One of the easiest portable 2M antennas to build for ARES/RACES work is the 300 Ohm Twin-Lead J-Pole antenna. It can be rolled into a very small coil for transportation and storage yet perform almost like a 1/2 wave dipole. Claims of antenna gain range from 2.4 dbi (1/2 wave dipole) to 5.1 dbi, depending on who you wish to listen to. I'm skeptical - I believe it is more like a good 1/4 wave ground plane. The radiation pattern is omni directional and very much like a 1/4 wave antenna with an excellent ground plane.
The simplest description I have found is documented at WB3GCK's site, while the most comprehensive dissertation is at W4RNL's site.
Other links of interest are:
If you find any other interesting links or information, let me know and I will include it here.
I have seen this basic diagram in at least five different sites. I do not know who did the original, but there are variations in several of them. The dimentions here seem to work as well as any (diagram is not to scale [duh]).