Actually, I believe I could build a nice 5 watt Hf rig set to 2182 Khz with an automated repeating SOS on CW that would run for about 12 hours or so. Someone is always listening on that frequency, and you would be DF'd with a fairly tight fix after the first hour or so of transmitting. Get into trouble? No problem, just flip the switch and let the little black box do the job from just about anywhere in the world. Try finding a good PLB for $50 these days.

You could do the same on 500 Khz, but I think the ERP at 2182 Khz is going to be better from a pocket portable device. I'd probably try and scavenge a bar mag antenna from an AM portable receiver and see if I could get a decent Z at frequency with a Hi-Q disc cap or some such.