System Gain

[Dave Sublette <arrl.net; k4to@xxxxxxxxxxxxxxxx>]

Good morning all,

My education concerning the art of microwave systems is continuing. In 
reading through some reference material I ran across a statement that 
said when the voltage is removed from the first amplifier stage of a 
transverter, the system noise should drop 15 dB.

My microwave transverters for 2,3,5,and 10 ghz are mounted remotely. I 
cannot turn off the power to just the preamps. So I am trying to devise 
an equivalent technique for measuring the system gain.

My first approach is to just unplug the IF cable coming from the box. I 
am using an SDR-14 with the Linrad program. The 144mhz IF comes from the 
transverters and is divided in a 3dB hybrid. One output goes to a 1.2 dB 
nf, 20dB gain LNA, then through a 144 mhz bandpass filter, then to the 
SDR-14 input. The other output of the hybrid goes to the 144mhz input of 
my K2 IF radio.

I am using the SDR-14 with the Linrad program to measure signal levels 
and background noise.

When I unplug the IF cable, I see little or no drop in system noise on 
the Linrad screen. I think I should be seeing 15 dB drop if I read and 
understood my statement in paragraph one.

So....the question is...is my test technique valid? If so, it would seem 
that I need an IF post amp in the output of all of my microwave 
transverters. Fortunately the DEMI design will accommodate this feature.

Your comments are invited.

Thanks, 73, and Merry Christmas everyone.

Dave, K4TO

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