[evlatests] Strange doings with EA13 at L-band

[Rick Perley]

     On August 13, we observed Cygnus A at L-band and P-band.  Two 
issues were uncovered.  The first (ea28 in RCP) has already been 
reported, and a likely cause identified.  This note is about the other 
-- much more mysterious -- effect.

     Antenna 13 -- and only this antenna -- demonstrated very strange 
changes in both power and correlated visibilities, in all spectral 
windows.  The behaviors in each SPW are grossly similar, but quite 
different in detail.  The bad behavior lasted about three hours, more or 
less exactly centered on meridian transit. Similar effects were seen on 
the calibrator in target source.

     The overall characateristics are these:

     1) The total power increased.

     2) The swtiched power was absolutely stable.

     3) The visibility amplitude declined.

     4) The bandpass shapes changed dramatically.

     5) Each spectral window showed different behavior, but the two 
polarizations in a SPW were similar (although not identical).

     Attached are some plots to illustrate the points:

A)  EA03-SPW07-SWPower.png

       Showing the switched power from ea03 -- the antenna adjacent to 
ea13.  Top to bottom:  PDif in RCP, PSum in RCP, PDif in LCP, PSum in 
LCP.   The color reflects the source:  Blue is the nearby calibrator, 
green is Cygnus A.  Shown in SPW 07 -- 1415 MHz.

     This is what we expect from a stable system:  The PDif panels (top, 
and third from the top) are stable.  The high noise for Cygnus A is a 
result of the increased power from Cygnua A (a factor of 4 above cold 
sky).  The 2nd and 4th panels show the total power.

     All antennas look like this *except* for ea13.

B) EA13-SPW07-SWPower.png

     The same as above, but for ea13.  Note the switched power remains 
beautifully stable.  This shows the receiver itself is not to blame for 
what follows.  The total power plots (2nd and 4th panels) show the power 
smoothly rising, then rapidly dropping to the level seen at the 
beginning (and end) of the run.  The pattern is symmetric about 
transit!  Note also that the calibrator powers (in blue) are also 
strongly perturbed.

  C) EA13-SPW08-SwchdPower.png

     The same as above, but for the next spectral window, SPW08. Note 
that the perturbations in power are much less than SPW07.

D) EA13-SPW7+8AmpGains.png

     This shows the calibrator gains for SPWs 7 and 8.  A rise means the 
visibility flux is too low.  Comparing this to the switched power plots 
shows that the visibility amplitudes are dropping more-or-less inversely 
to the rise in total power.  Yet the gains themselves (as shown by the 
PDif plots) are unchanged! This could be a saturation effect, but the 
rise in the power is no where near large enough to do this... Note that 
SPW7 (IF7) and SPW8 (IF8) have gain variations in opposite directions at 
Time = 05 hours).  SPW07 goes up and down on subsequent scanes, while 
SPW08 is reversed.

E) EA03-SPW+8AmpGains.png

     Same as above, but for the adjacent antenna.  The gains are 
beautifully stable!!!  (And the same is seen on the antenna on the other 
side of Ea13).

F) EA13-BPVar.png

     Showing the *differential* bandpasses (i.e., changes from the mean 
bandpass, averaged over the entire run), for ea13.  The (color-coded) 
variations are large (+/- 15%),, and very different between the two 
IFs.  They are similar in shape between polarization, but different in 
detail.

G) EA03-BPVar.png

     Same as above, but for the adjacent antenna, EA03. Variations here 
are minuscule -- 1%, and entirely due to noise.

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

     So what is going on?  The only rational explanation I can think of 
is a receiver saturation effect.  But the saturation must be 
out-of-band, since I can't see it in the spectra of any band.

     But, if a saturation effect, why is it nearly perfectly timed for 
meridian transit, and why is *only* ea13 (at W56) affected?

     All suggestions -- no matter how crazy, will be considered ...

     Rick


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