An excellent APT site is:http://poes-weather.com/
They have superb free APT decoding software. In the Articles box at the right of the home page, they have links to receiver and antenna articles. The double cross antenna works nearly as well as a quadrifilar, and is much easier to build. For best results, you will need a receiver with a wider bandwidth than a scanner, but if you already have a scanner, it can work in a pinch until you get something better.
Ham radio folks have recently discovered that certain European DVB-T dongles (USB tuners for European digital TV) can be used as an inexpensive front end for a software defined radio that covers 60 - 1700 mHz, with a few gaps, when used with appropriate SDR software in the PC. Most SDR software has user definable bandwidth, so this could work well for APT. These dongles used to be $10 on e-Bay, but since this new use has been found, they have gone up to $20-$30. You want a DVB-T dongle that uses the RTL2832U chip with an E4000 tuner. Google RTL-SDR and RTL2832 for more information. You would probably need one PC for the SDR software and another for the APT decoding software. Here is a link to my current favorite RTL SDR software, although since this is pretty new, things change rapidly.http://sdrsharp.com/