Internals.... Its a hybrid coupler, made for just a single band, you have two resonant circuits back to back and you get different phase signals out. You calculate the value of L and C you need to build based on the XC and XL values. (See previous posts for the links on those or buy the ON4UN book and wade through the maths). Note the C between the windings in the coils is important as well.
Built for way more power than I'd ever use but this will be usable up to KW power levels. A 100% QRP unit can and is much much smaller.
I have a good image "somewhere" that shows the phase differences between the signal in and the 4 antenna ports so you can see how the outputs change so you get constructive and destructive interference and thence gain in one direction or another. You can then select different antennas positions to act as 'primary' and so you 'move' the pattern of constructive/destructive interference around.
Anyway it works and does what I want it to.
I'll add the GP masts and wires bits here...