[evlatests] EVLA and VLA L-band tuning

[Rick Perley]

    Ken and I have done a number of tests over the past two days to 
explore the tuning ranges, and the sensitivities of the EVLA and VLA's 
L-band receivers.  The results are quite astonishing. 
    The complete frequency range we explored was from 1070 to 2010 MHz, 
spaced by 10 MHz at the ends, and by 50 MHz in the middle (where the 
characteristics are already well established).  Observations were loaded 
as correlation coefficients, so a 'CALIB' provides a direct measure of 
antenna sensitivity.  We used the spectral line mode '4', with either 
6.25 or 12.5 MHz BW. 

    1) VLA tuning ranges and sensitivites. 

    Remarkably, some VLA antennas gave sensible fringes as low as 1110 
MHz.  The sensitivities at this frequency are typically a factor of four 
worse than at band center.  All VLA antennas gave fringes at 1120 MHz, 
and higher.  However, the sensitivities are a sharp function of 
frequency, due to various resonances in the VLA's polarizer.  See the 
plot on page 9 of the 'VLA Observational Status Summary' which shows the 
array sensitivity from 1255 to 1145 MHz.  The 'good regions' shown in 
that plot are confirmed in our measurements.   In summary, these are:
       a) above 1220 MHz.
       b)  from 1190 to 1205 MHz
       c) from 1165 to 1175 MHz
       d) from 1150 to 1157 MHz. 
    At the high frequency end, the VLA antennas operate very well up to 
1900 MHz.  There is little loss in sensitivity for VLA antennas up to 
this frequency.  Limitations of the L6 synthesizer make it impossible to 
tune the VLA to any higher frequency. 
    There is a notable loss in VLA sensitivity -- by a factor of two to 
four -- in the range 1800 -- 1820 MHz.   Above this, the sensitivities 
return to 'band-central' values, up to the tuning limit of 1900 MHz. 

    2) EVLA tuning ranges and sensitivites.

    It seems that the EVLA antennas will tune, with good sensitivity (!) 
down to 1070 MHz.  This is a qualified statement, as these lowest 
frequencies were the first in the file, and not all antennas got to 
source in time (or -- they didn't lock up -- more tests will be needed 
to determine the difference). 
    With the exception of ea13, the sensitivities of EVLA antennas is 
flat between ~1150 and 2010 MHz, although there is considerable 
variation in the measurements below 1200 MHz between the two IFs.  The 
resonances in the VLA antennas are absent for the EVLA antennas, as they 
employ a different style of quadrature phase shifter. 
    Most remarkably -- all EVLA antennas are as sensitive at 2010 MHz as 
they are at the center of the band.  We do not know the top end of the 
tuning range -- 2010 MHz is as high as we tried.  Further tests will be 
run to see how high we can go.