I'm
a novice to radio in general and am looking for pointers to information
on choosing the right kind of antenna to that is small enough to fit.

In the car, I don't have much more space (I could use the car's antenna
instead), but it would need to be omnidirectional.



Getting any kind of directivity at 150MHz with small aerials is very
difficult. The wavelength at 150MHz in 2m so you can see that element
lengths will be quite long. One possibility might be a ferrite rod type
antenna if you can find some vhf ferrite. Some older pagers used them so
that may be a source. However you may still struggle to get very good
directivity.

Perhaps an active solution may be a better bet. Try Googling for
Handifinder, which is basically 2 small antennas that are switched at a fast
rate, this produces a tone in the receiver which vanishes when both antennas
have equal path length to the source.

Regards
Jeff