Mobile FM Transmitters — A Modulator Menace, or a Mountain out of a Molehill?

The traditional radio broadcast industry is getting pretty desperate lately. But while commercial radio has seen its fortunes slowly decline after squeezing out the consolidation profits, public radio has generally faired better. Nevertheless, there’s still some unregulated competition and interference coming from those little FM transmitters people use to pump their portable music players into their car stereos.

The NAB made a stink last month about FM modulators included in some satellite radios that apparently exceed the part 15 limits for unlicensed broadcasting. Conjuring up images of innocent drivers being bombarded with unwanted broadcasts of potty-talk from Howard Stern’s Sirius satellite morning show on their morning commute, the nation’s largest broadcast lobby has been pushing the FCC to force a recall.

The NAB did their own study on a wide variety of FM modulators and found, surprise surprise, that 76% exceeded Part 15 output limits. Despite the NAB’s history of releasing loaded research reports to kill off technologies it opposes (like LPFM), I don’t doubt their findings. Part 15 limits on the FM band are really low, and most of these cheap transmitters are not precision devices. In fact, my experience is that the ones that operate under Part 15 limits tend not to be particuarly useful, due to very limited range.

But, because Part 15 limits are so low, it’s hard to see these transmitters which operate at less than 1/10 of a watt at most can be a real threat to a local broadcast station in any significant way. I can’t imagine one of these transmitters posing more problems than power lines and other sources of electrical interference.

To their credit, NPR decided to investigate in a more real-world situation, setting up an antenna and spectrum analyzer alongside several major highways in the Washington DC area. NPR’s study [PDF link] found that as many as .91% of vehicles at any given time were operating a mini FM transmitter, and that sometimes a majority of them were operating above Part 15 limits. This amounts to as many as .39% of vehicles measured broadcasting with a noncompliant FM transmitter — about 4 in 1000.

While the NPR report confirms that there are noncompliant modulators operating in the real world, it still doesn’t address the claim that they’re causing interference with licensed stations. NPR’s report suggests that interference to distant stations would be possible — the measured field strength of some transmitters (at 48 – 68 dBμV) matches the field strength of the outer reaches of a station’s “rural service” area (at 56 – 60 dBμV).

But that’s still theory. RadioWorld’s report on the issue quotes some DC-area listener complaints about hearing Stern, specifically, instead of Baltimore NPR station WYPR. Yet that’s a pretty unique situation, with people trying to tune in a station nearly 50 miles away, on a frequency (88.1 FM) that is commonly used by these transmitters.

For the user of the transmitter there’s a strong disincentive to using it on frequency occupied by a local station, since it will be easily overwhelmed by any full-power transmitter. At most, what we’re really seeing, then, is a few fringe-area listeners being momentarily inconvenienced while trying to listen to a distant signal that isn’t strong enough to overpower a tiny transmitter that might be broadcasting a little outside the limits.

To me, the brouhaha is just another example of the broadcast industry dedicating its efforts to stomp out any possible tiny bit of competition rather than focus on providing a valuable service. NPR’s a little different here, in that it really is more listener than profit focus. Nevertheless, NPR didn’t shy away from joining the NAB in trying to stomp out LPFM over bogus cries of intereference.

Nevermind that NPR stations are some of the biggest users of translator stations, that can use up to 2.5x as much power as LPFM stations and are allowed to be crammed in closer to full-power licensed stations than LPFM can.

Frankly, I like the idea of thousands of little unlicensed transmitters operating all over the nation’s highways. None are strong enough to overwhelm a local station, but any one of them might just provide something more interesting than what’s on the rest of the dial.

In a way, it’s akin to Tetsuo Kogawa‘s idea of Mini-FM radio broadcasting ultra-local radio to neighborhoods, or just a city block.

Over the years lots of radio activists and hobbyists have talked about creating networks of small Part 15 transmitters. So maybe the “modulator menace” is a spontaneous version of that idea, more the result of a critical mass than any sort of organized movement. Perhaps it’s even a form of unintentional civil disobedience (an oxymoron, I admit), if it’s true that these transmitters are operating mostly above legal limits.

About two years ago the blogosphere was all atwitter over the discovery that one iPod transmitter accessory could be easily modified to broadcast much further. This led to all sorts of blogger fantasies of turning their iPods into mobile microbroadcast stations, even advertising their frequency on bumper stickers.

Of course, that fad is soooo 2004, and unwanted Stern reception seems to be the big threat here. Even if thousands of people took up the call to use their little modulators to bring something a little more intentionally unique to the airwaves I can’t see this as a great threat to FM broadcasters. But that won’t stop the NAB from calling out the calvary.

If they succeed in pushing the FCC into action, then you might have to buy one of these “high power” transmitters while you still can.


One response to “Mobile FM Transmitters — A Modulator Menace, or a Mountain out of a Molehill?”

  1. Complaints grow about micro-broadcast devices…

    From “Wireless devices send mixed radio signals,” by Jim Puzzanghera, Los Angeles Times, 1 November:

    “Todd Lockwood, a 56-year-old writer in South Burlington, [Vermont, USA], often finds an annoying hitchhiker in his Audi when he tunes in to his …

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