I read in an Amateur Radio magazine several years ago of a method to improve both transmit and receive for VHF and UHF handhelds. With these frequencies, the higher the antenna, the better the coverage.

The article mentioned that if you had access to the feed line of an external antenna of the frequency range needed, you could mount a rubber duck (the small flexible handheld antenna) to the end of the feed line where it would normally attach to the radio. Then hold the handheld's rubber duck close to that other rubber duck. Both transmit and receive would be improved because it is passively using the external antenna.

Before you ask why not just connect the handheld directly to the feed line, of course that would be best, but maybe you don't want to have to connect and disconnect the handheld each time to use it elsewhere.

You could utilize this approach with the 770 and a Cantenna by attaching the removable antenna from a wireless PC card to the feed line of the Cantenna and placing the antenna near the body of the 770.

The idea is basically the way a through-the-glass cell phone antenna works. No modifications to the 770.

Disclaimer: I've never tried this myself so I have no idea whether it will work or not.