[evlatests] antennas 15B and 18C

[Rick Perley]

    ea15, in IF 'B' was not functioning properly at any band on Friday 
evening -- the problem looks like a DTS issue. 

    ea18, in IF 'C' has a more curious problem, with phases stable for 
only a few seconds at a time, then abruptly changing to a completely 
different value.  Amplitudes at the high frequencies also abruptly drop 
to very low values, for most of the data.  But a small fraction look 
completely correctly.  This behavior is seen at all bands tested (X, Ku, 
K, Ka, and Q).