[evlatests] Ant 21

[Rob Long]

The issue with antenna 21 appears to be weak output from the X-band 
receiver.  IF A/C RMS values at the deformatter are running about 1/2 
what they should with all attenuation set to '0'(RF=0 & ch1=0).  IFs B/D 
still have older downconverters (T304s) with slightly higher gain.  The 
higher gain in B/D allows some extra signal strength, however we are 
still running all 4 IFs with a higher noise floor than we should be.  In 
compensating for low receiver output, we are essentially turning up the 
noise in order to get enough power into the digitizer.  We have 
encountered this problem on other antennas and need to come up with a 
permanent fix.

Rob