A spectrum analyzer is a phenomenally useful piece of test equipment when working on RF circuitry. While used, several generation old spectrum analyzers can be found fairly reasonably priced, network analyzers are usually out of reach for a home lab. However, with a little bit of additional equipment some of the measurements that would normally require a network analyzer can be done on a spectrum analyzer.
For a project I am currently working on (to be detailed shortly), I needed an oscillator capable of covering 8 to 12 GHz. There are quite a few of the miniature Stellex units on eBay, but they generally don't go much past 10 GHz. There are also tons of Avantek modules, in their standard round format, but a lot of them are custom part numbers with no specifications known. Since this particular project wasn't very sensitive to any of the parameters, I waited for a cheap one to show up with the right range and went for it. With a little investigating, it wasn't very difficult to determine the approximate specs for the module I purchased.