What bandwidth do you need? What field strength is the transmitter developing at your required listening position, and hence what antenna efficiency and bandwidth are required?
These are the drivers for this sort of discussion.
Ah I don't really want to design anything. I just wanted some kind of reference graph (perhaps to put in a publication/handbook about bug detecting/covert espionage devices). Touchy subject but being able to detect these things, probably made by someone alot richer then you, is invaluable towards having peace of mind (and if countermeasures are widely available/known such a device is less likely to be used due to paranoia of getting caught).
Like, if you are looking at a small object, and you only have a spectrum analyzer and antenna handy, what frequency ranges should you look at given the circumstances. (now its fairly common/cheap and useful to own this equipment, compared to buying specialty bug detection gear.. spectrum analyzers are well proliferated). If something is chirping/using occasional transmission, its very useful to know where to look.
Also of course, the peaceful application would be for a design engineer, or systems engineer, industrial designer, product designer etc, that has a vision of what his design will look like, and wants to write a specification for it... giving them a bit of rule of thumb knowledge about RF, so they know that yea., in this doodad im imagining its only practical to use frequencies between X and Y. ( i noticed alot of people really look at form factor/size/industrial design when it comes to selling a product.... the last thing you want is to get through a bunch of design work to find out that some specification is not capable of being met well with your existing thing that looks really appealing, customers liked, etc). A bit of 'rf engineering for the common man' kinda