microwave 14.5hhz waveguide

I need some help in designing a 4-port TE10mode rectangular waveguide at 14.5GHZ.
Specifications:
WG/18 Freq: 11.9Ghz - 18.0Ghz
Dimension: 16.0mm x 7.9mm
Coupling factor: -20dB
Direction: -20dB

I need to know how many holes and the dimensions of each hole to drill on the waveguide.
Any references is welcome.
Thank you!

You can use a book "Microwave engineering" by Pozar.
This book you can find in the Electroda site.

I suggest a book "Microwave engineering" by Annapurna Das, Sisir K Das.

MC Graw Hill, page 191-206.

I make code, butterworth type multi hole coupler.

I used Octave software. It's freeware.