Would it be possible to use this as the infrastructure for a clandestine, low-bandwidth, text-based mesh network? An internet for nerds kind of thing.
Firstly, it's not a new idea. Folks use JS8call (formerly FT8call). It's quite sophisticated and can work even below the noise floor. Folks have even gotten it to work in pretty ridiculous conditions. It has comically low bandwidth but works at comically low power.
That said, the only place I think this would be "clandestine" is in urban environments where there is a lot of reflection. Modern electronics and antennas get the benefit of very high fidelity simulation, so finding a signal with professional gear (especially if it's cutting edge and understands modern maps) is pretty wild. I saw a demo of a very expensive system that used Google Earth as its UX a few years ago, and it could do some amazing things.
Even in cluttered environments with low power transmitters, it still isn't very secretive. Amateur radio does this thing called "fox hunting" where they use a variety of handheld elements to quickly track down low power signals. Some of these contests get pretty technical, with the target transmitter being highly directional on some axis and deliberately casting false reflections, and lately they're entirely solar/lifepo4 so they're power constrainted.
But folks with a bit of training and practice can find them really quick.
If you want to be clandestine, then actually what you want to do is overlay your network over existing networks so that your traffic is difficult to isolate from a computationally infeasible flood.
Depending on how creative one was about the locations and operation times of the transmitter, I'd expect it'd be pretty difficult to find.
What about a transmitter temporarily sent aloft on a balloon, model rocket, or drone, then making its brief high-bandwidth transmission and then parachuting or landing, being retrieved by its operator who then disappears until the next transmission from another location?
It might even be disposable, and potentially either deliberately sink itself in to an ocean or other difficult to access place, and/or self-destruct to make recovery and forensics more difficult.
This isnt true. High data rate requires large channel size, which is certainly more commonly found in >1GHz portions of radio spectrum that require line of sight. But, a 10MHz channel is just as fast at 400MHz as it is a 4GHz. HDTV doesnt require line of sight to gets tens of megabits on VHF/UHF channels.
And even worse than the answer to that (nowhere) is that in the USA the data rate limit is actually in terms of symbol rate, not bandwidth used, to a mere 300 baud for the bands that have regular non-line of sight propagation.
I never implied you could? Its certainly possible in VHF/UHF, though (for licencees).
> And even worse than the answer to that (nowhere) is that in the USA the data rate limit is actually in terms of symbol rate, not bandwidth used
Sure, but those limits only apply for HF and longer-wavelength frequencies. You're welcome to do 256QAM in the microwave range, for example.
>You're welcome to do 256QAM in the microwave range, for example.
Which gets back to line of sight propagation (tropo is not common) and everything I said in my first post.
I've tried the home to car omnidirectional link thing on 902-928 MHz. All that matters is line of sight. I tried a 25w bidirectional amp (ham general license) and that didn't help. I tried bringing the ubiquiti transceivers w/broadband hamnet firmware all the way down to QPSK modulation and the narrowest channel I could. I even tried using narrowband 56k telemetry dongles. I customed designed and built my own microstrip based filters for the front ends.
The only thing that really helps, and it's not enough, is height above terrain. When I go park my car on the hills outside of town megabits are possible. But the slightest hill or valley losing LOS kills it.