Another way could be the use fo a modern integrated receiver with RSSI capibilities such as TI CC1101 (sub-1 GHz) or CC2500 (2.4 GHz).
The idea is based on the capability of measure the received signal amplitude inside the IC bandwidth (usually you can trim it, so you will have a RBW parameter).
TI has developed a project (with a student, I think) available in their web site.
To enlarge the available bandwidth my idea is to build a kind of superhet receiver using a wide tuning synth and a downconverter (and a SAW optional), like the one used in basestation, in front of the receiver.
Mazz