[evlatests] P-band Dipole Misorientation/Cross-hand Leakage

[Rick Perley]

     The calibrator structure run, taken late in October, included 
copious observations in P-band -- not so much for calibrator structure 
(which is certainly not expected to change very much over time), as for 
for the polarimetry.

     Calibration of the P-band data, including the basic polarimetry, is 
completed.  I'm pleased to report that the strongly polarized hotspots 
in DA240 and 3C303 are largely unresolved in A configuration, meaning we 
have a means of ensuring full polarimetric calibration.  Many practical 
details remain to be worked out however, and the problem of de-rotating 
for the ionospheric RM remains.

     At this band, polarization leakage (coupling of signals between the 
X and Y channels) has two likely origins:  The most obvious is a 
mis-rotation of the dipoles.  The other is a 'leakage' due to the 
electronics.  This ought to be small, as some effort has gone into 
ensuring the signal chains are kept isolated from each other.  A clear 
sign of a rotation is that the leakage amplitude found by the 
calibration program should have equal amplitude, with no frequency 
dependence.  (The angle of rotation is 1/2 the leakage amplitude).

     Examination of the leakage results (using the program PCAL on 
strong unpolarized sources) shows the expectation for dipole rotation is 
met -- in all cases, the X and Y cross-polarization amplitudes are the 
same, and have no slope with amplitude. Additional, the phases are flat 
with frequency as well.

     Below is the list of leakage amplitudes, using ea27 as the 
reference.  This antenna, along with ea02, 03, 06, 11, 15, 19, 24, and 
26 have been found (via numerous observations) to always have low 
cross-coupling with respect to each other.

     Note that, since these observations were taken at the beginning of 
the move, three antennas are missing (having just been  moved to their D 
configuration stations, and one was in the barn):  ea09, 12, 14, and 25.

     Antenna    Cross-Pol %

--------------------------------------

     1                    6

     2                    2

     3                    3

     4                    5

     5                    4

     6                    2

     7                    11  -- a long-term poor performer

     8                    4

     9                    out

     10                   8

     11                    2

     12                out

     13                    6

     14                out

     15                2

     16                6

     17                10

     18                9

     19                3

     20                9

     21                8

     22                8

     23                4

     24                4

     25                out

     26                2

     27                12

     28                ref

----------------------------------------------------

     Anything less than 3% is excellent.  3 -- 5% is 'acceptable. 5 -- 
9% is 'concerning', and anything at or above 10% really needs some 
attention ...

     Rick