D-STAR/Analog 10 kHz Adjacent Channel Experience
Gary Pearce KN4AQ
October 3, 2009
I just had some experience with interference between a D-STAR repeater and
an analog repeater, 10 kHz apart, that I think may be educational. Here's
the story.
This morning I was a SAG Wagon on a BikeMS public service event in South
Carolina, just outside Charlotte, NC. We were tracking about 300 riders who
were on the Century route, a 100 mile ride in one day. Most of those riders
would ride another 100 miles tomorrow. Almost 2000 more cyclists opted for
shorter routes that day.
We were using the Union, SC repeater on 145.15 MHz. The repeater was a few
miles south of the midway point the course, and about 35 miles from the
starting line in Rock Hill, fringe coverage for 50 watt mobiles.
I was about 10 miles into the course when the Net Control station started
having trouble raising the station at the starting line. I was heading back
there looking for late starters, and soon I began to have trouble receiving
the repeater. The interference pattern was unusual. Kind of like desense,
with the Union repeater sounding noisy, even though my S-meter read
mid-scale. Odd that I didn't remember hearing that on my first pass down
this road, when the repeater was noisy but readable. By the time I got to
the starting line, the Union repeater was unreadable most of the time,
which is why the station there wasn't responding to Net Control's hails.
I tuned my Yaesu 8900's receiver around 145.15, and the interference peaked
on 145.14. It was strong. And there was something familiar to it. Wossssssh
peep. Wossssssssssssssssh peep. And then I remembered. That's what D-STAR
sounds like on an analog receiver. The little peep gave it away.
Finally my brain kicked into gear. I tuned my ICOM 2820 to listen to the
Charlotte VHF D-STAR repeater on 145.14 MHz. A couple guys had begun
shooting the breeze, and were unknowingly blowing away the Union repeater's
signal in the process.
Here's why. The Charlotte D-STAR repeater was about 28 miles from the bike
tour starting line - a little closer than the Union repeater. And the
D-STAR machine was over twice as high, 1000 feet AGL. And the repeaters are
only 10 kHz apart.
So we have all the ingredients. Let's taste the soup.
These repeaters are in SERA territory. The Southeastern Repeater
Association is the frequency coordinating body for eight states in the
southeast, including North and South Carolina. Like many areas, SERA feels
pressure to find spectrum for new D-STAR repeaters in the crowded two-meter
band. They are testing the concept of slipping D-STAR repeaters in between
the 20 kHz channels that we have at the bottom end of two-meters. To make
this fly, they require geographic separation between the repeaters, at
least 50 miles. In this case the repeaters are about 65 miles apart, needed
because both repeaters are pretty high up, but especially Charlotte at 1000
feet.
Well, just because everything meets specs doesn't mean there can't be a
problem. SERA knows it, and they aren't pretending it can't happen. I
haven't heard any complaints, though. In many areas it seems to be working OK.
Why was this different? In this case, we were operating on the fringe of
the Union analog repeater's coverage, right in the direction of the
Charlotte D-STAR machine, and more or less in their prime coverage area. It
was a worst-case-scenario.
The lesson, though, isn't that a D-STAR repeater can interfere with an
analog repeater 10 kHz away when the receiving station leans to being in
the D-STAR repeater's coverage area. That's really a no-brainer.
The real lesson is that it took me so long to recognize the problem. Me,
Mr. D-STAR, one of the few people within 100 miles who could even possibly
correctly analyze what was going on, and "solve" it by asking the guys on
the D-STAR repeater to cool it for an hour or two (which they did, with no
complaint).
I had attempted to listen to adjacent-channel analog/D-STAR interference on
a few other occasions, but never quite got the mix of repeater use and my
location to come together. This was my first real taste, and it didn't
click for a long time. D-STAR interference in an analog receiver is an
unfamiliar sound. D-STAR's digital signal doesn't change in relation to
modulation. It doesn't get stronger (like AM), and it doesn't change how
much spectrum it uses (like AM and FM). It's pretty much a steady-state,
like power-line noise, which is what my mind locked on first. Listening to
the analog repeater signal, it just seemed like it was getting noisier and
noisier, even though my S-meter was still mid-scale.
It's going to be some time before more people have enough experience to
recognize when an adjacent-channel D-STAR signal is causing trouble with
their reception of an analog signal.
I'll wrap this up by remembering something Riley Hollingsworth K4ZDH said
at many FCC Forums: we are not guaranteed a clear frequency. Sometimes
there will be interference. Usually it is not malicious. Our job is to
cooperate in those circumstances when it occurs.
If we recognize what's happening.
73,
Gary KN4AQ
ARVN: Amateur Radio//Video News
Gary Pearce KN4AQ
508 Spencer Crest Ct.
Cary, NC 27513
<mailto:[email protected]>[email protected]
919-380-9944
www.ARVideoNews.com
