[evlatests] RFI in C-band

[Rick Perley]

    Dan:

    Thanks for this.  I can certainly confirm detection of the 6615 MHz 
RFI in my test Hercules A data taken a few days ago -- I should have 
noted this in my query to you about the 6255 MHz signal. 

    I think all of these RFI sources will be a significant concern only 
for C and D configurations, due to the rapid fringe winding we get for 
the long spacings.  But we'll have to wait until we get to C config. to 
test this hypothesis. 

    rick
   

Dan Mertely wrote:
> 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
>