Having etched a couple of circuit boards when I was a kid I thought I was probably up to building the 433MHz transmitter based on Hugh's schematic. With a little help from a friend I created a veroboard design and got to work with the soldering iron! I'm really quite bad at soldering but the components went together easily enough and works quite well. Chris Eilbeck, a very helpful regular on uk.tech.rocketry, suggested that I use a dipole aerial rather than a simple whip as it gives greater range.
Although the homemade transmitter works well enough it is not particularly compact and - especially given the doubts over the quality of my soldering - of unproven reliability! I started to look for small, commercially made alternatives and came across Marshall and Biotrack. Pete Davy spoke highly of the Marshall kit so I plumped for a 173MHz device from garlands@dial.pipex.com. The transmitter arrived the day after order and is very effective. It also has the advantage that most Brit rocketeers seem to use 433MHz transmitters so I'm less likely to get in the way of another tracker.
Several websites show yagi designs for home construction but with limited free time I decided to buy one. Note that yagis are "frequency specific", with the spacings of the elements differing according to the frequency you wish to receive. Biotrack sell a few yagis and appear to act as a reseller for Lintec Antennas Ltd as well as producing their own. If you can get one direct from Lintec you will make quite a saving, however they don't seem to hold much stock. I went for a yagi with flexible elements which seems to work well with the Yupiteru scanner and Marshall transmitter.
The technically minded or those on a tight budget might be interested in:
Cheap Yagi Aerials
Yagi Antenna Design
Basics