The FCC’s mandate that all two-way radio systems operating below 512 MHz convert to 12.5 kHz channels from the traditional 25 kHz channels was a boon for many two-way radio dealers whose customers decided to meet the requirement by upgrading their systems from analog to digital.

“Their thinking was, ‘If I have to buy new radios, I may as well buy digital,’” said Steve Guller, vice president of Warner Communications in St. Louis.

But the influx of all those new digital systems may be at the heart of a new problem that some users are experiencing, as digital systems increasingly are causing harmful co-channel and adjacent-channel interference to analog systems in some locations. According to Guller, St. Louis is one of those places.

“The digital noise is rendering some of the analog channels useless at times,” said Guller, who added that the problem is affecting both public-safety and commercial entities in his area. The net effect is that, while an analog transmission still can be heard, it’s garbled to the point of being unintelligible.

What Guller finds particularly interesting about the situation is that sometimes the interference affects an analog operator, but sometimes it doesn’t—there doesn’t seem to be any rhyme or reason to it.

“It’s very spotty,” Guller said, adding that one potential cause would be digital repeaters that operate continuously.

The city of Atlanta is another area where digital-to-analog interference is causing problems, according to Steve and Dave Keller, owners of Radio One. They said that one potential cause of the interference is that TDMA-based digital systems can trick analog receivers that are using certain Continuous Tone Coded Squelch System (CTCSS) tones—also known as PL tones—to open and receive the interference from a digital radio, when they shouldn’t be receiving anything at all.

This circumstance isn’t new, as the Public Safety Communications Council—an amalgam of the four public-safety frequency advisory committees—sent a letter on the topic two years ago to the FCC’s Public Safety and Homeland Security Bureau.

 “Another observed source of interference is false triggering of certain CTCSS tones in analog receivers by (we presume) harmonics of the TDMA framing rate which fall on, or are very close to, certain CTCSS tone frequencies, thus opening receivers and causing constant noise in affected systems,” the letter stated.

There is nothing peculiar about an LMR digital radio signal, compared with an FM signal, that would cause this interference, as long as the system is using a continuous phase modulation scheme, which most LMR digital radio systems (including P25 systems) do, according to Jay Jacobsmeyer, president of Pericle Communications in Colorado Springs, Colo., and a longtime contributor to IWCE’s Urgent Communications.

There are several potential causes for such interference, including unlicensed operators, systems that are transmitting at power levels high enough that the digital signal overpowers the analog signal, and signals that escape the emissions mask, due to equipment malfunction or poor engineering, Jacobsmeyer said.

Of the last possibility, “it would be more likely to occur with earlier generations of digital radios, because the manufacturers have better tools now to keep that under control,” Jacobsmeyer said. “But there are still a lot of first-generation radios out there.”