I recently built a RF detector device, complete with Basic Stamp, LCD display, 2-stage LNA amps, selectable filters, mixers, microprocessor-programmed frequency synthesizer for generating the LO, an RF detector, and a directional antenna. You can select narrow or wide bandwith (5 to 20 MHz) and any frequency from 750 to 1800 MHz. It's got really good sensitivity and can pick up cell towers from several miles away. The entire package cost around $800 and took about 6 months to develop. Unless you have alot of time and expertise, I suggest you buy a commercial device.