For portable operation in the 14-30 MHz range, I use a small, handheld
transmitting loop. Here is a web page describing it and how to build
one from inexpensive materials. The page also links to other pages
with good information on loops.

http://dt.prohosting.com/hacks/antenna/magloop.html

I chose the loop because it is actually possible to operate while
walking around. Obviously if you are able to erect a larger antenna
at your operating site, you would be able to get better performance.
My loop is about 30% efficient at 14 MHz, theoretically delivering
about one S-unit less than a half wave dipole.

David
http://www.qrz.com/n5izu