[evlatests] Characteristics from L-band, using 30 hour run

[Robert Hayward]

Rick,

It would be interesting to compare the diurnal effect you see in the
Switched Power between new EVLA style receivers (where the noise diode and
post-amps are located inside its new "RF Box") and the Interim-style
receivers (where the noise diode and post-amps are mounted on a printed
circuit board inside the old Card Cage).

As for changes in the attenuator settings, how much of that can we blame
on the severe RFI environment at L-Band? Since the Total Power Detectors
are seeing the full 1 GHz bandwidth, depending on sidelobe levels and
such, it is not unrealistic that the T304 Downconverters on some antennas
might want to twiddle their attenuators more than on others. This
situation would likely be the case more often at L-Band than at other
receiver frequency bands where less RFI is present.

-Bob


> Reduction of the 30-hour flux densities run for the L-band data has
> revealed a few interesting issues:
>
>     1) Switched Power.  As noted in earlier reports, the script had to
> be aborted about 9 hours into the run.  Upon return (40 minutes later),
> many of the receivers at many bands changed attenuator setting,
> typically by 1 dB (about 25% in power).  This is not too surprising.
> But at L-band, the change in attenuator was often greater than at any
> other band -- often by 3dB or more!.  This is unreasonably (even
> ridiculously!) too much.  Unless one of the two settings at which the
> power was measured was at an elevation of less than 10 degrees, it's
> hard to see how the actual power of the L-band system can be so
> different.   The change inserted must be erroneous -- the total power
> also changed (so the attenuator change was not to compensate for some
> remarkable change in system temperature), and in some cases, one
> polarization changed by 3 or 4 dB, while the other polarization didn't
> change at all.
>     Only a very few antennas had no change in power at this moment, and
> for these, the diurnal change in power noted at other bands was seen.
> However, the amount of change varies remarkably widely:  A list showing
> the ratio of maximum to minimum PDif, for those antennas in which no
> jump was seen:
>     1LCP   1.5%
>     3LCP   ~1%
>     6LCP   3%
>     7LCP   5%
>     11LCP   2%
>     13 LCP and RCP:  No diurnal effect noted, but rather a steady
> decline of 12%
>     21LCP   8%
>     22LCP   <2%
>     25LCP   4%
>     Note that only LCP was jump-free.  Every antenna (except 13) showed
> a power jump in RCP.  Something fooled the system ...
>
>     Application of the PDif did not provide us with <1% amplitude gain
> stability, however (unlike S, C, and X bands).  Residual gains of 2 --
> 3% (in amplitude, so ~5% in power) were seen on many antennas.  This is
> likely due to errors in the PDif measurements, but I have not yet proven
> this is the case.
>     As noted earlier, we have some evidence that the SY tables are not
> correctly labelled,, as subband 4, which contains the strong GPS signal
> provides beautifully smooth power measurements.  Some effort to prove
> this assertion is needed.
>
>     2) Bandpass.  L-band is infamous for its interference.  But this did
> not prevent excellent BP solutions from being generated, using the ~30
> observations of the four key calibrators used in this project.   Only
> the satellite downlink subbands (1520 through 1620 MHz) failed to get a
> good solution!
>
>     3) A fit for elevation gain dependence was made -- no antenna showed
> a dependence greater than ~1% over the range of elevations employed (18
> through 80 degrees).
>
>     4)  R-L phase:  Good polarimetry *requires* at least one antenna
> with stable R-L phase (unless you like doing painful transfer of the
> phase difference from a strongly linearly polarized source).  Plots of
> R-L phase show a distressing variance amongst the antennas:
>     a) many antennas showed a change after the script abort mentioned
> above.  This is bad for those who would like to utilize solutions from
> other observing sessions ...
>     b) many antennas showed significant changes in differential phase
> for different scans.  Notable are:  ea02:  25 degrees, ea13:  6 degrees,
> ea20:  15 degrees (single scan only), ea21: 10 degrees, ea23: 25
> degrees, ea24: 15 degrees.
>
>     5) Polarimetry results:  These are quite stunningly excellent!  I
> have now the run of polarized flux and position angle for 3C48, 3C138,
> and 3C286 (assuming the P.A. of 3C286 is fixed at 33 degrees).  (TECOR
> was run to help remove ionospheric Faraday rotation -- I can't claim it
> helped, since I don't know the correct answers in advance, but it
> certainly didn't hurt!).  Notable is that good polarimetry was
> accomplished within *all* subbands except around 1240 MHz (GPS), and
> between 1520 and 1630 MHz (satellite 'zone of death').  In particular,
> no problems were encountered in the DME zone 1030 -- 1130 MHz!
>     Antenna polarizations are high below 1200 MHz, but this does not
> appear to have prevented reasonable results (albeit on strong sources).
>     One significant oddity:  Antenna 5's polarization solution for RCP
> shows a strong resonance in both amplitude and phase -- the period is 5
> MHz, and the pk-pk amplitude is nearly 10% (!).  This resonance is
> completely absent in LCP.  The amplitude increases with increasing
> frequency -- it's barely visible at the bottom end of the band, and
> completely dominates above 1600 MHz.
>
>     6) There is a 25-MHz-wide slot, centered at 1015 MHz which is
> completely free of any RFI.  As a test, I extracted these data for
> imaging, with excellent results (other than SNR, which is low since most
> of these antennas do not yet have their wideband OMTs installed).
>
>
>
>     6) The narrow window between
>
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