[evlatests] C-Band Sensitivity Troubles

[Rick Perley]

    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.