[Microwave] Silly Rig Modification and Noise Temperature Ramble/Question

[Dr. Gerald N. Johnson, electrical engineer]

Considering that the thermal noise floor for a 500 Hz bandwidth is
something on the order of -145 dBm, finding receiver sensitivity in the
-136 dBm region makes it a long ways from "state of the art" That 4.5 dB
noise figure you compute was eclipsed by tubes 40 years ago with NF
under 2 dB at 2m. The rice box is DEAF.

Part of the rub of adding all the gain at the antenna is that there is
little selectivity so that any local signal between 100 and 240 mHz is
liable to overload the preamp. High power pagers at 154 MHz and TV plus
local FM are prime causes, not to mention amateur repeaters and packet
or aprs nodes.

The mods for the 726 are trying to change the gain distribution so that
the RF stage, not the mixer controls the noise threshold of the
receiver. The gain distribution is such that the mixer noise or IF stage
noise sets the receiver threshold, not the RF stage. Yet there is some
selectivity in the receiver, far more than the typical preamp input, so
that out of band signals aren't quite as much of a problem.

You probably need to (for dynamic range reasons) to reduce the gain from
your preamp to the point that the system NF isn't hurt, but the gain is
the absolute minimum for the preamp to set the system NF.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.