Upon closer inspection ...
I notice that all four EVLA antennas, at 1275 MHz, provide very
noisy Tsys values -- they are fluctuating by about 10% of the mean
value, for both polarizations (IFs A and C). IFs B and D were tuned at
the same time to 1375 MHz, and there are no unusual fluctuations with
these. No VLA antennas show this phenomenon.
Any ideas?