the scheme for waveguide coupler

waveguide coupler 7db

I need design the waveguide coupler for 8mm,the coupling is -4dB and -7dB,the direction is -20dB or -25dB.I want to know which type maveguide coupler can be used.give me some suggest.

I suppose You want couple signals coming from WG's rectangular ports, and TE01 is exiteded.
The solution adopted in precision coupler is the following:
The coupler is basically build as 2 waveguide with almost 1 common wall.
Il may be the H or the E wall.
1/4 lamda spaced holes provide the coupling.
As the hole diameter is great and as the wall thikness is small, as the coupling il large.
The most easy value of coupling spans from -30 db to - 10 dB.
Larger copling value (i.e. -4 or -7 dB) require largr holes and became machanically and geometrically hard.

Another solution is the "T junction" and its variants:
1) T with central post (i.e. a screw)
2) Y junction with a thin metal foil in the half of H plane

In any case, is hard to get 25 dB of directivity with so large value of coupling.
If you'll find good results, please let me know.