Hacker News Reader

Comment by myself248 | Hacker News Reader

With some filtering on the output to knock down harmonics and aliasing noise, and as long as the power output is low enough, this could actually be legal in the US. IANAL, etc, but this is my understanding:

It's actually ridiculously tricky to measure transmitter power output in the terms that the FCC regs are written in, but the rule of thumb is that, given the receiver sensitivity of a typical radio, if you lose the signal when you're more than 200 feet from the transmitter, you're probably in the clear.

This means a perfectly legal power level would be more than adequate to cover your house, office, hackerspace, or whatever.

https://www.fcc.gov/media/radio/low-power-radio-general-info...

View on HN

If by "ridiculously tricky" you mean "requires specialized equipment and domain knowledge". Like say, skiing.

With a cheapish Rigol spectrum analyzer and homebrew receive antennas and DIY wooden/pvc tripods you could probably do 3m open-air measurements to within 3dB of what you would measure in an anechoic chamber at a test laboratory if you were very careful. I can 100% assure you that is possible with a commercial dipole kit and horn antenna, neither of which is that expensive if you're getting paid. This in in the FM band so you could use dipoles and a low-end analyzer to the tenth harmonic.

You can absolutely hook an antenna to a spec-an, but how do you calibrate the result? I can get a power measurement of mV or dBm at the instrument input, but that's not how the regs are written.

They're written as field strength in free space (microvolts per meter), which as I understand it, either requires a calibrated field probe, or a calibrated antenna. I'm sure there's a way to perform such calibrations from first principles, but that's where I'm invoking "tricky". If you've got a trick to it, please share.

In practice, Part 15.23 is pretty generous about home-built equipment, and I think since the "200 foot" thing came from the FCC's own mouth, if you can show that you walked the perimeter with a portable radio and confirmed that you're good, that probably shows good faith on the power issue.

...

Absolutely yes a spectrum analyzer is helpful to make sure your harmonic filtering is working. I would put that forth as "good engineering practice" under 15.5, and it's far more than any hobbyist could be reasonably expected to do even just a few years ago; this equipment has gotten mindbogglingly affordable and accessible.

You get a field strength the same way the pros do - from the antenna factors for your antenna and the cable loss. Look at a public FCC test report for an exposition of the method. DIY dipoles and even horns are not going to be very much different from ideal. Calibrated antennas are typically not more than a dB or two off ideal. Dipoles not even that unless they're broken. Horns and log-periodic antennas, enough that it can matter - but fine for hobbyist practice and even most pre-compliance work. I once discovered an RF engineer was using ideal gain numbers for a horn antenna instead of the calibration data for pre-compliance - it fortunately did not make a practical difference. The Roberts balun for measurement dipoles is standardized and I have instructions for building one around here somewhere. That is literally the reference for calibrating other antennas.

The primitive method for getting an antenna calibration is the three-antenna method. If you are particularly motivated, you can build three Roberts dipoles and check them. For a number of reasons, you might want to have three dipole kits anyway if you are doing a lot of these measurements.

"Tricky" in the sense that skiing is tricky. You will struggle learning from books and the internet. Starting from scratch with DIY equipment would be quite challenging. But it's all in standards and the open literature.

> The Roberts balun for measurement dipoles is ... literally the reference for calibrating other antennas.

Solid gold search term right there, thank you!

> The primitive method for getting an antenna calibration is the three-antenna method.

More good stuff. Seriously, I hadn't gotten here and this is suddenly demystifying a lot of fundamentals.

Wave if you see me on the bunny hill! ;)

I have to admit that having a ham radio license that would be easy to take away is one reason why I wouldn't mess with anything questionable radio-wise.

I wonder though if you could trust the published specs of a commercial FM transmitter to calibrate a power meter on it from a distance of a mile or so...

The FCC doesn't revoke ham radio licenses except in egregious circumstances. Most of the time, it's someone who is deliberately interfering with public safety or commercial frequencies, and is told to stop, and doesn't.

Even the douche canoe that intentionally interfered with firefighting efforts recently only got a hefty fine. He still has his license.

Ham radio licenses are granted by the FCC with the understanding that the the license-holders intend to experiment and explore radio technology. And further understanding that while licensees are expected to adhere to all of the rules, mistakes will sometimes be made. Hams are therefore given wide latitude in terms of self-policing and forgiveness for accidentally or unknowingly breaking them from time to time.

Basically if anyone receives it that wasn't expecting to it's illegal. That is at the core of the FCC rules in the US, you can't interfere with the lawful use of the band, nor can you cause "interference" on unused bands. That said, the process for prosecuting you is long and somewhat convoluted as they have to file a report, then the FCC will confirm the report, Etc.

But this kind of project (and a similar I saw that broadcasts on every FRS[1] channel at the same time) is a great example of something you can do with SDR that you cannot (easily) do with transistors and boring old analog radio.[2]

It is one of the reasons I enjoy it (playing around with SDRs and DSP) so much as I keep learning new things.

[1] Family Radio Service

[2] I know, I know, building 20 modulators isn't all that tough yadda yadda.

> if anyone receives it that wasn't expecting to it's illegal.

Are there any countries where that isn't illegal?

I don't think so, but it doesn't apply to international waters apparently?

> if you lose the signal when you're more than 200 feet from the transmitter, you're probably in the clear.

With the FCC maybe, but someone far worse will be interested, the RIAA. It's copyright infringement by illegal broadcast.

If it's intended for personal consumption it's fine. It's not copyright infringement to hear my neighbor's stereo if it's a bit loud. It is if he's using it commercially for hosting a block party selling tickets it's a problem.

Ah, but if your neighbor's stereo is faintly audible in the distance while you record a screencast, your video will get taken down off Youtube very quickly.

Posting it publicly to YouTube is no longer for personal consumption.

Do it with a private video, it won't be taken down.

That’s not true. Why do you say that? Content id takes down private videos too.

Apologies, I had done that in the past and it didn't get any copyright strikes despite using obviously copyrighted audio.

I guess they've since changed their policies.

From a copyright perspective there's no issue with uploading your own personal videos with copyrighted material in it to a site like YouTube so long as they're still private. I guess YouTube just assumes if you're uploading it there, you're going to eventually want to make it public. IMO they should just show the results of their scans to clue you into potential copyright claims but not actually do enforcement if the video isn't available publicly.

Note: an unlisted video is still a public video.

Thats because Youtube doesnt want to broadcast copyright work in any way as a courtesy for rights holders' trade groups that have asked, not because of regulatory or legal consequences though.

Nah, don't kid yourself. Tech lik€ this is NOT fine. You're a douche if you deploy this. It may be technically interesting but just a dick move to use it.

I was just speaking from a copyright perspective here. If you're causing interference on your neighbor's radio you're causing a problem, fully agreed.

> It's copyright infringement by illegal broadcast.

And I'd bet they would make a good attempt at fining you for each channel that you modulated for.

Small, low-power FM transmitters have been around even prior to Mr. Microphone. Hooking to your Hi-Fi so you can get your music on the radio in the garage is as old as Jarts.

Why do you think they would be interested?

And a wonderful April Fool's prank, too!

The main problem with this line of reasoning is that it neglects important parts defined elsewhere making it seem like this is legal when its not. IANAL, but I do hang out with a lot of hammies. You can't help but absorb a lot of technical stuff with them around.

Like for example, https://www.fcc.gov/enforcement/areas/jammers

It need not saturate the bands, you technically are in violation the moment a non-licensed entity has caused interference to a licensed entity.

Willful is a pretty low bar since general intent can be derived from any related negligent acts. Its pretty safe to say transmitting on these bands without a license is illegal, and yes its a fairly big deal.

Most of the public radio spectrum has been monitored, and archived for later retrieval if circumstances dictate since the 60s. There are services that aggregate this data and make realtime info available globally from satellite systems. Trilateration of signals is fairly trivial in most cases.

You may not get caught if you always stay below the noise floor, but you can't really have any real use of the spectrum in doing so, and algorithmic scanning of structured signals is always improving.

It is really negligent to make it seem like these things are legal, when they aren't, and the author should get in trouble for that.

A simple disclaimer isn't going to cut it when then primary purpose is effectively breaking the law in many civilized nations.

How do you interpret Bulletin 63?

https://transition.fcc.gov/oet/info/documents/bulletins/oet6...

For what it's worth (which isn't much), all of that hinges upon the FCC's motivation and capacity to enforce its rules.

If amateur and citizens band radio shenanigans over the last few decades have proved anything, it's that you have to be an absolutely massive pain in the radio-wave-receiving public's ass to really get their attention.

If you had this device on 14.300, 7.200, or channel 6? Meh. No one's gonna care.

In my town there was a guy who was absolutely obliterating 88.5 as well as 88.3 and 88.9 broadcasting some Hispanic hip-hop with some grossly overpowered local transmitter. He overpowered the stations on the entire east side of town. It took over 6 months before it was shut down. I think the FCC penalties are so harsh because their enforcement arm is so small that they can only go after the most blatant offenders. Not that I would want to risk fate given how harsh those penalties are.

I hold a ham radio licence in a couple of countries, although what's happening here is far outside of the remit of ham operations.

I'm pretty certain this is not legal in my locale, but I don't think the author of the article is making this out to be legal - and I don't think they should get in trouble for any of this publishing [assuming you're adressing them, not the commenter]. Perhaps the equivalent of a script kiddy could get up to no good with it, nothing new from the last 20 years of computer hacking then.

I'd far rather see enforcement and ire directed at the proliferation of poorly shielded junk that spews noise all over the spectrum, at surprisingly high power, that seems to have no trouble being sold online or imported for sale in many countries - and is often, strictly speaking, not legal with regards to necessary EMC, and probably safety, standards.

... reminds me of the times I've taken my HT to Southern Cal and heard things on the radio (in the ham band and elsewhere) that I haven't heard any place else, like the ham who would hold court on the repeater with background music.

Eh, if they're so low power that they're not stepping on their neighbor's reception I'd personally say it's fine. Tree falling in the woods situation and all. That definitely varies based on where one is playing around with such things, and it's always important to understand how far your transmissions are actually going (which can be surprising if you don't have much experience with it!).

Someone in a shed on a few acres playing around with an SDR? Go ahead, I don't care. Playing around in an amateur anechoic chamber, have fun if you're sure you're containing things. On a desk in a random apartment in Manhattan? What an asshole, call the FCC.

Is it potentially against the law to do this? Yeah, probably. How bad is it? Somewhere around jaywalking on an absolutely empty street and dumping waste in a park, greatly depending on multiple factors.

Reply on news.ycombinator.com