14GHz directional coupler

Very poor specs! What about bandwidth, directivity? What do you mean by "frequency effects"?

Very good specs you can get by using waveguide structure.
Look at the Pozar's("Microwave engineering") and Matthaei's("Microwave filters, impedance-matching networks...") books. These books are the best for such problems.

use slot line with about 15 dB coupling and 5 dB att ( to improve the directivity)

why don't you use microstrip one?

Stripline directional coupler is good for your spec.

Do you need your coupler in waveguide?

stripline have more loss then microstrip, so i suggest overlay coupler

That's right , but is tolerable for general case.

you can use lang coupler with fr4 microssrip line or wilkinson coupler.
for more information to design this coupler you can:
1- search throgh the google
2- download book "RF circuit design by:ludwic"