[evlatests] Update on Strange R-L phase behavior

[Sanjay Bhatnagar]

I have been thinking about this issue from geometric/optics point of 
view, and was waiting for the investigations for the source being in the 
electronics/temp. variations etc. to settle down.

"over the top" tests would be quite useful, I agree.  From what I can 
make out, none of the on-sky tests so far included correction for 
antenna pointing offsets.  If so, I suggest that in the next set of 
tests we include pointing correction.

With antenna pointing offsets, which can be significant, celestial 
sources trace a curve about the nominal optical axis. Such a trace (by 
hand -- so not accurate but good enough) on antenna R- and L-voltage 
patterns from holographic measurements show a R-L phase difference of 
2-3 deg. which is in the ball-park of Rick's measurement.  Examination 
of the Rick's plot with and without correcting for pointing offsets will 
help determine if intrinsic R-L phase in the antenna aperture 
illumination pattern could be contributing (partly or entirely) to 
Rick's R-L phase plots.

Regards,

sanjay

On 3/30/22 10:26 AM, Rick Perley via evlatests wrote:
>
>
> Barry has opined for an antenna-based problem (something within the 
> electronics which is strongly elevation-sensitive).  But, in an 
> experiment run by Paul two days ago, no elevation-dependency on the 
> 'auto-cross' phase was seen.  (This monitors the phase difference in 
> the injected noise-diode signal -- and so is not an astronomical 
> observation).  Arguments based on a temperature effect in my data are 
> hard to sustain, as the outside temperatures on the night of my 
> observations were  exceptionally uniform throughout the period -- and 
> it was quite breeezy as well.   These results argue for an origin 
> preceding the injection of the noise diode signal.
>
> So -- what to do next to isolate the cause(s)?
>
> I'd like to try the 'over-the-top' observation.  If the effect is 
> truly due to an elevation-dependent effect within the antenna, then it 
> should continue to increase as the antenna is tilted 'over backwards' 
> -- the antenna elevation is then greater than 90 degrees.  This should 
> cleanly separate effects due to elevation from those due to HA or 
> parallactic angle.  Observing OTT also reverses the orientation of the 
> R and L 'squint' beams, so should be definitive in eliminating that 
> origin.
>
> I suggest we do this with sources which transit both to the north, and 
> to the south of the zenith.  All my current examples are from sources 
> which transit on the south side.
>
> Doing this with the current 'A' configuration might also illuminate 
> any dependencies on antenna placement -- despite all our antennas 
> nominally having parallel azimuth axes, sources will transit at 
> slightly different times.   I don't think this is an issue -- but who 
> knows?  We might be surprised ...
>
> This is a fair investment of time -- a few hours.  But I think we need 
> to do something like this to make any progress in isolating the 
> origin(s).
>
> Rick
>
>
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