[evlatests] L band focus

[rperley at nrao.edu]

I checked for correlations between the antennas listed below, and my
sensitivity values derived last week -- there is good correlation for
some, but unfortunately, I don't have all the data here on my laptop (as
I'm in Vancouver now) -- especially the high frequency data, were the
effects are most pronounced.

Check EVLA Memo 109, figure 9, to see the effect of focus error on L-band
efficiency.  At the high frequency end, a 7cm error will drop the
efficiency from 45 to about 32%.  A 5cm error drops it from 45 to 38% --
significant losses, indeed!  The effects at the low end of the band are
much more modest -- a couple percent at most.

The above-mentioned figure also shows very clearly that the focus should
be set for a frequency of about 1800 MHz (unless we're going to try a
variable focus, depending on the requested frequency).

It would be interesting and useful if somebody looked at the variation of
the correlation coefficient as a function of focus setting and frequency,
to see if the relations shown in Memo109 are confirmed (they should be!)

Rick (currently working at 'NRAO-FarWest')



> I wrote a script to determine optimal focus at L band.  This
> was done at the default observing frequencies around 1400 MHz.
> Most antennas were found to have significant focus offsets
> and all, except 17 which was out of service, have been adjusted.
> ea26 was the worst of the lof, off by about 1/3 of a wavelength.
> This should noticably improve Rick's efficiency estimate for
> many of the antennas.
>
> Further work is to to this at the band edges.
>
> Ant   New    Old   Diff
> 11   -2.5   -1.5   -1.0
> 13   -4.0   -3.35  -0.65
> 14   -6.0   -3.0   -3.0
> 16   -9.0   -8.0   -1.0
> 17   ----
> 18   -3.0   -3.0    0.0
> 19   -5.0    0.0   -5.0
> 21    0.0    5.0   -5.0
> 23    0.0    0.0    0.0
> 24   -5.0   -3.0   -2.0
> 25   -3.0    2.0   -5.0
> 26   -7.0    0.0   -7.0
>
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