[evlatests] L-band receiver output levels

[Keith Morris]

Brent Willoughby and I visited antennas 5, 6, and 15 -- three of the six
"most compressed" antennas from Rick's Cygnus pdiff test.  Of the
remaining three, one is in the barn, one was not yet recovered from the
scheduled power outages, and one I intend to keep the way it is, as a
control, for further testing.

We installed 10dB attenuators at the outputs of the room temperature
section of the L-band receivers on the three antennas.  For two of the
antennas, 6 and 15, this brought the power, as measured at the T302
output/T304 to around -40dB, which we believe is within an acceptable
tolerance of the specified value.  The third antenna, ea05, required
additional attenuation (6dB) between the T302 and the T304.  This may be
due to a difference in T302 build.

We should re-run the Cygnus pdiff test, or at least a subset of the test,
to determine whether these changes affect the measured compression ratios.