[evlatests] Antenna 8 polarization

[Jim Jackson]

We'll get you a wrench. You know where the receivers are :-)

Jim



At 03:39 PM 4/30/2009, Rick Perley wrote:
>     The 6cm receiver on antenna 8 was correctly re-oriented yesterday
>(having been found to have been mounted 90 degrees away from the
>standard position), and a quick observation of an unpolarized source
>made at noon today.
>     The cross-polarization between antenna 8 and all the other wide-band
>antennas is now around 4%, rather than the 15% that was seen on the
>weekend data.
>
>     But we can do a little more with these data.  Presuming that all the
>observed cross-polarization is due to the receiver (and that none of it
>is due to the antenna), rotating the feed by 90 degrees flips the sign
>of the cross-polarized contribution from that antenna.  So, if on a
>single baseline observed an unpolarized source, the two antennas both
>have their feeds oriented normally, the RL correlator data, after
>calibration for parallel-hand gains, gives an output given by:
>
>RL = (Dr1 + Dl2*)I
>
>where I is the total intensity.
>If antenna 2's receiver is rotated by 90 degrees, the relation becomes
>
>RL = (Dr1 - Dl2*)I
>
>The sum and differences give us the actual D terms amplitude and phase.
>
>I tried this, by hand, on a single baseline, 28 x 8.  I get:
>
>Dl28 = 5.8% at pa = 113.
>Dr8 = 8.6% at pa = -52.
>
>Note that these are fairly high, and that their phases are nearly in
>opposition (meaning, physically, that the R and L antenna ellipticities
>are nearly orthogonal).  This explains why, when in the correct
>orientation the polarization is seen to be low (the vector sum nearly
>cancels the contributions), while when one feed is rotated by 90 degrees
>(which rotates the D-term by 180 degrees), the observed polarization is
>very high (the vector amplitudes very nearly add tip to tail).
>
>This procedure can be done, using the data in hand, to derive the
>absolute cross polarizations for every antenna in the array.  If we want
>to do this for every band, we'll need one antenna's feeds to be
>similarly rotated, for a little while ...
>
>
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