[owenversteeg]

So, this raises a question I've been meaning to ask for a while. Let's say you want to transmit a very small amount of information over a very long distance (say 80 bytes over a minimum distance of hundreds to thousands of kilometers.) What options are there that are relatively small-sized?

Let's say maximum receiver size is roughly 10cm by 10cm, and power consumption can't be over 10 watts, but the "base station" can be as large and powerful as you want.

[edit] cause this is getting some upvotes, here's some thinking I was doing: you can't go through the ground (unless you're at ultra low frequencies and those need antennas measured in tens of kilometers long). So you pretty much have to bounce stuff off the atmosphere, right? When you see stuff like this [0], with thousands of kilometers of range, I imagine that's what happening. Especially as that's 10MHz, which is 30 meters wavelength, which bounces off the atmosphere pretty well IIRC. The only thing with that is that the powers are incredibly low and the antennas are huge. What if you increased the power but decreased the antenna size, for example a 10cm x 10cm antenna with 10W? 100W? 1kW? Would transmissions across thousands of kilometers still be possible?

[jdietrich]

The Shannon-Hartley theorem is your friend. The theoretical maximum data rate of a channel is the product of bandwidth and SNR. With a sufficiently low data rate and/or a sufficiently wide bandwidth, we can effectively communicate at well below the noise floor.

There are many propagation modes to choose from. HF frequencies (~3MHz to ~30MHz) will propagate via the skywave mode, bouncing off charged layers in the ionosphere. By using codings with extremely low data rates (JT65, QRSS), amateurs routinely make contacts over thousands of miles on a fraction of a watt. Unfortunately, this mode relies on sunspot activity and we're currently at a minimum in the solar cycle. Frequencies below 3MHz will diffract around obstacles and follow the curvature of earth; unfortunately an efficient antenna at these wavelengths is enormously long, so a small system would have absolutely vast antenna losses. NVIS propatation is usable between about 3 and 8MHz, although you're limited to about 600km in a single hop. Multi-hop propagation is possible, although the path loss increases exponentially. Satellite is the other obvious propagation mode and is surprisingly accessible to amateurs.

Solutions to the Shannon-Hartley equation that involve very wide bandwidths are sadly underexplored by the amateur community, because of an FCC requirement to use the narrowest possible bandwidth and the relatively meagre frequency allocations available to amateur operators. The extraordinarily wide bandwidth of a modern direct-conversion transceiver offers some tantalising possibilities.

[newman8r]

take a look at https://en.wikipedia.org/wiki/QRP_operation - there's an entire community dedicated to this pursuit. Also take a look at https://en.wikipedia.org/wiki/LowFER - you can even use 160-190 kHz under FCC part 15 in the US.

The data rates would be very low, but for experimental purposes it's viable. The cool thing is there's a ton of simple circuits you can build on your own that other enthusiasts have put online.

The requirement for a small antenna is hard to accommodate, but a lot of people find creative ways to deploy something with a low profile.

I also think the idea of using drones for microwave links has some potential - haven't seen much on that yet but I'm sure people are doing it.

[PoachedSausage]

What you are describing there is basically longwave AM radio broadcasting. Large transmitter, small transistor radio receiver.

Low speed data can be and is sent using such transmitters, it is done by phase modulating the carrier. The data can be used to switch electrical appliances on and off.

https://en.wikipedia.org/wiki/Radio_teleswitch

https://en.wikipedia.org/wiki/Droitwich_Transmitting_Station

Droitwich puts out 500kW and reception range is on the order of 1000km.

[owenversteeg]

Right, but that's only one direction. I thought I put it in my comment, but I'm only curious about bidirectional stuff. The best path forward here seems to be WSPR- like stuff, but that requires larger transmitters on the order of tens of meters.

[PoachedSausage]

That's the problem, an asymmetry in transceiver size does not really lend itself to bidirectional unless its relatively short distances like cellular phone networks.

[fulafel]

To game the unlimited base station rule in these criteria: you could use an EMP weapon type of transmitter. Apparently you can get tens of terawatts with flux-compression generator + vircator/reditron systems, or more with nuclear devices. The pulses are quite short so I'm not sure how the transmitted payload would be encoded, but I'm sure a RF engineer could tell you.

[LeoPanthera]

You could use magnetic loop antenna for transmitting. They're pretty small but only work at once very specific frequency.

[tlrobinson]

Take a look at the various amateur radio HF digital modes like JT65 and WSPR.

[pmorici]

Iridium SBD?