[evlatests] C-Band Sensitivity Troubles

[Kevin Ryan]

If you guys can correlate Tsys losses to inches of water,
we can always use them as rain gauges.

Kevin

On Jul 16, 2008, at 1:02 PM, 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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