[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.