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Comment by jdietrich | Hacker News Reader

Emissions must be kept within the 88.0 to 108.0 MHz band

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.

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While I agree with everything you say, this:

"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...)

While it may be true that jamming a VOR signal is unlikely to cause a crash without other co-incidental bad luck, consider this: The localizer portion of ILS operates on those frequencies, too.

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.

I agree that if you intentionally try to jam it using something like this you may be able to, however to unintentionally as a side effect of mucking around with FM transmissions seems unlikely.

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.

Agree.

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.

I fail to see how a dummy load and a SWR meter will help to make sure you don't disturb other (portions of) RF band(s). To assess this you must look into the spectral purity of your signal. You do this with a spectrum analyzer (or equivalent). The SA is already 50 Ohms terminated so you at most need attenuators for your transmitter. The SWR meter will tell you if your transmitter is matched to your antenna. If not you might kill the final stage of your transmitter. SWR doesn't tell you something about radiating unwanted frequencies.

You are correct about the SWR only measuring the matching of your output to the antenna. But consider this: a signal with lots of harmonics (read: outside of your band intended RF signals) is going to make a 1:1 SWR match harder to achieve. If you've cut your antenna to the right length and you're not getting close to the match you need, something is wrong. It's a rough measurement to be sure, but decent in a pinch.

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.

Surely you can put a maximum on whatever you might be accidentally transmitting. Those GPIO pins are 3V, and you are not going to get many amps through that board.

How many "couple of watts" are we talking about? I don't see this being capable of more than single digits.

Does it matter? The UK legal interference limit is 50 NANOwatts (prior to the Wireless Telegraphy (Exemption) (Amendment) Regulations 2006 ALL unlicensed FM transmitters were strictly illegal). This is almost certainly capable of exceeding that, perhaps even with just a bent wire antenna (and almost certainly with a tuned antenna). There are good reasons why those limits exist, and infringement can carry penalties up to a £5000 fine.

Oh, I am certainly not saying it is legal.

Low-power amateur radio (QRP) enthusiasts communicate many miles apart with well under 5 watts.

I guess my hangup here is that it seems like a <10W (with a raspberry pi's GPIO and no amp, we are probably talking <1W in reality, but throw on a factor of 10 for safety) RF circuit with an antenna a few cm long seems really easy to make by accident, let alone on purpose.

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.

That's no justification for doing it on purpose.

I'm not trying to justify what they are doing, just get a handle on how much damage they could be doing by breaking the rules in this way, not how much damage they could be doing by breaking the rules in general.

On much different frequencies.

How do you have a moral obligation to get licensed? Isn't the moral duty not to be an ass an interfere with others? What does being licensed have to do with anything?

In this case being licensed means getting an independent opinion that you know what you're doing. If you don't, you might "be an ass and interfere with others" without even knowing it.

A couple of watts on 109MHz could take down a light aircraft.

Only if the pilot is a fuckwit.

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