[evlatests] Changing P-band Dipole orientations

[Ephraim Ford]

I won't be able to take a look at antenna 1 but perhaps others could.

I overlaid some transparent polar coordinate grids and, for what that is worth based on my crude photos, it confirmed what I see: The worst case was noticeable and you could argue that there is a 5 degree difference. However, I would have estimated EA11 being less than 10 degrees off center. Put another way, I doubt I would have noticed EA23 being misaligned, but I did notice EA11 being misaligned by what appeared to be 5-7 degrees.

From: Rick Perley <rperley at nrao.edu>
Sent: Thursday, September 12, 2024 8:48 AM
To: Ephraim Ford <eford at nrao.edu>; Rob Long <rlong at nrao.edu>; evlatests <evlatests at nrao.edu>
Cc: Clarke, Tracy E CIV USN NRL WASHINGTON DC (USA) <tracy.e.clarke2.civ at us.navy.mil>
Subject: Re: [evlatests] Changing P-band Dipole orientations

Could you tell if the difference between them is about 5 degrees?  Our method for determining misorientations is necessarily based on the reference antenna, which may well be significantly misaligned.  The difference between ea11 and ea23 will be independe of the reference antenna orientation.

The reference antenna used in the calibration was ea01.  Might you be able to take a look at this one?

Rick

PS  I should note here that because it is rather easy to correct for misalignments in the software, the only important reason for wanting a correct alignment is to ensure that our polarization maps are correctly aligned.  In principle, this requires that only one (or preferably a few) antenna have correctly aligned dipoles.   It would of course be good if we users knew in advance which antennas have correctly aligned dipoles ...
________________________________
From: Ephraim Ford <eford at nrao.edu<mailto:eford at nrao.edu>>
Sent: Thursday, September 12, 2024 8:39 AM
To: Rob Long <rlong at nrao.edu<mailto:rlong at nrao.edu>>; Rick Perley <rperley at nrao.edu<mailto:rperley at nrao.edu>>; evlatests <evlatests at nrao.edu<mailto:evlatests at nrao.edu>>
Cc: Clarke, Tracy E CIV USN NRL WASHINGTON DC (USA) <tracy.e.clarke2.civ at us.navy.mil<mailto:tracy.e.clarke2.civ at us.navy.mil>>
Subject: RE: [evlatests] Changing P-band Dipole orientations


I did a quick climb up on EA11 and EA23 since they were close enough to allow a lone-worker visit. Attached are photos of the dipole positions. EA11's 10.3* rotation was clearly more noticeable than EA23's 5.2* rotation.



Ephraim



From: evlatests <evlatests-bounces at listmgr.nrao.edu<mailto:evlatests-bounces at listmgr.nrao.edu>> On Behalf Of Rob Long via evlatests
Sent: Wednesday, September 11, 2024 3:44 PM
To: Rick Perley <rperley at nrao.edu<mailto:rperley at nrao.edu>>; evlatests <evlatests at nrao.edu<mailto:evlatests at nrao.edu>>
Cc: Clarke, Tracy E CIV USN NRL WASHINGTON DC (USA) <tracy.e.clarke2.civ at us.navy.mil<mailto:tracy.e.clarke2.civ at us.navy.mil>>
Subject: Re: [evlatests] Changing P-band Dipole orientations



What Rick mentioned is correct. Unfortunately, it is difficult to keep the dipole aligned "perfectly", dependent on which direction the sub-reflector was last rotating. There are nylon guide blocks at the apex of the antenna trombone assembly which do loosen/wear over time and allow more slop in the dipole orientation. CCW vs CW rotation of the sub-reflector can leave the dipole in slightly different positions. Usually, this is within a couple of degrees.



Rob



On 9/11/2024 10:12 AM, Rick Perley via evlatests wrote:

I noted yesterday some discrepant dipole orientations, and noted that I had the impression these were worse than at earlier times.



By happenstance, I'm calibrating some old (2019) P-band data for Tracy Clarke, which allows me to judge the correctness of that statement.



The statement is wrong.   Misorientations were worse in 2019 than now.



Tracy's data were taken twice - in March and in September of 2019.  The distribution of orientations were in fact worse than now, and also were very different.  Antennas misoriented now were better (or worse) in 2019.  Indeed, the correlation between the two 2019 observations is not very strong.



I believe these conclusions are not surprising, as the dipoles are known to be subject to rotation caused by the rotating subreflector.  Perhaps Rob Long can confirm this statment.



Rick



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