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

[Rob Long]

Bob is correct about the attenuators setting when in L-band. RFI causes 
the attenuators to set incorrectly frequently. Since it is strictly a 
total power measurement, large fluctuations due to RFI in L-band can 
wreak havoc. Not always the same from antenna to antenna either (as I'm 
sure you already know).

Rob

Rick Perley wrote:
>     Bob (et al.):
> 
>     For the first point -- I'll need to know which antennas have what 
> suite of FE electronics.  But, due to the big power jump, and also from 
> the general rattiness of the power (likely primarily due to the RF 
> switch problem), we have very few antennas with clear diurnal signatures 
> at L-band.
> 
>     For the second:  The RFI environment has got to be a part of this 
> problem.  I can get a rough estimate by looking at the power variances 
> at all the subbands (some of which are truly awful), and do an 
> appropriate weighting ...
> 
> Robert Hayward wrote:
>> 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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>>   
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