[evlatests] R-L Phase Differences -- a 'simple' explanation

[rperley]

There is a simple explanation for the R-L phase differentials -- a 
differential tilt between the two antennas.  If the two antennas' poles 
point in slightly different directions, the parallactic angles seen by 
each are different, which results in a different measured phase for the 
two polarizations.

For the 'RR' correlation, the phase difference is -(delta par angle), 
for the 'LL' correlation, it is +(delta par angle), so the R-L 
difference is twice the difference is parallactic angle between the two 
antennas.

This effect is independent of band -- it is purely geometrical.

To show how these differentials vary with source declination, I 
generated plots for the four sources observed:  3C286 (dec = 30.5), 
OQ208 (dec = 28.5), 3C287 (dec = 25.2), and 3C273 (dec = 2.1).  I 
generated curves for a E-W tilt, and a N-S tilt.

Attached is the resulting plot, showing the predicted R-L phase in 
degrees as a function of Hour Angle, for the four sources.  Solid lines 
are for an E-W tilt, Dashed lines for N-S tilt.

The match to the observed data is extremely good.  (To be fair, the 
match to the largest of the 'even' tilts is extremely good).  But I bet 
that a suitable combination of E-W and N-S tilts would give a good fit 
to almost all of the data.

There is only one problem -- the amplitude of the tilt required to give 
the size of the observed phase is about 5 times larger than the largest 
measured tilt.  The plots were generated with a tilt differential 
(between the two antennas) of 6 arcminutes.

So, if this model has any relevance, it begs the question:  How do we 
measure the antenna tilts?  Are these tilts different than those used in 
the model?

Rick
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