[evlatests] Strange differential bandpasses

[Vivek Dhawan]

Wondering if this is the 4-band system at the edge of the
subreflector-main reflector converging rays. Surely someone
did a bandpass before and after 4-band was installed?

On Thu, August 26, 2021 15:41, Rick Perley via evlatests wrote:
| One of our DAs, Edward Starr, has shown me some strange bandpass
| effects, whereby properly calibrated data, after application of
| calibration, shown undulating sinusoids with frequency.  There appear to
| be (at least) four separate observations affected by this, all taken in
| July or August.
|
| Only one of these is accessible to AIPS at the present time.  I loaded
| this one (21A-033, S-band, observed 23 July 2021) into AIPS to investigate.
|
| What I found is most peculiar:Â  The bandpasses of nearly all antennas
| systematically change over the course of the observation (13:30 to 17:35
| IAT).  The effect is very easily seen if one uses the initial scan of
| 3C48 to calibrate the bandpasses, then generate 'differential' bandpass
| solutions on the subsequent data (following normal calibration).  By the
| end of the run, the observation of 3C84 shows bandpass ripples up to
| 2.5% in amplitude, and 2 degrees in phase.
|
| The character of the change in bandpass is exactly as Edward originally
| described -- a nearly perfect sinusoid, which steadily grows in
| amplitude over time.  The period is very close to 43 MHz, which
| corresponds to 3.5 meters in free space, if due to a standing wave.  The
| characteristics shown in the plot make it virtually certain that this is
| a beat phenomenon due to a reflected signal.
|
| I have attached an example -- the most spectacular antenna is ea05. 
| This is the differential bandpass, from 3C84, observed at the end of the
| run, using the bandpass from 3C48, taken at the beginning.
|
| All antennas look like this, but none have amplitudes as large as this
| one.  The 'beat' pattern is the same in all antennas on which the effect
| can be discerned.  Other antennas which have the largest effect are
| ea08, 12, 20, and 26.  There is no spatial relationship of these -- they
| are evenly spread about the array. This is not an elevation effect, as
| the growth of the pattern in the phase calibrator (J0204+1514) neatly
| fits the final observation of 3C84.  The last observation of J0204+1514
| was taken at an elevation of 32, the subsequent observation of 3C84 was
| taken at elevation 54.  The original calibration, on 3C48, was at
| elevation 79.
|
| The two polarizations give identical results -- both in amplitude and
| phase, including the frequency location of the peaks and troughs.
|
| I've seen sinusoids before, but nothing quite like this case.
|
| Ideas?
|
|
|
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