[evlatests] Azimuth-Dependent Noise Power Variations on Antenna 4, at K, Ka, and Q bands

[Dan Mertely]

... Perhaps far-out sidelobes (90 deg off beam) rotating through
the nearby (DW8) antenna & seeing 300K?  Were previous measurements
taken in the D array?  The range between DW8 & DW9 is only around
100m.  Figure 7 on page 19 of Vivek's, "Far Sidelobes of 25m
Radio Astronomy Antennas at 18 cm" (Interference Memo #25) seems
to indicate that (at least for VLBA antennas @ L-band) variations
of up to 20 dB can be found in Az cuts 90 deg off of boresite.

-Mert


Rick Perley wrote:
>     Bob Hayward and I journeyed to antenna 4 last Thursday (19 June) to 
> antenna sensitivity tests at K, Ka, and Q bands.  These include tips, 
> which are taken at an azimuth of -20 degrees.  The antenna is located on 
> pad W9.
>     After the completion of our last tip, at 23560 MHz, with the antenna 
> pointed to the zenith, we rotated it to the 'stow azimuth' at Az ~ 80.   
> We were monitoring the receiver total power in the T304 1-2 GHz TP 
> monitor, and we noted very distinct changes in power at Az ~ 26 and 50 
> degrees.  The changes were by about 1K , or 2% of total system power.  
> We reversed the rotation, and noted the same power bumps at the same 
> azimuths -- these are not due to any receiver gain changes!  We then 
> drove from an azimuth of -20 through to azimuth of 180 -- and noted the 
> same pair of noise bumps (but stronger now -- 2K-- than the initial 
> passes), plus a very wide 'bump' extending over 40 degrees of azimuth, 
> centered near Az ~ 140. 
> 
>     We then changed frequency to 33160 MHz (Ka-band), and drove from Az 
> = 180 through to -20.  Three distinct 'bumps' were seen, at azimuths of 
> 147, 100 and a double-bump near 59 degrees.  This latter pair were very 
> narrow in width -- less than 10 degrees.  All are of about 2 K in amplitude.
> 
>     We changed frequency to 40072 MHz, in Q-band, and rotated from -20 
> through +180.  Again, power bumps of about 1K amplitude were seen, at 
> azimuths of 70 and 140, with numerous others -- both 'up' and 'down', 
> along the way. 
> 
>     These were all seen in the 1 GHz-wide TP monitor in the T304.  When 
> we had completed these scans, we retrieved the analog 'Milhouse' data, 
> which was running with a 100 MHz-wide filter.  The same patterns were 
> seen as in the T304 data -- strongly indicating that the source is 
> wide-band noise.  But where from? 
> 
>     We note that in our previous testing at K and Q bands (in 2005), no 
> such variations were seen (we did the tests in an identical manner) -- 
> but were were not in the D-configuration at that time. 
> 
>     Any ideas?  Any suggestions for useful further tests?  These can be 
> done remotely, as there is no need for the analog Milhouse system to 
> record the data. 
> 
>     I have plots in my office, for those wishing to see the scans.
> 
> 
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