[evlatests] C-Band Sensitivity Troubles

[Dan Mertely]

Hi Rick.  The FE guys just found an inch of water in the
K band feed on one of the bad antennas.  Can you tell us
which of the C band antennas showed the worst Tsys?  They
may be able to look down the feed to the window of that C
band & see if we have a similar situation.  (You mentioned
the best performers, but didn't say which one was worst.)

-Mert



Rick Perley wrote:
>     I used a few minutes of maintenance time this morning to roughly 
> calibrate the Tcals for the antennas at C-band, and hence get an 
> estimate of the system temperatures.  This was done by observing Cygnus 
> A ( a strong source of known flux density), noting the reported rise in 
> system temperature, and adjusting this by the expected rise. 
> 
>     Cyg A provides about 495 Jy at 6cm.  If we make the assumption that 
> the efficiency of the antenna at this frequency is same for all 
> antennas, and is equal to 0.55, then the expected rise in Tsys will be 
> about 50K.  Although there will be some variation in antenna 
> efficiencies, these are most unlikely to be greater than a few percent 
> -- a far smaller error than the observed spread in Tsys.  So I expect we 
> should be able to calibrate the Tcals to perhaps 5% -- certainly 
> sufficient to judge whether the observed poor sensitivity at C-band is 
> due to high Tsys.
> 
>     The results of this exercise are as expected (sadly).  After 
> correction by this procedure, the 'cold sky' system temperatures for all 
> antennas (EVLA and VLA) are typically 40 K to 80K, and correlate very 
> well with the observed sensitivities (as derived from correlator 
> coefficients, which are independent of the measured Tsys). 
>     Some details:
> 
>     1) There is no difference in the mean Tsys for EVLA and VLA antennas 
> -- about 60K. 
> 
>     2) The lowest Tsys values are from EVLA antennas 14, 16, 18, and 4 
> -- about 35K in both RCP and LCP.  Antenna 13 (which also has decent 
> sensitivity) did not fringe in these tests, and gave a zero degree 
> increment on Cyg A.  .   In 2005, Bob Hayward and I measured antenna 
> 13's Tsys (by hot/cold load tests) to be 24K, with an efficiency of 
> about 0.55.  Presuming 13 is similar to the others, the Tsys appears to 
> have degraded by at least 10K since then -- or the efficiency to have 
> dropped to about 0.40. 
> 
>     3) All other EVLA antennas have Tsys values higher than 50K -- 
> that's twice the expected (and required) values!!!
> 
>     4) The three VLA antennas with remarkably good sensitivities have 
> the lowest system temperatures amongst the VLA antennas -- 20, 22 and 27 
> all have Tsys values about 45K. 
> 
>     Although not a precise substitute for proper measurement of Tsys 
> (via correct values of Tcal), these high Tsys values are very unlikely 
> to be caused by deviant system efficiencies.  The strong (but tentative) 
> indication is that there is something seriously amiss with our C-band 
> EVLA receivers. 
> 
> 
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