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

[Rick Perley]

    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