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#300134 - 10/04/21 04:55 AM Re: Tramper rescued after finding pocket of cell cover [Re: Doug_Ritter]
paulr Offline
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Registered: 02/18/04
Posts: 495
I wish PLB's were a little more flexible, so they could be used for general purpose 2 way comms (like InReach) rather than simply summoning rescue at coordinates XYZ without even indicating what kind of emergency it is. They are also quite expensive. On the other hand, if they were widespread, people would be activating them foolishly all the time.

There seems like no alternative to satellite comms in the open ocean; but on land near some semblance of civilization, I've been wondering whether low speed digital ham radio modes could be used for this purpose. A pocket sized tranceiver with a wire antenna would likely be able to send and receive text messages at very low speed (a few seconds per character) with 100+ miles range. So it would be a matter of organizing protocols and receiver networks for emergency and personal comms with these things.

The transceiver would be something like this: https://hackaday.com/2021/09/25/the-simplest-ft8-transceiver-youll-ever-build/

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#300135 - 10/04/21 11:48 AM Re: Tramper rescued after finding pocket of cell cover [Re: Doug_Ritter]
Ren Offline
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Registered: 11/05/07
Posts: 443
Loc: Wales, UK
Onland you can just increase cell phone coverage.

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#300137 - 10/04/21 05:36 PM Re: Tramper rescued after finding pocket of cell cover [Re: paulr]
chaosmagnet Offline
Sheriff
Carpal Tunnel

Registered: 12/03/09
Posts: 3689
Loc: USA
Originally Posted By: paulr
I wish PLB's were a little more flexible, so they could be used for general purpose 2 way comms (like InReach) rather than simply summoning rescue at coordinates XYZ without even indicating what kind of emergency it is. They are also quite expensive. On the other hand, if they were widespread, people would be activating them foolishly all the time.


PLBs take advantage of public infrastructure (the COSPAS/SARSAT system, installed as instrument packages on GNSS and weather satellites operated by the USA, Europe, India and Russia). It was a big investment to get it going and the instrument packages would need to be replaced or at least upgraded in place before something like that could work.

As far as I know, all SENDs use Globalstar (a publicly held satellite communications company) or Iridium (also publicly held but at one point at least propped up by the US government). These are dedicated platforms and use newer technology than than the COSPAS/SARSAT program envisioned when it was started in 1979.

Quote:


Now you're making me want to build a pocket FT8 transceiver kit to try out.

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#300138 - 10/04/21 07:35 PM Re: Tramper rescued after finding pocket of cell cover [Re: Doug_Ritter]
paulr Offline
Addict

Registered: 02/18/04
Posts: 495
That's interesting about the satellites, thanks.

FT8's technology is brilliant but the FT8 protocol itself is crazy imho. If you're not familiar, it's an almost pure contesting mode: every contact is exchange callsigns and signal reports, done. There is NO place for any personal communication, even "hi!", in the template. This is alluded to in the hackaday thread. But there are related modes like JS8call and Olivia that are more flexible. I do think a pocket tranceiver with a pocket antenna is feasible, especially if the other end has a bigger antenna.

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#300141 - 10/04/21 11:26 PM Re: Tramper rescued after finding pocket of cell cover [Re: Doug_Ritter]
chaosmagnet Offline
Sheriff
Carpal Tunnel

Registered: 12/03/09
Posts: 3689
Loc: USA
Receiving is a whole different game from transmitting effectively and generally way easier. The following summary is based on needing both.

Small antennas are always a compromise, but less so for higher frequencies (VHF and higher). But those frequencies are very difficult to use beyond line of sight. It can sometimes be done (moonbounce and Sporadic E come to mind) but may not be possible at all with portable antennas and even less so when using very low power.

By contrast a PLB will typically be using 5W of transmit energy to a fairly short antenna (3 feet or longer, I think) using UHF to transmit to satellites within line of sight.

Lower frequencies, such as the various Ham HF bands, can get truly spectacular propagation, especially with digital modes, even at very low power. This uses skywave propagation, but only with a much longer antenna than typically used for mobile VHF and higher frequencies.

Portable antennas for HF can work, but they require long wires. The rule of thumb is that the lower the frequency (meaning the longer the wavelength) the longer your wire needs to be to work well. I have portable long-wire antennas for HF and while they *could* fit in a pocket, most of them do not — thicker wires, wire winders, Baluns, Ununs, traps and so on add to weight and bulk.

Long wire antennas work way better when they’re up off the ground. 10-30 feet should be considered a minimum.

EDITED TO ADD: There are backpack "portable" antennas that work without needing as much in the way of long wire or height off the ground, but they are not pocket portable.

Skywave propagation changes drastically between day and night due to the Sun’s affects on Earth’s ionosphere.

The rule of thumb is that the Ham 20M band is often at least somewhat usable day and night. Higher frequencies often work better during the day and have very poor propagation at night. Lower frequencies often work better at night and have very poor propagation during the day.

My crayons-on-napkin estimate is that an emergency transceiver using a digital protocol somewhere close to the 20M band would work “okay” day and night, but would require significant terrestrial infrastructure to give reliable coverage. Your end-user would have to be able to get a long length of wire up off the ground for this to work at all. If we expect our hypothetical end-user to get the wire ten feet off the ground, we’ll need a lot of listening stations to get good coverage. At thirty feet off the ground we’d need fewer stations.

I like the idea of this a lot — I have a small HF rig designed for backpack portable use — but I think the end-user expertise level may leave this as a solution for hams rather than the general public.


Edited by chaosmagnet (10/05/21 01:30 AM)
Edit Reason: clarity

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