You can calculate the optimum antenna length if you know the formula, but if you are going to be transmitting on the aircraft emergency frequency (121.5 MHz) then I would try to jury rig a dipole antenna with end-to-end length approximately from my left shoulder to the fingertips on my outstretched right arm (approximately 3.65 feet). In an emergency situation I doubt I would be capable of building a directional antenna (and if I did, it would be a Yagi, not a parabolic dish).
I have a VHF aircraft h/h from my days as a private pilot, and I have been known to throw it in my backpack if I'm going backpacking. Mine is from Sporty's Aviation (
www.sportys.com) but as Doug says, if you're not going to use it for its intended purpose, you're better off spending the money on a PLB.
If you're a pilot and have one, it should always be with you in the plane. (Except I'm sure the TSA would probably find it suspicious - I'm waiting for the day when they refuse to let the flight crew board the airplane because they find it suspicious for three guys with pilot's licenses to be travelling together <img src="/images/graemlins/wink.gif" alt="" /> )
Far more useful than being able to rig a better antenna, though, would be to know the frequencies in use. Airliners are supposed to monitor 121.5 but only if they have a spare radio. Aircraft at altitude will, however, be communicating on the Center frequency. (Swing by your local flying club and they will be happy to give you a list of useful frequencies, especially if you listen to a brief sales pitch <img src="/images/graemlins/wink.gif" alt="" /> )
(Go to the ETS Home Page, scroll down to the link ?A Question of Survival?, and read William Waldock?s excellent article.)
Also, if you're able to travel at all, you might be better going up than down - from the east side of the Canadian Rockies, you might be able to contact Ground Control in Calgary, but from the west side you'd be lucky to contact an airliner flying overhead.
Also, if you do have to transmit, make sure you say which frequency you?re using. In one case that I know of, a US military AWACS on a training flight picked up a distress call, then had to spend several minutes trying to figure out which frequency the pilot was using, out of the dozens of frequencies they were monitoring. (He was using 121.5 - ?Victor Guard?, which was probably not a huge surprise to them, but there are many other emergency frequencies: 245 MHz for airborne UHF, another one for marine radios, another one for HF, etc.) The pilot in that instance did survive, although whether he ever figured out just how lucky he was to be able to contact anyone after flying into a mountain canyon in bad weather is unknown. <img src="/images/graemlins/wink.gif" alt="" />