Microwave absorber design

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?

HI, I use absorber before. As long as all the side walls of the cavity is deposited with absorber, your circuit will work fine.

Since your housing is quite small, you can consider higher quality absorber, request free sample from Cuming Microwave

Hi, I don't know about your circuit, but the idea to avoid propagation of umwanted waveguide mode is to keep the cut-off waveguide mode (TE01) well above the operating frequency of your circuit. For microwave housing design, usually the circuits (such as filter, amplifiers) are contained in narrow channels to keep the waveguide mode frequency at least 3 times higher than your operating frequency. For example, for 15GHz edge coupled microstrip filter etched on ceramic hardboard, we keep the side wall of the filter housing just about 5-6mm, height.

You may search triquint 's milimeterwave IC application " Design Guidelines for microwave cavities" for detailed discussion.

Alternatively, the Artech House book " Microwave Component Mechanics" should be able to address your problem.

My point is: Don't rely on microwave absorber. I have reverse engineered so many commercial microwave components. Hardly seen ones which use absorber. 8)

http://www.emersoncumingmp.com/ sells a variety of materials and has application notes.

http://www.kellysearch.com/qr-product-119163.html has links to 12 other suppliers who probably also have application notes.

Hi, I uploaded the triquint application note for your reference :)

I think most absorbers are design for absorbing plane waves. You can't simply put a bit of it in a box and expect the waveguide modes to be absorbed effectively. So I resonate Guanchoon's point.

How to design a microwave absorber?

Yeah, commercial components only work in a very narrow bandwidth, so you can toy with the housing dimensions to suppress waveguide modes. In broadband designs, you need to keep the housing width small, AND use absorber on the cover bottom.