[evlatests] Preliminary K, Ka, Q band tests on antenna 2

[Rick Perley]

    Bob Hayward and I performed the usual suite of antenna sensitivity 
tests on antenna 2 at K, Ka, and Q bands last Friday, August 21.  This 
is the 2nd antenna outfitted at Ka band -- this particular antenna was 
chosen because it was known to be a good performer at Q-band, which is 
good evidence its optics and antenna surface are in good alignment.   We 
were fortunate in that Friday was a clear, dry day, with no clouds and a 
dew point in the low 40s. 

    The motivation behind these detailed tests (we normally only do 
these for a new band/receiver on a single antenna) is that our June 
tests done on the first Ka-band receiver, on antenna 4, showed clear 
evidence for an excess 7 to 10K system temperature that does not 
originate in the receiver, nor from the the atmosphere.  This excess was 
present in K, Ka, and Q bands.  In addition, we found -- again at all 
three bands -- an apparently external source of variable broadband 
emission, which caused system temperature to erratically vary at the 1 
to 2 K level. 

    The results from our Friday tests are preliminary, but are 
sufficiently clear that this short note is warranted. 

    1) The `spillover' power on antenna 2 is 5K to 10K *less* than on 
antenna 4 at all three bands.   The other power contributors (receiver 
temperature and atmospheric emission) are similar on both antennas. 

    2) There is no sign whatever of a variable external broadband signal 
on antenna 2.  

    3) The Ka band efficiency for antenna 2 is about 45%, similar to 
that of antenna 4. 

    In short -- antenna 2 is a spectacular performer (easily meeting the 
project requirements for sensitivity) at all three bands. 
    There is thus strong evidence that antenna 4 has two issues in want 
of explanations:

    1) The source of the excess 'spillover'.  A possible, worrisome 
candidate is resistive losses on the subreflector.  In other words, it 
is not a perfect reflector.  It takes a very small reduction in 
reflectance to notable increase system temperature -- a 7K increase 
would be due to only 3% of the subreflector surface being replaced by a 
blackblody emitter.   Careful holography might be able to detect the 
reduction in taper illumination which would accompany such an effect. 

    2) The source of the erratic broadband emission.    It is surely 
external to the antenna -- but if so, it is either far enough away from 
antenna 2 (at E5), and close enough to antenna 4 (W9), or was 
conveniently not emitting on Friday.  If the former hypothesis is true, 
it is evidence the emitter is close to W9, and likely located on the 
west side of that antenna.