[evlatests] R/L phase differences.

[Claire Chandler]

Hi Vivek,

Don't forget that the masers in Orion are polarized and have complicated 
structure, which could account for at least some of the time-variable R-L 
phases.

Claire

On Mon, 18 May 2009, Vivek Dhawan wrote:

>             1. Summary of previous tests:
>
> 3MHz continuum, IF1 = IF2 in frequency, source= Orion maser.
>
> Q and K band showed short-term changes in (RR-LL) phase of ~5 deg.
>
> Apart from the polarizer D-terms, such effects may be caused by cross-
> talk between various paths in the RF chain (unlikely, since these are
> well padded with isolators); or in the LO output fanout of the L301 or
> L302 (neither of which are as well isolated).
>
> Similar variations are seen in the (IF1-IF2) phases, K & Q. This is
> (R1-R2) phase, so the D-terms cancel exactly as IF1=IF2 in frequency,
> implicating the LO fanout path.
>
> Most antennas show something, not just a few bad apples. Looking at
> VLA vs. EVLA differences, a puzzle (or clue?) emerges:
> VLA antennas are just as bad as the EVLA at K band. But
> VLA antennas are good (in the noise, <1deg) at Q band.
>
>
>
>            2. New tests:  6.25MHz continuum, 3C84, L, C, X bands.
>
> L,C,X all show the same variations as above, at about 1-2deg. VLA ants
> are a bit better, but the SNR is getting low, I can't really tell.
>
> There are a couple really bad outliers, at all bands. Possibly, the
> T304's are old models or something like that: EA16 IF D; EA17 IF B.
> Perhaps also EA14 B,D (intermittent?)
>
>
>
>            3. Next step: in consultation with Hayward & Jackson,
>               the plan is, in rough sequence,
>
> A. Do Ka band for completeness. (Test scheduled for tomorrow)
>
> B. Put in isolators on the L302 outputs of a select few antennas, see
>    if that cleans up X band. I pick EA24 and one (or more) of EA1,2,3,4.
>    as hardware permits. Most useful if both L302's are done on the same
>    antenna. Isolators can be left in place for normal use.
>
> C. Put in isolators after the L301 power splitter at Q band. Also can
>    be left in for normal use. See if this cleans up Q band. Same antennas
>    as in A, to ensure good L301/T304 isolation.
>
> D. More invasive diagnostics - Feed same RF input to several T304's to
>    narrow down the location of trouble, e.g. cables vs. T304's. This must
>    be undone for normal astronomy.
>
> Vivek.
>
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