[evlatests] C-Band Sensitivity Troubles

[Rob Long]

I think Kevin has a great idea!

Rob

Kevin Ryan wrote:
> 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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