[evlatests] C-Band Sensitivity Troubles

[Robert Hayward]

It will probably be easier to drop the receiver to check for moisture on 
the vacuum window that it would be to remove the Escolam radome (and 
possibly making any potential leak worse).

-Bob

Dan Mertely wrote:
> 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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