### 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
Message:
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from Mr. Rod Raburn at rraburn1@attmail.com.

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
subject.

```

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