[evlatests] Origin of Antenna Polarization

[Rick Perley]

We have long believed the majority of the observed (on-axis) antenna 
polarization is due to the receiver.   Evidence for this comes from EVLA 
Memo 170, where Bob Sault and I compared the absolute antenna 
polarization results derived from two different methods -- the 'receiver 
rotation' method, and the '3C286 rotation' method.  The former 'sees' 
only that part of the antenna polarization from the feeds onwards, the 
latter sees the entire signal chain, including the antenna itself.

We found excellent agreement between the two methods, indicating that 
indeed the bulk of the observed antenna polarization is due to the 
receivers.

Here I report on a different comparison -- comparing the 'relative' 
antenna polarizations between the 3-bit and 8-bit systems.  The data 
were taken at the same time, at the same frequencies, and were 
calibrated identically, with the same 'reference' antenna.  Any 
differences in polarization should be due to the different signal paths 
through the T304.

Attached is a plot showing the results for ea15 -- typical for all 
antennas.  The yellow trace is from the 8-bit path, the green trace is 
from the 3-bit path.  Red colors indicate identical results (hence the 
axis labelling is completely red).  Note the yaxis is in tenths of 
percent -- '100' means 10%.

The upper panel shows phase in the narrow window, and amplitude in the 
wider window for RCP.  The lower panel shows the same in LCP.

There is broad similarity in the traces, but also remarkable differences 
of up to 4% in amplitude, and perhaps 20 degrees in phase.  Presumably, 
different reflections within the T304's signal paths for 3 and 8 bit are 
responsible for the polarization differences.

Note also the very high cross-polarization -- about 12% at the lower 
frequencies.

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