Unlicensed broadcasts on the FM broadcast band (88 to 108 MHz) are limited to a field strength of 250 µV/m at a distance of 3 meters from the antenna. This is equivalent to 0.01 microwatts.[2] Emissions must be kept within the 88.0 to 108.0 MHz band under Part 15 rules.
Without a spectrum analyser, you have absolutely no idea how much energy is being emitted on other frequencies. A crude unfiltered transmitter of this sort is probably emitting more spurious energy than on the intended frequency. Ignorance is no defence and it is the obligation of the operator to ensure that their transmission is compliant.
Lots of serious, life-or-death stuff depends on radio. The FM broadcast frequencies are directly adjacent to the VHF Airband, either side of 108MHz. The lowest part of the Airband is allocated to VOR and ILS navigation aids. Transmit a couple of watts on 107MHz and you're just going to interfere with broadcast radio; A couple of watts on 109MHz could take down a light aircraft. If you're going to experiment, you have a moral duty to get yourself an amateur license and ensure you have the skills and equipment to be a good neighbour.
Amateur radio operators have dummy loads and SWR meters to make sure that the signal they are putting out isn't stomping on some other critical portion of the RF band. Experimentation is a good thing, but this is the equivalent of someone being disturbingly careless experimenting with chemicals in their home "lab." The chance you could cause a serious problem with contamination for your neighbors is small but definitely non-zero.
That said, you should always use a dummy load when working with a new transmitter project. SA is always a good idea, but I've been in a lot of shacks that didn't have one.
Of course, this guy used a FM radio to check his signal. He should have used a scanner and checked out the multiples of his center freq.
How many "couple of watts" are we talking about? I don't see this being capable of more than single digits.
It is illegal sure, and these people shouldn't be doing it intentionally, but I have a hard time seeing it causing damage measurable in actual dollars.
"A couple of watts on 109MHz could take down a light aircraft."
is without doubt the sort of unnecessary hyperbolic statement that leads directly to the modern security-theatre over reaction. Sure, it might lead to navigation problems which combined with enough other co-incidental bad luck might lead to a crash, but the reductio ad absurdum statement "$30 worth of widely available electronics could crash a Cessna" would surely lead to an entirely justified call from Cessna's legal department if published prominently enough.
Yeah, fucking around with poorly controlled transmitters in the FM broadcast band isn't a great idea. but telling people "you'll crash planes!" is a sure-fire way to have the rest of your message's reliability questioned.
(And, for the record, I'm going to try this out with my RaspberryPi and see if I can get some sort of handle on unexpected emissions using my FunCube software radio to listen above and below the tx frequency...)
I don't think it's all that far-fetched to imagine that an unfiltered transmitter like the one described could generate signals which 'trick' the localizer receiver and guide a low-flying airplane into an obstruction which is technically outside, yet near, the approach path. (The wikipedia page on ILS describes how it works, so I won't repeat it here.) AMing the right carrier with 150 and 90 Hz tones (which really means generating carrier + and - 150 Hz and carrier + and - 90 Hz) would cause the localizer to show the plane centered on the glide path, even if it isn't.
Also I suspect the actual ILS transmitters are a lot more powerful, so unless you have this on the plane it probably wouldn't do anything.
Only if the pilot is a fuckwit.
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