[evlatests] PDif Compression on the Moon

[rperley]

A polarization-based test run two weeks ago had us observing the moon at 
L-band.  This source approximately doubles to triples the system 
temperature.  From work done long ago, I expect to see the infamous (and 
still unexplained) 'P-Dif compression'.  This is an artificial drop in 
the observed switched power when the antennas observe a source which 
raises the power by factors of more than ~2.

The recent moon data show that 'PDif Compression' is still with us.  In 
SPW08, virtually every antenna shows the effect, with an artificial 
depression of up to 5%.  Most antennas are 2 -- 3 %.  There is modest 
correlation between RCP and LCP -- if one polarization is more highly 
compressed than the average amongst other antennas, then the other 
polarization is likely to be as well.

Four antennas are worse than the others:  ea14, ea21, ea23 and ea25 -- 
all about 5%.

For some antennas, the effect is barely notable:  ea03, ea07, ea08, 
ea12, ea18, ea20.

One antenna is notably different:  ea22 has 'PDif Expansion' by a large 
factor -- 20%.  But as noted in the earlier email, something else is 
wrong with this antenna.