[evlatests] Effects of T304 attenuator settings on bandpass shapes

[Michael Rupen]

The "0,0" levels for the three antennas in this test were as follows:

             Input attn     Output attn
      ea01        7             18
      ea18        9             14
      ea25       12             15

Rick, would you send around a PDF/PS of your BP plots?  I'd like to see
whether the 7->8 and 15->16 changes had any greater effect, as Jim (?)
thought they might.

                 Michael

>     Earlier WIDAR tests have shown that changing the T304's attenuator
> levels causes changes in the bandpass amplitude shape by ~1%.  To
> determine which attenuator is responsible, whether particular settings
> have a more dramatic effect, and to otherwise illuminate the
> characteristics of the problem, Ken and Michael ran a specific
> experiment.   In this, they stepped through five different input and
> five output attenuator settings, spending one minute on each pair.  This
> was done on three antennas (1, 18, 25), the others were fixed.  The
> observations were of 3C84 in one continuous observation.  The attenuator
> pairs which were sampled were (in order):  (0,0), (-2,0), (-1,0), (0,0),
> (1,0), (2,0), (0,0), (0,-2), (0,-1), (0,0), (0,1), (0,2), (0,0).  All
> values are in dB.
>
>    The data were calibrated and the bandpasses determined for each
> antenna, for each of the paired settings listed above.  The AIPS program
> BPLOT was used to show the *differential* results.  (This means that the
> average of all bandpasses is made and subtracted.  The results displayed
> are the fractional deviations from the average).
>
>    Results:  Not as simple as had been hoped.
>
>    1)  The three antennas which had the attenuators changed clearly
> show larger changes in their bandpass shapes.  The maximum deviations
> are ~2.5%.   No commonality nor trends are seen in the bandpass changes.
>    2) The seven antennas with fixed attenuators showed much smaller
> deviations, much less than 1% for nearly all.   Changes that are seen
> are gradual, indicating some physical change over time.  Antenna 19 is
> by far the worst amongst these, with a 1.5% change over 13 minutes.  The
> amplitude pattern has a ~100 MHz periodicity, indicating a standing wave
> set up over ~1.5 meter length.
>    3).  Two of the changing antennas -- 18 and 25 -- showed by far the
> biggest deviations (>2%) with the setting (0,1) and (0,2) -- the output
> attenuator seems to have the biggest effect.   This is also true for
> antenna 1, but the amplitude of the bandpass change for this antenna is
> very much less (~1%).
>    4) For the three antennas with attenuators that changed, the four
> (0,0) settings all showed different bandpasses.  These changes are
> typical for the temporal variation of the seven antennas with unchanging
> attenuators.
>
>    It thus appears likely that we have two simultaneous effects
> confusing the issue:  A temporal change, operating over timescales of
> tens of minutes, and an attenuator-depending effect.  Both are of
> similar magnitudes -- up to 2% in bandpass amplitude.
>
>    All the above applies to the amplitudes only.  I'll determine the
> phase effects after running FRING to remove residual delay effects (this
> should keep my computer busy over the lunch hour ...)
>
>
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