[evlatests] [Fwd: Re: Antenna 4 -- Gone Wild! ... Fixed!]

[Dan Mertely]

Hi Rick.  There had been reports of problems at X (especially),
C, and P bands on ant04.  A bunch of us went out there with a
spectrum analyzer yesterday and today & determined that the
P-band problem was due to very strong, intermittent RFI at
340.125 MHz.  It seems to be coming from the SE, the direction
of the WSMR.  I will be checking with them to find out if they
are doing anything new.

The problems at C and X band seem to have been caused by a
wildly-oscillating P-band front end.  It was putting out many
spurious signals throughout X, and C.  (We didn't check L,
but considering the superfluity of signals we saw from
7 - 9 GHz (looking via the X-band FE), I wouldn't doubt that
the P-band FE was generously providing RF down at lower frequencies
too).  The P-band was seen to be generating a strong tone in
C-band just above 5 GHz as well.

The P-band has been de-powered until John McClendon can find
and install a replacement.

PS:  The 340 MHz P-band RFI was still there after we pulled
the plug on the naughty P-band front end.  (We had a UHF antenna
and pre-amp to go with the spectrum analyzer, so we weren't
relying on the VLA P-band receiver for our look at the UHF
world.)

-Mert


Rick Perley wrote:
>     The 'stress test' has revealed some very peculiar behavior with VLA 
> antenna 4.  It makes little sense to me -- I describe what I see, in the 
> hope that others can see through the fog ...
> 
>     a) P-Band:  There are no fringes, and all data are flagged.
> 
>     b) L-Band:  Short dropouts are seen on all four IFs.  The duration 
> of any drop is a single .416 second integration.  The amplitude of the 
> drop is typically a factor of two.  There is no phase effect.  The 
> frequency of the drops is about once every 10 seconds -- but there is no 
> regularity.  All four IFs drop at the same time.  These are the same 
> characteristics that I have noted before. 
>           In addition to these, there are 'extra' phenomena on IFs A and 
> C only -- short (few record) amplitude *rises* of a couple seconds 
> duration.  Similar effects may be in B and D, but are less clear.  These 
> perturbations may be correlated with very odd Tsys behavior. 
>           None of these bad data were flagged. 
> 
>     c) C-Band.  Everything looks quite normal at C-band -- except that 
> IF 'A' shows a regular 'beat' in amplitude, dropping by about 3% over ~5 
> seconds, then abruptly returning to the normal level.  The 'beat' period 
> is about  20 seconds.  There is no corresponding effect in Tsys, so this 
> originates in the correlation coefficient.  No data were flagged. 
> 
>     d) X-Band:  IFs B and D are entirely normal.  But in IF 'A', we 
> again see the 'beat', but the sense of it is reversed -- the antenna has 
> about 10 seconds in the low state, then about 5 second in a higher, 
> normal, state.  The amplitude of the beat is much higher -- about a 
> factor of 2.  In IF 'C', the gains are low by a factor of 10, and there 
> are intermittent short (few seconds) pulses of normal amplitude.  Tsys 
> values of IF 'A' are fixed at 203K, and are large and erratic in IF 
> 'C'.    No data were flagged. 
> 
>     e) U, K, Q bands.  All is normal at these bands. 
> 
> 
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