenna,
it sounded like an FM unit. The problem with that design is that the AGC
voltage was derived from the RF stage with its relatively low selectivity,
in addition to the I.F. strip. Strong off channel signals would capture the
AGC loop and desense the whole system. Remember the old term "bleed over"?
You do have a good point about keeping the RF gain ahead of the mixer as low
as possible, since any gain ahead of the 1st mixer degrades the dynamic
range by that same amount.


The objective is not low gain but low input impedance. Closer to the
impedance of the feed, to keep the first impedance transformation as
small as possible. With a common emitter, the only way to do that is
by reducing the gain. And just at the first RF stage, not necessarily
everything else in front of the first mixer.


In my last contract with Motorola, we were using
mixers that had an IP3 of +40dBm so we were able to get away with having
some gain ahead of that mixer.


Most of the problems I've had with mixers came not from the mixers but
from unbuffered oscillators. Anywayz.....

I guess the question is if the radio works well enough as it sits. If
you can hear a signal buried in the band noise then that's about as
good as it gets. The only way I know to improve it is by matching the
impedance of the first RF to the antenna. Beyond that you'll need to
get a directional antenna.