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Is it possible to transmit a short message across the Atlantic ocean from the UK to the US via shortwave? From what I understand, long-distance shortwave communication has lower bandwidth than other forms of communication such as microwave which uses line-of-sight. So by a short message, I mean maybe around three bytes.

Please excuse any poor phrasing of my questions/lack of knowledge. I am new to Amateur Radio.

Some questions:

1. To send a message of around three bytes over such a long distance, would this require an antenna that uses alot of power?

2. Are there companies that enable users to rent space on antennas that would be capable of transmitting messages over such long distances? Or would this require a completely custom infastructure(e.g. buying land and building a tower(s) with the necessary antennas)?

3. How reliable would this be? I am aware that messages transmitted via shortwave may be subject to scrambling/interference due to sunspot activity/ionosphere conditions and poor weather.

If there are any other things to consider for this theoretical scenario (e.g. licensing, costs etc), I would appreciate any information on them.

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    $\begingroup$ Hello, and welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Dec 17 '19 at 23:46
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  1. To send a message of around three bytes over such a long distance, would this require an antenna that uses alot of power?

No. There's a huge variety of tradeoffs here, but as a quick calibration, WSPR sends 50 bit messages (so, a bit more than twice what you're asking) all around the world using power levels usually ranging from several milliwatts to ten watts. Its main disadvantage is that it takes almost two minutes to send such a message. There are digital modes that are quite a bit faster, that can still do the job with less than 100 watts (which is considered an "ordinary" amount of power for an amateur).

Are there companies that enable users to rent space on antennas that would be capable of transmitting messages over such long distances? Or would this require a completely custom infastructure(e.g. buying land and building a tower(s) with the necessary antennas)?

Hmm, maybe? I'm not familiar with rental, but I imagine it's possible. However, you might need far less in the way of land and antennas than you think, and no towers at all. It's hard to get into specifics without knowing details of your situation, but a simple wire antenna on a rooftop or a "small transmitting loop" antenna could potentially get the job done.

How reliable would this be? I am aware that messages transmitted via shortwave may be subject to scrambling/interference due to sunspot activity/ionosphere conditions and poor weather.

Less than 100% reliable — sometimes solar flares and the like make HF pretty close to completely unusable. But on most days, there will be at least several hours a day when there is at least a 90%-reliability path from the UK to a given part of the US. When those hours occur and which bands they occur on depend on solar cycle, time of year, and location (the US is a big place).

If there are any other things to consider for this theoretical scenario (e.g. licensing, costs etc), I would appreciate any information on them.

Licensing is probably the sticking point. I imagine the communications you want to do are probably not within the rules of amateur radio, so another form of licensing will be necessary. I'm not especially familiar with the UK but it seems like there's no out-of-the-box license type for this kind of use, and requests are handled case-by-case by Ofcom. In this regard, working with a third party is probably to your benefit.

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    $\begingroup$ Love this answer! Last point, most importantly. I'd imagine it being practically impossible to get a commercial HF license – that spectrum's been divided, eons ago, and every stakeholder resists change with great vigor. And, technically, any satellite paging system would be more reliable and probably use less power. $\endgroup$ – Marcus Müller Dec 18 '19 at 14:42
  • $\begingroup$ Apologies for such a delayed response, and thanks for a great answer. With regard to WSPR, would you be able to explain why this takes almost 2 minutes to send? I would have imagined the actual wave propogation would be roughly 3x10*8m/s, so is this latency value for signal processing either side of the transmission and reception? My main objective is to make this long-distance, short message transmission as fast as possible. $\endgroup$ – p.luck May 31 at 8:53
  • $\begingroup$ @p.luck no, the transmission itself takes two minutes (actually just over 111 seconds), at slightly less than 1.5 baud. The low rate is what makes it reliable — a decoder can sample several hundred milliseconds of every symbol to try to decorrelate it from the noise. $\endgroup$ – hobbs - KC2G May 31 at 9:09

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