[evlatests] Receiver gains

[Barry Clark]

I have been speculating about whether receiver gains are sufficiently
similar that we could use a standard attenuator setting at the low
bands and thereby avoid getting confused by setting attenuators in the
presence of interference.  The tentative answer is 'no'.

I ran a couple of the standard setupfiles, which use 'SETLEVEL' to
control the attenuators in A and C IFs, and looked at how much
attenuation was put in by the T304s for several antennas.

At L band, the attenuation ranged from 11 dB in ea08-t304-c to 34 dB
in ea09-t304-c.  (The antenna 8 receiver is low in both IFs.  Excluding
ea08, the range is from 20 dB in ea26-t304-c to 34 dB in ea09-t304-c.)

At S band, the situation is a bit better, but only a bit.  The range is
from 16 dB in ea27-t304-c to 31 dB in ea09-t304-c.