microwave designer
Newbie level 2
The problem is to avoid parasitic resonaces of a metal box containing a microstrip layout. The box size is about 30x50x7 mm. The microstrip layout has er=2.6 and the thickness is about 0.5 mm. Since the box size is large enough the volume resonances appeared inside the frequecy range of interest (20-28 GHz). I decided to use microwave absorber layer deposited on the box cover above the microstrip layout. I am going to use polymer bonded carbonyl iron, but the question is how to select proper thickness of the absorbing layer and concentration of the carbonyl iron to obtain a good performance inside the desired frequency band?