[evlatests] C-Band Sensitivity Troubles

[Charles Kutz]

It was not an inch of water! Although there were a few drops on the 
window inside the feed.

In order to maintain some modicum of order and accuracy in information 
flow, I would highly prefer that information flowed from the appropriate 
channels.

I was waiting on the full debrief from the FE crew this afternoon prior 
to disseminating their findings.

Chuck




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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