[evlatests] Amplitude stability on antennas 24 and 26, at X-band

[Rick Perley]

    I ran a 'holography' test file today, with the primary purpose of 
determining whether the odd EVLA phase effects noted in August and early 
September are still with us. 

    The observation was done at X-band.  All six EVLA antennas produced 
good fringes.  As there has been good reasons to think the BW/Fluke 
settings influence the phase behavior, I ran this test with three 
different BW/Fluke setups.  All three used the same frequency pair:  
8435 and 8485 MHz in AC and BD. 

    Many interesting phenomena were observed.  I will separate these 
into separate messages, so as not to overwhelm the reader.

    The data were taken in 'fast continuum' mode -- 0.417 seconds 
integration/dump time. 

    Amplitude Stability was in general very good, with only occasional 
odd changes.  The exceptions are:

    1) Antenna 16, IFs A and D were dead.  I believe this is a broken 
L302. 

    2) Antenna 24, IF D.  The amplitude gain has two semi-stable states. 
  The higher amplitude state is 1.37 dB (in power) above the lower.  The 
higher state lasts 18 seconds.  (I cannot determine the lower state 
duration easily, as the holographic pattern changes every 10 seconds). 
   The transition between up and down is very fast -- about 0.5 to 1.0 
seconds.  There is no phase effect at all. 

    3) Antenna 26, IFs B and D.  There is a remarkable bi-stable state 
on these two IFs, with a period of EXACTLY 1.66666 seconds.  The higher 
amplitude state lasts exactly 3 integration periods, the lower state 
exactly 1 integration period.  (One integration period is 0.417 
seconds).  The amplitude gain ratio between the two is 1.47, 
corresponding to a power ratio of 3.3 dB.  There is no phase effect 
whatever.