At first I thought the "unmanned tunnels" description was just a way to avoid broadcast regulator scrutiny, but it does look like it's genuinely designed to be used underground as part of an emergency alert system. That led me to "leaky feeders", a type of broadcast antenna used in mines and tunnels.
I did a more aggressive internet search. This seems not possible given physics, as well as not documented (at least in the US) in the CDC Mine Accidents Database [0], which has been recording mine accidents since before the discovery + invention of AM radio.
Edit: The physics
- (lambda) = c / 1,620,000 Hz = 185 m :: 1.62 MHz is what I derived as a near max possible accidental frequency able to be produced by AM equipment
- 185 m / 2 = 92.6 m :: this is half a wave length
In order to resonate (let alone have enough power to "cook", which I didn't even look at because the wave can't even resonate), a tunnel must be at 92.6 m (fundamental) or 185 m wide or tall (2nd harmonic). Most tunnels are ~5m/3m wide/tall at most.
Dusted off my physics from my minor in college so someone feel free to correct me.
Bollocks the wavelengths are on the order of hundreds of meters, there is no way you get microwave like heating out of that. Even at 30 MHz you're still looking at 10 meters wavelength, 3 meters at 100 MHz.
This system operates according to TFA up to the end of the AM band at roughly 1600 KHz, so 180 meters and change.
The danger is more likely there because someone might enter the tunnel and hit the feeder, which depending on the design can carry considerable power.
This is basically hilarious. Leaky Feeders are a few watts. and even a high powered multi-kW AM radio with 200m wavelength wouldn't resonate much in a rough walled tunnel multiple sq meters in cross section. It's both too large for there to be significant power density, and much less than a wavelength in diameter except in length where the tunnel passively attenuates the signal.
I must be missing something ... Why broadcast speech over AM in an 'unmanned' tunnel ?? who's gonna hear/receive it ?? I wish the repo had some sort of use case summary or something...
Edit: I'm also sorta puzzled by the choice of AM in any sort of 'alert' context...Do people still listen to/use AM radios?
AM radios are extremely simple and utterly fool proof, they can be made with only a handful of simple parts. Even so, these systems are archaic and these days you can make much more complex radio systems using a SDR which effectively reduces the whole radio to a pre-amp an ADC, an embedded CPU and some software.
I would assume because restarting would be saying "it's ok if the control system in our emergency transmitter sometimes fails, so let's just pretend it didn't happen and ignore it! yolo!".
I think a reasonable approach would be to have a redundant system, that gets activated if failure occurs in the first, and blasts a full alarm message to "abandon the tunnels, emergency system has failed!" type message.
Could obtain better quality at the higher channel counts by phase shifting the audio for each channel such that the modulation peaks do not exactly align for each (as they do now). Even inverting the audio for half the channels would help.
This seems like a crude way to do it.. why not provide all 110 carrier frequencies by using a polyphase channelizer? The bandwidth of the entire AM broadcast band is pretty low..
Yes, that's very true. It would make much more sense to hit all channels, at the same time, maybe they want to use some of the channels for comms even during an emergency?
https://en.wikipedia.org/wiki/Leaky_feeder
Edit: The physics
- (lambda) = c / 1,620,000 Hz = 185 m :: 1.62 MHz is what I derived as a near max possible accidental frequency able to be produced by AM equipment
- 185 m / 2 = 92.6 m :: this is half a wave length
In order to resonate (let alone have enough power to "cook", which I didn't even look at because the wave can't even resonate), a tunnel must be at 92.6 m (fundamental) or 185 m wide or tall (2nd harmonic). Most tunnels are ~5m/3m wide/tall at most.
Dusted off my physics from my minor in college so someone feel free to correct me.
[0]: https://wwwn.cdc.gov/NIOSH-Mining/MMWC/MineDisasters/Table
This system operates according to TFA up to the end of the AM band at roughly 1600 KHz, so 180 meters and change.
The danger is more likely there because someone might enter the tunnel and hit the feeder, which depending on the design can carry considerable power.
Edit: I'm also sorta puzzled by the choice of AM in any sort of 'alert' context...Do people still listen to/use AM radios?
Why?
Also, surely they don't mean unmanned, who is listening to the AM emergency broadcast then?
I think a reasonable approach would be to have a redundant system, that gets activated if failure occurs in the first, and blasts a full alarm message to "abandon the tunnels, emergency system has failed!" type message.
This seems like a crude way to do it.. why not provide all 110 carrier frequencies by using a polyphase channelizer? The bandwidth of the entire AM broadcast band is pretty low..