[evlatests] L-Band results

[Rick Perley]

    Here are the L-band results -- some of which are quite interesting ...

    1) General:  Antnenas 13, 14, 16 and 18 gave good stable fringes on 
all four IFs.
             One oddity noted:  antenna 13 (only), on IFs B and D (only) 
went bonkers for 2.5 minutes.  The recorded Tsys values went crazy 
during this same interval, for that antenna and IFs.  Very doubtful this 
is RFI, as nobody else skipped a beat.  Something internal ...

    2) Bandpasses:  They show the VLA polarizer 'suckout' (which will go 
away when we get the new super-OMT).  Other than that, great. 

    2) Return to Phase:  None of the EVLA antennas, on any IF, returned 
correctly to phase.  However, we should not jump to the conclusion that 
the L302 is still not fixed.  The time interval between the two 
observations is well over an hour, and the antennas were changing 
elevation.   There may be something else afoot.  I note that Vivek is 
tearing out his hair over large phase residuals in the baseline data, 
presumably due to a similar long-term phase instability. 

    3) Short-term stability:   Just great, as for the other bands.  One 
minor deviation -- antennas 16 and 18, on IFs B and D only, had a ~ten 
degree 1-minute long 'wobble'
(at the beginning of the scan, and not at the same time -- antenna 16 
did it on the first scan, antenna 18 on the last) of ~10 degree 
excursion, in the phase.  Some VLA antennas show similar effects, but 
it's doubtful the EVLA wobbles are atmosphere. 

    4) Sensitivity.  This is the interesting part. 

       On IFs A and C (1465 MHz), antenna 18 is better than nearly all 
VLA antennas, while antennas 13, 14, and 16 are at or below the median. 
    However -- for IFs B and D (1385 MHz), I get the amazing result that 
the four EVLA antennas are *better* than all VLA antennas, with antenna 
13 leading the pack by a significant amount. 
    Now, I've noted in past tests that IFs B and D are better than A and 
C at L-band, and placed the explanation on the changing efficiency of 
the horn (although the case is hardly convincing -- I didn't have a 
better suggestion).  But never have we seen the L-band sensitivity 
significantly better than the VLA antennas until now.  The elevation of 
the observation was modest -- 35 degrees, which certainly degrades the 
VLA system temperatures. 
    More investigation (as suggested by Ken earlier) will be needed. 

    5) System Temperatures. 

       a)  Antenna 18 claims to have a Tsys of 3K.  We wish it were so.  
Clearly, this antenna is not returning useful Tcal data. 

       b)  It's clear the recorded Tsys values (discounting ant 18) are 
not realistic. 
Antenna 13, which is easily the best in IFS B and D according to 
sensitivity measures, has a recorded Tsys of over 50K, far more than 
many other, less sensitive, antennas.   Furthermore, in IFs A and C, 
where antenna 13 is very, very bad (nearly the worst in the array), the 
recorded Tsys is lower than in IFs B and D.  Either there is a 
spectacular difference in antenna 13 efficiency between the two 
frequencies, or these Tsys values are far from the truth.  I'm betting 
on the latter. 
    Antennas 14 and 16 give 'reasonable' values of Tsys, but given the 
evidence above, I don't think we can useful statements based on the 
values.  They do seem stable.