[evlatests] C-band Sensitivity Oscillation

[Emmanuel Momjian]

Multi-frequency and multi-band test observations have been carried out 
on Sunday to look into the oscillation in sensitivity which was 
initially found on the EVLA antennas in C-band.

The test included observations at L, C, X, K and Q bands. The observed 
frequency range in each band was set by appropriately scaling the effect 
seen at C-band to the other frequency bands (i.e. scaling by the sizes 
of the feed horns). For proper sampling, each of the frequency ranges 
were observed using ~30 different frequency settings.

The results show that the oscillation in sensitivity phenomenon, which 
is on the scale of 10%, is seen only on the EVLA antennas and only in 
C-Band.

Emmanuel

Rick Perley wrote:
>     We have long noted that IF#1 at C-band had slightly (~5%) poorer 
> sensitivity than IF#2 for EVLA antennas.   Recent tests showed that this 
> is not an effect due to the IF -- swapping frequencies caused the 
> sensitivity difference to change.  Furthermore, the effect is not seen 
> at X-band, nor for the VLA antennas at C-band. 
>
>     Emmanuel ran a test last night to clarify the picture.  He tuned the 
> array from 4.80 through 5.05 GHz, in 5 MHz steps, using 6MHz continuum.  
> The sensitivity for each frequency was derived via the correlation 
> coefficients. 
>
>     We find a remarkable sinusoid oscillation in sensitivity, for all 
> IFs, for all EVLA antennas, over the entire bandwidth surveyed.  The 
> amplitude of the oscillation is about 10% (in Tsys/effic), the period is 
> close to 90 MHz.  If due to a standing wave, the length is 1.7 meters -- 
> about the length of the C-band horn? 
>
>     We stumbled upon this because the default frequencies -- 4885 and 
> 4835 MHz, just happen to lie on the peak and trough of the standing wave...
>
>    
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>   

-- 
**************************************
Dr. Emmanuel Momjian
National Radio Astronomy Observatory
P. O. Box O
Socorro, NM 87801
tel:(505)835-7452
http://www.aoc.nrao.edu/~emomjian
**************************************