[evlatests] Strange differential bandpasses

[Frazer Owen]

Bandpasses were checked for the first few antennas which had the 4band 
dipoles installed and no impact of the dipoles was observed. Of course, 
the dipoles are not at the edge of the subreflector but are suspended 
from the feedlegs well out of the geometric optical path.

---Frazer

On 8/26/21 8:25 PM, Vivek Dhawan via evlatests wrote:
> 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.Â
> | 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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