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.