The Optoelectronics, Inc. line of Nearfield Receivers include the Xplorer and X Sweeper.
Our nearfield receivers lock on signals that are 15dB above the background RF noise floor. The quieter the noise floor the further the pickup distance will be. Typical pickup distances for 5 watt radios are around 800 feet, and 200 feet for 50mW transmitters.
Both the Xplorer and X Sweeper are great tools for going into an area and quickly identifying signals that are in the nearfield area. Both units sweep their entire frequency ranges in about 1-2 seconds, ensuring that if there is something transmitting you will be able to locate it very quickly.