[evlatests] L-Band tuning -- How high can we go!

[Rick Perley]

    In short:  2250 MHz. 

    Details:

    Although the EVLA's LO system seems to permit tuning the L-band 
receivers to any frequency, the FEs have a 800 -- 2200 MHz filter 
following the first amplifier which will limit the actual range 
obtained.  In addition, we have the OMT response to factor in, which 
will in general be more limiting. 

    We completed our exploration of the practical tuning range this 
morning by attempting observations at 50 MHz intervals, starting at 2000 
MHz.  The results are in keeping with the comments made above:

    For EVLA antennas, *** other than ea14 *** , the sensitivity drops 
by ~50% between 2000 and 2150 MHz, (and appears relatively constant, at 
the reduced level, between 2050 and 2200 MHz), then drops dramatically 
above 2200 MHz.  At 2250 MHz, the sensitivities are down, from those at 
2000 MHz, by factors of four to 16.  At 2300 MHz, no coherent signal at 
all is seen. 

    For ea14 -- which has the prototype OMT -- the cutoff occurs between 
2100 and 2150 MHz.  Sensitivity at 2150 MHz is down by a factor of 100 
compared to that at 2000 MHz.