AM is quite easy to track, as well. In fact, the old marine direction finders sold years ago (before loran C and GPS) used land based AM stations for reference points. Any signal that can be received can be tracked, it's just logical, really. Spread spectrum is a bit harder because the tracker won't ...
One approach to reducing the physical length of the antenna is to play with the electrical length through the use of tuned loading coils. There are losses, of course (damn physics!) but you might find a trade off you'll accept.
Commercial units from Carl the AM Antenna Guy: http://antenna18431 ...
Along with posting some info about the PC Max card and a link to this site, I've added a tutorial and some other (I hope you'll find) interesting info.