In article ,
"Pete KE9OA" wrote:

Snip

As an example, suppose a receiver has an IP3 of +30dBm. Now, if a
preamplifier with 10dB of gain is placed ahead of the 1st mixer the
IP3 will be degraded to +20dBm. Typically, excess noise contributed
by the environment is in the 15 to 20dB range. Many shortwave
receivers have a noise figure in the 10 to 12dB range. The excess
environmental noise can be between 3 to 5dB above any noise
contribution that the receiver has itself. For this reason, most
modern receivers don't use an RF amplifier ahead of the 1st mixer.

Snip

Most of the better modern receivers have a switchable RF amplifier
because the background noise is higher at lower frequencies and as you
noted can reduce the intermodulation performance so with adjustable or
switchable RF gain you can use it at higher frequencies or actually at
any frequency as conditions permit.

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Telamon
Ventura, California