MadSci Network: Engineering

Re: Can you explain the principles of how microwave filters work in conjunction

Date: Fri Feb 5 20:31:35 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Engineering
ID: 917376744.Eg

I cannot answer this question adequately.  A good answer can be obtained 
from Mr. Rod Raburn at

My best answer is simply that the filter is chosen to propagate a narrow 
band of microwave energy at a certain frequency.  In some cases, a filter 
(very low power)will consist of a certain length, shape, etc. of highly 
conductive metal strip that forms the waveguide.  From what I remember, 
the dielectric that the board rests on can be a thin film to relatively 
thick separation between either air or another surface of metal.  The 
dielectric is chosen to have as little loss as possible so that the 
impedance is not attenuated.  In the case of the metal strip that has no 
metal on the other side of the dielectric that it is mounted on, the 
air/dielectric discontinuity forms a reflective (but not perfect)boundry.  
The thickness of the dielectric and the size of the dielectric constant 
define the natural frequency of microwave energy that can travel along the 
waveguide.  For example, if the frequency one wanted to filter was 2.45 
GHz, in air, its wavelength would be (3E8 m/s)/(2.45E9) = 0.12m or roughly 
12 cm.  Since the wavelength is a function of the square root of the 
dielectric constant, choosing a dielectric with a dielectric constant of 
100 would reduce the thickness of the dielectric by a factor of ten.  
Therefore, to filter out 2.45 GHz, the dielectric thickness would have to 
be a fraction of 1.2 cm.  Generally, if I remember correctly, this 
technique is a quater wave waveguide, so the board thickness would be 
reduced to 1.2/4 cm or roughly 0.3 cm.

If you need a more specific or clearer answer than that, which I suspect 
you do, then please consult the person mentioned above.  I worked with 
engineers at Los Alamos labs briefly who were experts in the design of 
microwave filters and gleaned a little information from them on this 

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