[evlatests] High frequency performance

[Rick Perley]

    Finally, the results from the U, K, and Q band observations, 
yesterday late afternoon.

    Weather was good.  Referenced pointing was done, and applied, 
successfully, except for antennas 13 and 25 (which were not working well 
at the time). 

    A)  U-Band. 

       Other than the misplaced first and last records in the scan 
(which was seen at all bands), all looks relatively normal for the 15 
remaining U-band systems. 

    B)  K-Band.

       1) Antenna 13 and 25 gave fringes so weak they were not useable. 

       2)  Antenna 14 was flagged, but the data were find. 

        3)  Some antennas showed unusual amplitude instability on short 
timescales -- see the comments under Q-band.

        4) Antenna 7 showed a slow, 10% gain change over 3 minutes, on 
all IFs -- also seen at Q band.  Looks like some pointing wandering.  
The ref pnt. solution was large (0.6 arcminutes), but looked valid.  
Collimation? 

       5)  All phases looked good, except for VLA antenna 27, IFs B and 
D, which showed a weird, 60 degree 'wobble', lasting about 30 seconds.  
No explanation is offered. 

       6)  Statistical weights show:
                Antennas 17 and 19 are a factor of about 2 less 
sensitive (in terms of G/Tsys) than the norm.  The listed Tsys for 
antenna 17 is an unbelievable 600K (if it were this hot, the weights 
would be done by a factor of 100).  For antenna 19, Tsys is given as 150 
-- about the value needed to reduce the sensitivity to what we see. 

          Antennas 26B and 24D also have very low weights, but for these 
antennas, the Tsys values are normal.  Something else is likely amiss. 

        C)  Q-Band.

            1)  Antenna 11, 13, and 25 were all flagged.  Antennas 11 
and 25 have no receiver.  Antenna 13 does (and the listed Tsys is fine). 

            2) Antenna 19 gave no visibility amplitudes, and was not 
flagged.  Tsys looks o.k., so it looks like the receiver (scheduled to 
be put in earlier this week) is indeed in place.   

             3)  Antennas 9, 10, 14, 15, 20, and 28 all gave rather 
unstable amplitudes (variations on timescales of tens of seconds of 
perhaps 10 -- 20 %).  The curious thing is that all of these are 
adjacent, along the E arm (from E4 through E24), except for 9, which is 
at N8.  Perhaps a local weather condition?  All IFs were unstable in 
exactly the same way.  No corresponding variation is seen in Tsys.  I'm 
guessing some gusty winds...

             4) Antenna 17 has the bi-stable amplitudes, as reported 
over, and over, and over, again.  It also has a recorded Tsys of 900K, 
which is just not possible. 

             5)  Statistical weights show low sensitivity (by a factor 
of about 1.5 to 2) on antennas 4, 14, 17, 20, 16.  Only antenna 4 offers 
an easy explanation  -- Tsys of about 110 is sufficient.  I've already 
noted antenna 17's outrageous Tsys -- it can't be correct.  All others 
listed here have normal Tsys. 

             6)  By contrast, antennas 21 and 23 (both EVLA antennas) 
have very high statistical weights -- and nice low Tsys values (50 to 60 
K) to match.

             7) 24 D has a very low weight, while 24B is very high.  
Neither can be explained by Tsys.  I'm stumped. 

    Despite the long list -- Q-band is actually working quite well!  
(One must remember that this is a tough band to make work well, and we 
shouldn't expect X-band like stability).