[evlatests] Antenna 22 at X-band

[Rick Perley]

    Astute readers will recall that we had noted last week the high 
cross-polarization of antenna 22 at X-band, and also the report  
yesterday that this antenna's gains fluctuated by 10s of percent, in an 
anti-correlated manner, such that when LCP gains dropped the RCP gain 
rose.   These characteristics are symptomatic of a common cause. 
    Bob Hayward immediately suggested that the origin is likely to be 
material lying on the vacuum window within the X-band feed horn.  
Changes in gain and polarization would occur as this stuff slides around 
at different elevations. 
    Chuck Kutz reported yesterday that an inspection has indeed found 
material on the window which (I believe) has now been cleaned up and the 
antenna returned to service. 

    Further evidence that this is indeed the cause of the high 
cross-polarization is provided, curiously enough, from the observations 
of Mars during the recent 'flux densities' run.  Mars Orbiter broadcasts 
a pure RCP signal centered at 8437 MHz (plus or minus 1 MHz or so).  In 
reviewing the flux density data, I find that the RCP correlations of six 
of the eight observations show the very strong signal (for the other two 
the spectra are clean, as the satellite is presumably behind the 
planet).   In the LCP correlations, the signal is very nearly invisible, 
*except* on baselines with antenna 22, where it is strongly visible.  
This is, of course, due to the high cross-polarization of that antenna. 

    Rick