[evlatests] Strange differential bandpasses

[Rick Perley]

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:  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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