[evlatests] Widened L-band tuning for VLA antennas

[Dan Mertely]

Hi Rick/Ken.  You've probably already already gotten the info,
but... my records show the following valid L6 lock frequencies:

____  3510  3610  3710  3810  3910
____  3540  3640  3740  3840  3940
____  3560  3660  3760  3860  3960
3490  3590  3690  3790  3890  3990

Terry Cotter can give you the details.

-Mert




Rick Perley wrote:
>     All VLA antennas have now had their low-pass and stop-band filters 
> -- originally installed in very early times to stop saturation from a 
> strong Forest Service link frequency -- removed. 
>     A test was run yesterday to measure system performance.  The test 
> was run in spectral line mode, at 16 frequencies from 1210 to 1960 
> MHz.    All VLA antennas can now tune above 1730 MHz (the previous lower 
> limit).  Some conclusions from the test:
> 
>     1) The previously-reported incoherency between the VLA and EVLA is 
> now known to be due to the executor's (mistaken) belief that the L6 
> synthesizers cannot bet set above 3090 MHz.   Hence, the VLA antenna 
> were fringing nicely at another (unrequested) frequency.  My earlier 
> tests (on antennas 5 and 10), taken under Modcomp control ( which 
> apparently suffered no such misconceptions) clearly show that 4010 MHz 
> is a legal and working frequency.  Ken is currently checking the 
> documentation to determine what the upper limit is. 
> 
>     2) VLA sensitivity at 1810 MHz (the highest frequency which worked 
> for the test) show a reduction by about a factor of four in sensitivity 
> (equivalently, Tsys is four times too high, or efficiency four times too 
> low) compared to the middle L-band frequencies. 
> 
>     3) VLA sensitivity at 1210 MHz is similarly a factor of four worse 
> than at the middle frequencies.  In this case, the cause is a waveguide 
> resonant cutoff, clearly visible in the bandpass spectra. 
> 
>     4) The EVLA sensitivities are nearly completely flat between 1260 
> and 1960 MHz.  There are some exceptions:  antenna 13 steadily worsens 
> with increasing frequency (this is a long-known problem), antennas 17 
> and 21 are less sensitive overall (likely due to high Tsys). 
> 
>     5) The calibration was done on 3C48, at an elevation of 25 degrees.  
> At this elevation, the EVLA antennas have better sensitivity -- by 
> roughly 15% -- than VLA antennas.   At higher elevations, the situation 
> reverses. 
> 
>    
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