There are great web sites out there for good information. For radio direction finding projects check out this one:
http://home.att.net/~jleggio/projects/rdf/rdf.htm
You can click on the link for antennas or go directly to the three element tape-measure Yagi at this link:
http://home.att.net/~jleggio/projects/rdf/tape_bm.htm
I use the Arrow Antenna four element Yagi for mounting on the car. Breaks down fast for storage, ridged enough to handle highway speeds. Have also used it for foot hunting but is very hard to handle in corn fields, scrub oak, trees, etc. The tape-measure Yagi is the way to go for tree and bush bashing as the elements are flexible. It is also cheep and easy to replace a damaged element (in a hotel room the night before the balloon launch). Very safe for children to use. It can be difficult to use if the wind is blowing as the elements will flex. Always use a good quality one inch wide steel tape and remove the paint prior to soldering. Trim the end of the elements with scissors to prevent sharp edges that may cut. For extra safety against sharp edges you can coat the ends dipping them into a plastic resin tool handle dip or tape them with electrical tape.
Another great site for ARDF is:
http://members.aol.com/homingin/equipment.html
And the home page is at: