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.

This method has been used in the real world for many years, and it is still
being used. Better ways?


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?

Of course!



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.

As long as we are on that subject, an RF stage isn't even needed at
frequencies below 30MHz. As an example, you can use a Mini-Circuits SRA-3
doubly balanced diode ring mixer, that has only 4.77dB conversion loss at
11M. You also have approximately 35dB of port to port isolation. The only
advantage that an RF amplifier would provide in this situation is minimizing
1st LO radiation through the antenna port of the radio.


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.

Agreed.