[evlatests] RFI in C-band

[Dan Mertely]

Hi Rick.  As I reported in the EVLA Coordination
Meeting this morning, we have located the source
of the 2nd strong C-band data link you saw at around
6240 -- 6270 MHz.  Western New Mexico Telephone Co.
has a C-band microwave link (data fiber back-up) at
Gray Hill (10 miles ESE of the VLA) that shoots over
the VLA to Mangus Mtn (39 miles W of the VLA).  From
what the WNMTC rep told me last year when we discussed
the 6004.5 MHz link *to* Gray Hill from Fox Mtn,  I
believe that this newly found link is the return to
Fox Mtn, by way of Mangus Mtn.

The signal is officially 30 MHz wide, and centered at
6256.54 MHz.  At the Gray Hill site (which we visited
last Friday with a spectrum analyzer), we saw the signal
coming from the southern tower (the shorter tower), with
6241 - 6285 MHz above our 100 KHz RBW noise level on the
spectrum analyzer.  The signal appeared to be bi-modal,
with 2 peaks, evenly spaced within the bandpass.  We
used a commercial X-band rectangular feed horn, which
seemed to indicate that the signal was horizontally
polarized (although we were probably 160 degrees off to
the north side of the main beam).

The documented EIRP is 71.4 dBm, which indicates a high
gain feed.  As a result, my mapping indicates that the RFI
from this emitter will be worst @ E48/AE6 and adjacent pads,
then at W40/AW5 and adjacent pads.  So, we haven't seen
the worst of it yet in our current A configuration.  For
most of the array, the RFI will be worst when pointed
low and to the east.

While at the Gray Hill electronic site we also noted a
narrower (around 5 MHz wide) signal centered around 6615
MHz, which seemed (from our vantage point) to be transmitted
from the north tower (the tall one).  The FCC licensing
info suggests that this signal is being transmitted from
Gray Hill (incorrectly identified as "Magdalena" in the
license data base) at 6615 MHz and an EIRP of +64.4 dBm.
The receiver is at the "Smith Bagley" tower, which is
just south of US 60, 10 miles west of Datil.  The VLA
antennas most affected by this signal will be those at
N28/BN7 when observing toward the SE. That transmitter
is listed as belonging to ALLTEL.  It is a new license
which only became active in March of last year.  Since
Gray Hill is not on US Forest Service property, we didn't
get a notification.

I will check with both WNMTC & ALLTEL shortly to find out
the importance of the links to their operations.  Our
previous query to WNMTC about their 6004.5 MHz Gray Hill
link yielded the estimate that it would take a very good
reason and funds in the range of "mid to high 6 digit
neighborhood at a minimum" to relocate or re-route the
link.  I suspect the answer will be the same for this
link as well.  I also ofter slim hope for relief from
the 6615 MHz ALLTEL link, since it was just installed,
and is licensed through February of 2018.

-Mert




rperley at nrao.edu wrote:
> I forgot to note in the previous email the observed state of RFI within
> C-band.
> 
> The bottom two subbands (covering 3952 through 4208 MHz) were filled with
> RFI, visible on all baselines.  The nature of the amplitudes and phases
> clearly indicates multiple origins -- as expected if the sources are
> satellite downlinks.  I doubt these two subbands are of any use to
> astronomy without some sophisticated removal techniques.
> 
> The pair of local RFI (telecommunication links) that I reported on
> yesterday are present.
> 
> Surprisingly (given the reports from others), I could see no strong
> interference between 7 and 8 GHz.
> 
> The entire 1024 MHz-wide bandwidth in X-band looks clear of RFI.
> 
> 
> 
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Rick Perley wrote:
 >    Dan:
 >
 >    There are two strong interferors seen in my C-band data which spans
 > 4.5 -- 6.5 GHz:  One of these I know about, the other I don't. 
Perhaps
 > you can uncover the origin of the other ...
 >
 >    1) 5990 - 6020 MHz.  We already know this is the Mangas to Grayhill
 > telecom link.
 >    2) 6240 -- 6270 MHz.  This has the same width as the other one 
-- is
 > it another link?  The power for this one is about 10 times higher than
 > the other one.
 >    Rick