[ctoth]

Okay, now, can you explain to me how an FM transmitter, transmitting somewhere between 87 MHz and 108 MHz will interfere with your emergency services? Potential harmonics? Can you point at one case of this ever happening, ever in real life where modern emergency communications were interrupted by a FM transmitter?

[rsaxvc]

> can you explain to me how an FM transmitter, transmitting somewhere between 87 MHz and 108 MHz will interfere with your emergency services

Jamming EAS(https://www.fcc.gov/emergency-alert-system). In the rural midwest, it's critical for distributing tornado warnings further from town than you can hear the sirens. Periodic tests are required, and there's a readiness report.

>Can you point at one case of this ever happening, ever in real life where modern emergency communications were interrupted by a FM transmitter?

EAS failures due to interference causing poor signal does happen. From the August 11, 2021 Nationwide EAS Test: "There were 78 test participants on receipt and 32 on retransmission that reported failure to receive the test message due to poor signal. Test participants attributed the poor signal to interference, a weak signal from their monitoring source, or a weather-related complication."

Also from https://www.govinfo.gov/content/pkg/CRPT-115hrpt843/html/CRP... ,

"Illegal pirate radio stations interfere with the Emergency Alert System (EAS). EAS is critically important to protect the public and national security. During national, regional, and local emergencies, the broadcast EAS system is essential to saving lives. Stations participating in the EAS system must be able to transmit and receive interference-free signals. Pirate stations do not participate in the EAS system and do not comply with FCC's EAS rules monitoring and broadcasting EAS alerts. Further, unlicensed illegal stations interfere with licensed radio stations. Such interference affects EAS alerts that are broadcast by licensed radio stations. Thus, consumers located near a pirate radio transmitter will not hear the legitimate station's EAS alert."

[bb88]

> Jamming EAS(https://www.fcc.gov/emergency-alert-system). In the rural midwest, it's critical for distributing tornado warnings further from town than you can hear the sirens. Periodic tests are required, and there's a readiness report.

Not just the midwest. Anywhere there is not cell service where there might be some kind of an alert that needs to be generated. AM, FM, and the weather bands are often the only reliable signals that can be received in some areas.

Flooding, tsunamis, avalanches, nuclear power plant warnings, civil defense warnings, etc all have SAME codes.

See page A-13 for all the codes that SAME provides. https://www.nws.noaa.gov/directives/sym/pd01017012curr.pdf

[brightlancer]

> EAS failures due to interference causing poor signal does happen. From the August 11, 2021 Nationwide EAS Test: "There were 78 test participants on receipt and 32 on retransmission that reported failure to receive the test message due to poor signal. Test participants attributed the poor signal to interference, a weak signal from their monitoring source, or a weather-related complication."

Out of how many test recipients? If I'm reading the report correctly, it was at least 19,302, which means interference caused failure for < 0.6% -- and the overall failure rate was slightly higher than 10%.

https://docs.fcc.gov/public/attachments/DOC-378861A1.txt

I understand that any interference could potentially cause loss of life, but I wonder how much illegal "pirate" broadcasts are really a factor.

[rsaxvc]

Dunno - they don't break it down any further.

This may have some amount of survivorship bias in that remediated interference won't count toward those failures.

[Gordonjcp]

Doing something like that on VHF is utterly idiotic if you need to cover a large area, unless it's as flat as a snooker table.

Do it on SW, around 6MHz, and you'll cover a 300-mile radius with 50 watts.

[RF_Savage]

But weather is more local. 6MHz might not get that good of a range with low powers like that thanks to the really high amounts of man made noise on HF bands.

So the inherently local nature of VHF complements the inherently local weather alerts. Floods and tornadoes are not an issue at a distance of 300 miles.

[rsaxvc]

That seems rather limited compared to EAS in practice, since some EAS stations will relay appropriate alerts, and since EAS runs over both broadcast FM, AM, and TV stations.

[jrockway]

Real life communications have been interrupted by faulty sump pump motors and bad LED drivers. Actually trying to build a radio is no guarantee of success.

A long time ago, we were acquiring the shadiest RF devices possible to test against our WiFi routers, which should avoid auto-selecting interfering frequencies. One device we got was an analog wireless security camera thingie bought right from Amazon. "2.4GHz" it declared, so it was the perfect test case. We turned it on and there was no WiFi interference whatsoever. We got out the test equipment and... its carrier frequency was right on the edge of the L1 GPS frequency. You turn the thing on, there goes GPS for the neighborhood. It was impressively disastrous and we did not test it any further!

That's a company trying to build a radio product, for sale to consumers in the US market. I don't have a lot more faith in random people trying to build FM radios. Sure, you'll know if it doesn't work, because you tune your car radio to it and you can't hear anything. But unless you're careful, you can radiate a lot of power in the sidebands and the harmonics. That will trash licensed spectrum users, which is Not Nice Of You. (As for "amateur radio" operators, how to build and test radios is part of the exam, so there are a lot of great homebuilt radios floating around out there. Also part of the test is knowing when you can use non-amateur frequencies, and what the punishment for doing so is. Needless to say, not a lot of trained hams are building pirate radio stations. So that brings the likelihood of doing a bad job even higher; by definition, only the unlicensed and untrained are even trying this.)

This comment is already too long but I want to relay another fun fact. Building a receiver can interfere with other users of the spectrum; a common design mixes the incoming radio signal by a higher frequency, filters it, and then mixes it down to audio frequencies. If you don't shield this well, then your receiver is actually a transmitter on some random other band. Be careful and test your design with a spectrum analyzer. It's not rocket science but it's not trivial either.

[JumpCrisscross]

> can you explain to me how an FM transmitter, transmitting somewhere between 87 MHz and 108 MHz will interfere with your emergency services

In isolation it’s a dice roll. Normalise it and the game becomes power: since nobody is coördinating spectrum, the loudest transmitter (noisily) wins. Waiting until that predictable point doesn’t make sense, particularly given the FCC seems to have warned this guy in the past without handing out a fine.

[lajupechere]

It’s not emergency, but i would consider an airport to qualify. The airport on Orcas Island, WA had harmonics issues from someone running an illicit FM transmitter in 2018.

[RF_Enthusiast]

Another one is in California: https://docs.fcc.gov/public/attachments/DOC-128793A1.pdf

More recently, a construction permit on 107.9 was cancelled after it was determined that it could not co-exist with an airport. (If I find the link, I'll edit this reply and post it)

[FL410]

The transmitters near John Tune airport in Nashville regularly interfere with avionics. Been that way for years, and no action....

[RF_Enthusiast]

John Tune airport should complain. Interference with air traffic is one of the highest enforcement priorities with the FCC. They'd probably show up as fast as they could drive there.

[Gordonjcp]

> Okay, now, can you explain to me how an FM transmitter, transmitting somewhere between 87 MHz and 108 MHz will interfere with your emergency services?

The fifth harmonic will land squarely in the chunk of band between about 435MHz and 500MHz, and being roughly 200kHz wide even assuming they're not overdeviating like hell it'll obliterate a huge chunk of the band. Given how poorly constructed many "pirate TXes are it's almost a given that they'll have ridiculous amounts of out-of-band radiation.

There's one very popular design available on the Internet which I won't link to but will leave as an exercise for the reader, which has a rather nasty sproggie that's only about 10dB down at around 156MHz when it's tuned for about 104MHz. Now, that chunk of the FM broadcast band is quiet around here, but the sproggie passes through the transmitter's largely untuned (at least, broad as a barn door) PA and out the antenna.

The whole thing is not terribly efficient at radiating something 200kHz wide all over the bottom end of the Marine VHF band, but it sure is noticeable.

Source: literally my day job.

[RF_Savage]

I have measured some cheap aliexpross specials and they ware remarkably dirty. A forest of spikes around the fundamental thanks to parts of the TX strip oscillating. I guess the manufacturers just pulled every last bit of gain and output power from their transistors with minimal amounts of additional parts. And the end result is something you cannot run even if you had a licensed FM broadcast band frequency.

[Anarch157a]

Some years ago, here in Brazil, I saw a news report on TV where they played a recording from an ATC tower where you could hear a pirate station talking over their transmissions. The pirate station was operated by a church.