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why multiple modes in rectangular cavity when it is coupling?

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When i am performing a simulation of 1 cm radius cylindrical cavity with comsol multiphysics, the obtained frequency eigen value for TM010 mode is 11.47GHz. But when I am adding the coupling structures ( coaxial cable) it get excited at two modes, one mode is around 10 GHz and other mode is around 12GHz.

Why this is happening?

simulation results are attached here: simulation results


1 Reply Last Post 13 feb 2020, 15:35 GMT-5
Robert Koslover Certified Consultant

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Posted: 5 years ago 13 feb 2020, 15:35 GMT-5
Updated: 5 years ago 13 feb 2020, 15:36 GMT-5

You changed the geometry by adding the structures and thus introduced new possible modes and/or new boundary conditions. The eigenmodes and eigenfrequencies of any system that exhibits oscillations, whether it is an RF cavity, an antenna, a swinging pendulum, a mass on a spring, a vibrating quartz cystal, etc., are not independent of the structures and/or environments that couple to them, unless those couplings are made weak enough that they can be neglected.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
You changed the geometry by adding the structures and thus introduced new possible modes and/or new boundary conditions. The eigenmodes and eigenfrequencies of *any* system that exhibits oscillations, whether it is an RF cavity, an antenna, a swinging pendulum, a mass on a spring, a vibrating quartz cystal, etc., are not independent of the structures and/or environments that couple to them, *unless* those couplings are made weak enough that they can be neglected.

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