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 Feb 13, 2020, 3:35 PM EST
Robert Koslover Antennas, Waveguides, Electromagnetics

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Posted: 1 week ago Feb 13, 2020, 3:35 PM EST
Updated: 1 week ago Feb 13, 2020, 3:36 PM EST

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.

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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