[evlatests] Standing Waves and X-band

[Rick Perley]

       The short test done last night provides some useful information 
into bandpasses.  I used only a single subband (to keep data volume 
down), but this is sufficient ...

      All bands show a wobble in the bandpasses (both amplitude and 
phase) of typically 23 MHz period -- this is clearly due to a standing 
wave set up between the receiver input and the subreflector:  the length 
corresponding to a 23 MHz period is 6.5 meters, and the distance between 
the subreflector and the secondary phase center is 6.6 meters. 

      The reflections at X-band are more pronounced than at other 
bands.  The typical pk-pk power amplitude of the standing wave is ~0.5 
dB, which I think corresponds to a ~-15 dB return loss.  Even more 
intriguing, for most antennas, the standing wave pattern is quite a bit 
larger at the high frequency end of the bandpass -- typically twice the 
amplitude at 11.5 GHz than at 8.5 GHz. 

    One antenna is particularly bad:  ea11, on the RCP side (IF 'B', at 
11.5 GHz), has a pk-pk amplitude of nearly 3dB in the standing wave (and 
about 20 degrees in phase).  The corresonding return loss is -8 dB.  The 
pattern is barely visible in the LCP side (at the same frequency).