[evlatests] New RFI in Antenna 14!

[Dan Mertely]

The network LAN stuff in that enclosure is quite noisy,
although I haven't tested it to the freq resolution you mention.
My test of last spring found significant spikes all the
way into X-band, with various spacings--commonly 6 & 12 MHz.
We used an RBW of 100 KHz, and 800 points in 1000 MHz.

I have attached the 1-2 GHz plot, and a 1.4 - 1.45 GHz
close-up.  The plots were labeled as "IP Phone LAN System", but the
LAN switches are the same ones used for all antenna LAN
communications.

The detrimental threshold line on these 2 plots was plotted
for the use of these LAN switch boxes as if used in the vertex
room without shielding, so I appears that only a reasonable
amount of shielding (20 - 30 dB) will drop the emissions
below the detrimental threshold.  The digital hash continues
beyond 5 GHz, and is especially dense between 3 and 3.5 GHz.

Clearly, we should be buttoning-up all these enclosures when
the LAN stuff is not being worked on, but then, we all already
knew that, didn't we?

-Mert


Rick Perley wrote:
>     I ran a 12-hour RFI check of antenna 14 last night 
> (Wednesday/Thursday), using the 'FFT Spectral Scanner' of Mike's and 
> Walter's.  The goal was to monitor the DME spectrum, to see if (as 
> anticipated) the 1.05 -- 1.15 GHz spectrum opens up late at night. 
> 
>     8192-channel spectra (resolution of 250 kHz) spectra were taken 
> every minute.  The results show a remarkable comb on RFI, *** which was 
> not present in that antenna a few months ago ***:
> 
>     There is a comb of strong, steady interfering tones, with spacing 
> 62.5 MHz, starting at 1000.0 MHz, and extending all the way to 1940 MHz. 
> 
>     I was in that antenna today (as part of a tour), and noted -- with 
> some surprise -- that the cover plate of the utility box was not 
> attached!  I suspect that the comb I saw (plus a lot of other steady RFI 
> in the spectrum) was emanating out of that gaping hole, from the 
> considerable digital stuff located within. 
> 
>     Is this spectrum of RFI expected from the hardware within? 
> 
> 
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