Most, if not all homemade detectors are omnidirectional. They are an antenna and a detector of some sort. They can give you an idea of the strength of the strike, and therfore a rough estimate of the distance away.
You can make one for free by tuning an AM radio between stations at the bottom of the band and listening to the static crashes. Those are lightning. (But you're a ham, you knew that.
The lightning network sites that I am aware of, like StrikeStar and the "homemade" European site at http://www.gearthblog.com/blog/archives/2006/08/homemade_networ.html
, use Boltek Storm trackers and either Nexstorm, or Lightning 2000 software.
Bolteks seem to have 2 or possibly 3 active orthogonal antennas in the housing, which via relative signal strength and polarity of the detected pulse indicate its azimuth and type. (The antenna cable is CAT5 which is 4 pair. That would be one pair for power (the antenna runs warm) and one pair each for a N/S, E/W, and horizontal loop. I'm not sure if the horizontal antenna would be useful or not.)
There are a lot of interface and conditioning electronics on the Boltek PCI card, which the software can interface with. I haven't really studied the card to decipher what chips and stuff are installed on it. They make a serial detector for laptops and other computers without a PCI slot, but it does on-board processing, and does not send as much data to the software on the PC.
Boltek seems to be the standard for anything more than a simple signal strength detector. I'm not sure there is anything else comparable, especially at the "hobbiest" price level.
All of this isn't to say that someone couldn't build or hasn't built their own direction and strength estimating detector. It would be an interesting challenge. I suppose one way to start would be with 2 1-wire detectors and 2 directional loop antennas at right angles, and write a little program to calibrate them and then estimate azimuth from the relative signal strengths. But that wouldn't tell you which direction along the line the strike was.
Hmm, maybe the Boltek does
have 3 antennas, and that's how it calcs direction.