[evlatests] Strange System Temperature Changes

[Bryan Butler]

water in the feeds, which has some odd elevation effect?  it rained like 
hell in ABQ last night - don't know about out at the site...


Rick Perley wrote:
>     I secured a four-hour observation early this morning (3 to 7 AM) to 
> further study EVLA polarization behavior.  As usual, I found other 
> problems, not associated with what I was seeking ...
> 
>     The observations were of a single source, cycling through four bands 
> -- L, C, X, and K.  Data quality in general is quite good.  (Details on 
> what is not will be reported separately).   Observations were made at 
> high elevation -- about 60 through 82 degrees, with meridian transit 
> three hours into the run. 
> 
>     However,  the C-band calibration revealed a very odd, global, change 
> in ampliutude over the 4-hour period, in that the antenna amplitudes on 
> ALL antennas (both VLA and EVLA) dropped by typically 3 - 6 % over the 
> four-hour period.    For some EVLA antennas (11, 16, 17, 23 and 24), the 
> effect was considerably less.  All VLA antennas showed the decline at 
> full amplitude.    The changes were smooth over the timerange.
> 
>     The decline is amplitude is clearly associated with an increase in 
> system temperature.  All antennas recorded a dramatic rise in Tsys, with 
> the same functional form as the loss in amplitude -- typically rising 
> from 30 to 50 K through the time period!  By filling the data as 
> correlation coefficients, it is clear that the SNR was declining 
> throughout the time period, and that the Tsys correction mostly -- but 
> not completely -- made the necessary adjustments. 
> 
>       I then checked the other frequencies:
> 
>        a)  No such Tsys variation is seen at L-band.  Everything was 
> steady. 
>        b) A similar -- and even larger -- effect was seen at X-band. 
>        c)  At K-band, there are antennas whose Tsys rose, and others for 
> which it fell, during the same time frame. 
> 
>     So what caused this? 
> 
>        - Weather can be eliminated.  The skies were reported to be 
> mostly clear, and there was no wind.  Perusual of the 'wunderground' 
> database shows the site temperature to be steady, and the winds light 
> throughout.
>        - It is not a purely elevation effect.  The antennas rose, then 
> dropped.  Plotting Tsys, or gain, against elevation shows there is no 
> direct correlation.  If elevation is involved, there has to be at least 
> a two-hour lag.    This cannot be due to opacity. 
>        - It's not due to pointing off the source.  Besides not being 
> able to make the Tsys rise, the effects are much less noted at K-band.  
> (See below). 
> 
>        I can only conclude that the antenna system temperature did 
> change as reported, that this increase in noise power decreased the 
> correlation coefficients, which were nearly, but not quite correctly, 
> adjusted by the synchronous monitoring. 
>     But as to *why* the system temperatures were changing so 
> dramatically at X and C bands, on a clear, calm night -- I have no clue. 
> 
>    
>       
>    
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests