Saw this homemade TV antenna design linked on Lifehacker, decided to try it out since it's simpler than any other DIY HDTV antenna I've seen so far, and I had the balun (the thing needed to hook up the wires to a TV or receiver) sitting around in my parts bin. It works suprisingly well and it's simple and quick to make using just aluminum foil, cardboard, a couple of wires and a balun.
I've seen more powerful antenna designs on the net, based on the Gray-Hoverman or other designs. This won't match the performance of those antennas, but it works well enough to compare well with the HDTV antenna I bought from Amazon and spent way too much on a few years ago when I was trying to get my family to cut the cord and drop cable.
I'm kicking myself for buying that "HDTV" antenna back then. It looked cool but never maintained a strong signal without drops in picture. If I'd known a DIY solution would have worked as well I could have saved that money. This antenna design doesn't look as good, but that's the only downside for me. I can just hide the antenna away behind the TV or out of sight, and that solves that problem.
If you don't have a balun you can probably use a coax cable and simply strip one end, then connect the inner wire to the aluminum on one cardboard square and the outer screening wires to the aluminum on the other cardboard square. I imagine that would work, but haven't tried it myself.
Using coax in place of a balun is what this guy did with his beer can dipole antenna. It's a clever use of an aluminum can, and while it's ugly as heck it's still effective and remains small enough to hide behind a TV. That design can be made more effective by putting an aluminum reflector behind it. I'm considering adding a foil reflector to my cardboard antenna and seeing if that could improve performance, but aiming it correctly towards the local TV towers picks up the signal fine.
It's amazing what you can do with aluminum foil. I've built simple paper/tape/foil reflectors to boost my WiFi signal and those are very effective, but I never knew you could pick up 1080i/720p HDTV signals over the air with just a simple aluminum antenna design. Duct tape and aluminum, these things make the world go round, for DIYers at least.