At one time Gunnplexers were popular, but there is a modern equivalent available at low cost (HB100). These can be bought on eBay for example.
Although I have not looked recently, I expect someone (or many?) have made 10 GHz WBFM transceivers with them.
Similar modules are also available for 24 GHz.
Wideband FM 10 GHz transceivers could be made with the 10 and 24 GHz modules without any microwave engineering at all. At the focus of a dish the 10 GHz ones have been used for ATV contacts over 100km.
If you know of WBFM transceivers designed around these, I'd love to take a look.
A quick look on eBay showed the 10 GHz units were available for just over £2.
See https://sites.google.com/view/g3xbm4/home/vhfuhfmicrowaves/homebrew/10ghz .
UPDATE 1535z: The link Jan has put in the comment below (in French) looks interesting and is just the sort of thing I had in mind.
