[evlatests] C-Band RFI

[Dan Mertely]

Hi Rick.  We checked up on the 6 GHz signal a few
weeks ago.  I had previously believed that the signal
was from a Glenwood-Brushy to Frisco Divide microwave
link, based on what I found in the FCC database.  However,
when we got our RF Direction Finding System (RF-DFS)
working above 2 GHz for the first time a couple of weeks
ago we found that the source bearing was just a few degrees
south of due west--the same bearing that Steve found when
he tried using a EVLA antenna as a DF system.

Confirmation of a bearing to the west and not the south-west
got me digging deeper into the FCC database.  It turns out
that the real culprit is a Mangus Mtn to Grayhill microwave
link @ 6004.5 MHz.  Mangus is 60 Km almost due west, and Grayhill
is just north of AE8/E64 on the east arm.  Their link shoots
right over W48, then on to the east arm.  The Effective
Isotropic Radiated Power (EIRP--includes TX antenna gain)
is +71.4 dBm, and the maximum utilized bandwidth is 30 MHz.
The link is owned/managed by our friends at the Western New
Mexico Telephone Co.  Although our monitor sees variations
in signal strength, and occasional drop-outs, I think we can
be sure that it is essentially a continuous link.  I've
attached an RF-DFS plot of the bearing to the 6004.5 MHz
signal.

As for the "6580 MHz birdie" to the west:  I don't yet have a
confirmation bearing using our system.  I do note from the
FCC database however that there is a Grants to Datil (Davenport
Mtn) microwave link @ 6585 MHz.  If that is the signal source,
it ought to be strongest to the north, not the west.  It is
approved for max 5 MHz wide, 70.3 dBm EIRP, & a 1.3 deg
"beam-width" (3 dB down beam-width I would assume).  The VLA is
"only" off by around 16 degrees from the center of their beam,
so we're probably picking up side-lobes.  The link is owned/managed
by AirCell LLC.  Bradford (IPG Coop) and I will verify bearing
once we get the RF-DFS back up (we had servo problems during the
wind storm of last week).

The band from 6.1 to 7 GHz is full of such microwave links.
These 2 are just more noticeable because their TX antenna points
toward our array.

-Mert



Rick Perley wrote:
>     Bob and I conducted the final C-band efficiency/system performance 
> tests on antenna 24 last week.  While conducting these, we noted some 
> interesting (and strong) RFI.  Using Walter's bandpass tool, we captured 
> some spectra for subsequent review.  Below is a summary. 
> 
>     1) A weak *internal* birdie appears at about 3605 MHz (outside the 
> C-band passband).  The strength is about 4 dB above the noise in a 2 MHz 
> channelwideth.  This is of no consequence to observing, but may indicate 
> some internal issue. 
>     2) A narrow, and strong, birdie near 6580 MHz is seen -- only at low 
> elevations towards the west.  The width is no more than 2 MHz, and the 
> strength is up to 15 dB over the noise.  The birdie is undetectable 
> above 20 degrees elevation, and gets stronger as elevation decreases. 
> 
>     3) An extremely wide and strong 'birdie' is seen between 5990 and 
> 6020 MHz.  This has already been detected by Emmanuel in his 6cm 
> sensitivity tests.  The signal is seen in all our observations, and 
> appears to be at least as strong when at the vertical as when pointed to 
> the horizon.  It is likely time variable.  Its strength is 15 to 20 dB 
> above the noise over its 30 MHz width -- thus contributing about as much 
> power as is in the entire 1 GHz band on cold sky! 
> 
>     I think it is important that we learn the origin of the last two 
> signals -- particularly the last one, which if omnipresent, will prevent 
> future observations within that frequency window. 
> 
>    
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