On Sat, 24 Feb 2007 18:12:49 -0600, "Pete KE9OA"
wrote in
:

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?


Several.

Long story short, the power-to-voltage ratio of a signal is always
higher than the power-to-voltage ratio of noise. Most RF front ends
are voltage amps. But a -power- amp on the left can dig the signal out
of the noise on the order of 2-4dB, sometimes more. I like using a
common-base for the 1st RF, but you can re-bias a common emitter and
make pretty good improvements. And, as I stated before, a low input
impedance will reduce or eliminate the impedance transformation prior
to amplification.


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.


You can do better with discretes from Radio Shaft, which is really sad
when you realize that those are their lab numbers. The only advantage
I've seen to Mini-Circuits is the size. For performance, their stuff
sucks.


The only
advantage that an RF amplifier would provide in this situation is minimizing
1st LO radiation through the antenna port of the radio.


It also serves as a buffer to the mixer, which is essential for
reducing mixer IMD. The RF amp is generally a good idea.