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microwave designer
Joined: 15 Apr 2004
Posts: 2
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 22 Apr 2004 9:39 Microwave absorber design |
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| 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?
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activewei
Joined: 19 Sep 2002
Posts: 127
Location: UK
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22 Apr 2004 11:27 Microwave absorber design |
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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
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guanchoon
Joined: 16 Jan 2004
Posts: 15
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23 Apr 2004 0:07 Re: Microwave absorber design |
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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. 
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flatulent
Joined: 19 Jul 2002
Posts: 4926
Helped: 328
Location: Middle Earth
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23 Apr 2004 1:17 Re: Microwave absorber design |
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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.
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Google
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23 Apr 2004 1:17 Ads |
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lguancho
Joined: 07 Jan 2004
Posts: 266
Helped: 5
Location: Singapore
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23 Apr 2004 4:43 Re: Microwave absorber design |
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Hi, I uploaded the triquint application note for your reference 
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loucy
Joined: 26 Aug 2001
Posts: 433
Helped: 10
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23 Apr 2004 14:11 Re: Microwave absorber design |
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| 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.
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cassiopeta7
Joined: 28 Jan 2008
Posts: 6
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28 Jan 2008 7:48 Re: Microwave absorber design |
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| How to design a microwave absorber?
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biff44
Joined: 24 Dec 2004
Posts: 1959
Helped: 261
Location: New England, USA
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28 Jan 2008 11:43 Re: Microwave absorber design |
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| 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.
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