[evlatests] C-band Sensitivity Oscillation

[Rick Perley]

    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...