D-STAR/Analog Interference Experience

D-STAR/Analog 10 kHz Adjacent Channel Interference Experience

Gary Pearce KN4AQ

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.

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 cyclists would ride another 100 miles tomorrow. Almost 2000 more 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. The starting line was fringe coverage for 50 watt mobiles, but the repeater signal improved quickly on down the course.

I was about 10 miles into the course when Net Control 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 very 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.

My first thought was some kind of power line noise, or maybe a digital paging transmitter that had gone spurious. 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. Wossshhhh peep. Wosssssssssssshhhhh 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, at least for stations near the BikeMS starting line.

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. Both repeaters were coordinated. 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. 10 kHz between D-STAR and analog is a serious compromise that was adopted because that's about all we can do given the current distribution of repeaters in the two-meter band. It relies on the fact that the D-STAR signal is narrow - about 8 kHz wide compared to analog's 16 kHz on voice peaks - and that geographic separation. But the passband of most analog receivers is almost 20 kHz. So if the adjacent channel D-STAR signal is very strong, the analog receiver won't reject it much. That's exactly what was happening to me.

I haven't heard any other complaints, though, from repeaters that are trying it. 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 Charlotte's 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 we don't have that mix of repeaters where I live, and while traveling through areas that do have those adjacent channel machines, I never found them both busy at the same time. So this was my first real taste, and it didn't click for a long time. Adjacent channel D-STAR interference in an analog receiver is an unfamiliar sound. D-STAR's digital signal doesn't change in relation to modulation the way FM does. 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.

As it happens, the owners of the Charlotte D-STAR and Union analog repeaters had already talked about the potential for interference, and agreed to give it a try. This specific circumstance shouldn't kill the 10 kHz + distance concept. But it makes me more aware of the real-world interference possibility, and the need to be able to recognize it when it happens.

73,
Gary KN4AQ

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