There seems to be quite a bit of cross-over on this board between ham radio and HSM enthusiasts. Here's a 'not-quite-finished' project that has been keeping me busy, and may be of interest to those getting into microwave experimenting...
It's a slotted line, for measuring impedance (including SWR) from about 400mhz up to about 4ghz.
In a nutshell, it's like a piece of coaxial cable that has been opened up and flattened so a small antenna (barely visible in the third photo) can travel along down inside the gap between the two slab sides.
By connecting a level detector (a glorified crystal set for those who built them as kids) to the antenna through the fitting on the top side of the carriage, the rf level on the copper center conductor can be measured at any point along the line by sliding the carriage back and forth.
It's sensitive enough to measure input impedances on things like amplifier circuits, and robust enough to measure SWR on antennas and feed lines - very handy for all sorts of things.
Spacing and sizing of the center conductor and side slabs is critical, as is keeping the little antenna centered in the slot.
To keep the impedance constant between the air insulated part (between the slabs), and the teflon insulated cables/connectors on the ends, it was necessary to fabricate a tapered cone affair (brass cone inside a teflon 'funnel'), which also serves to hold the whole thing straight, while making the electrical connection to the input/output connectors on the ends.
Its been a fiddly job getting the tapers right, and getting the carriage to slide easily without slop - still a few high spots to polish down, and some 'prettying up' to do. That said, initial testing indicates it's close to a perfect match - once a series of tests are done, its intrinsic swr can be worked out, and a required 'adjustment' value calculated. From that point, very accurate measurements are possible at remarkably high frequencies.
While the technology has been overtaken in the commercial world by computer controlled vector analyzers and such, Hewlett Packard did well over the years selling a very similar device which is still an economical solution for those without kilo-bucks to spend on exotic test gear. I borrowed heavily from their (50 year old) technical journals to figure out how to build this one.