[evlatests] EA01 beam cuts -- before and after

[rperley]

To see the effect of 'tilting' (by 0.5 degrees) and translating (by 12 
mm) the subreflector on ea01, I retrieved the beam cuts made on Jan 30, 
to compare to the ones made this month.

The 'executive summary' for the impatient is:

A) Ku and K bands have gone from quite bad to nearly perfect.

B) Ka band has gone from perfect to really bad

C) Q-band is about as bad now as it was before, but in the 'opposite' 
sense.

The only way I know of to explain the discrepancy between the bands is 
that the feeds themselves are not properly positioned.  ea01 is not the 
only antenna with this characteristic -- ea15 and 18 (and probably a 
couple more) also have this problem.  If we want optimum performance at 
high frequencies for these antennas, we must explore how to move the 
feeds laterally -- likely by 10cm or more.

------------------------------------------------------

The details:

The attached files show the story.  Each has two plots -- the first is 
the vertical cut through the beam, the second is the horizontal cut.  It 
is the latter we are concerned with here.

When the Q-band feed was installed on ea01, more than 20 years ago, it 
was found that the beam showed enormous coma lobes in the azimuth axis, 
large enough that referenced pointing failed.  The solution was to 
horizontally displace the subreflector, as far as it could go 'to the 
left'.  The resulting beam cuts are shown in the plot files with 'JAN30' 
in their names.  It will be noted that there is considerable asymmetry 
in the sidelobe ratios for Ku, K and Q bands.  The sense of the ratio 
(higher on the right) indicates that further motion of the subreflector 
'to the left' is needed to balance the sidelobes.  The curious exception 
is Ka-band, which is just about perfect!  The only explanation I know of 
is that the feed itself has been positioned to offset the subreflector's 
incorrect position.

As reported earlier, we cannot move the subreflector any further than it 
has already been moved.  So the solution adopted is to 'tilt' the 
subreflector a small amount, and horizontally translate it.  The result 
of this is shown in the files with 'MAY' in their names.  It will be 
seen that Ku and K bands are now much more symmetric in their horizontal 
cuts.  Q-band is better than before, but the sidelobe ratio is now in 
the opposite sense -- it indicates that we need to move the subreflector 
'to the right' about 8 mm.  Ka-band, as before, stands alone' -- it is 
much worse than before, clearly because the feed itself is in the wrong 
location.

Recommended Actions:

1) The new K and Q band cuts both show a 'negative' sidelobe ratio -- 
the left side is higher than the right.  Both cuts indicate the same 
translation -- about 8 mm to the right, in order to balance the 
sidelobes, and optimize gain.

2) This motion will improve the Ka-band beam, but will still leave it in 
a non-optimum state.  So far as I know, the only recourse is to move the 
feed horizontally.  We need to investigate the feasibility of this.  As 
noted above, there are at least two other antennas with a similar 
problem.

Rick
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