RF MODULE

Coaxial to Waveguide Coupling
This model shows how to use assembly pairs and the Port boundary conditions to feed
a rectangular waveguide.
Introduction
Feeding a waveguide from a coaxial cable is a straightforward way to get
electromagnetic waves inside the waveguide. The coaxial feed must be modeled with
enough length, so reflected waves either are damped out in the cable (evanescent
modes) or turn into propagating modes. Propagating modes are efficiently terminated
with a matched absorbing boundary condition, called the Port boundary condition in
the RF Module. Due to its small size and circular shape, the cable contributes
significantly to the overall size of the problem. It is therefore necessary to keep the
cable as short as possible. One approach is to add the cable part as a separate assembly,
connecting it to the waveguide using the port condition for pairs. This port condition
can excite a wave into the master side of the pair while only the reflected waves exiting
the master side enters the slave side. The port condition can be placed directly at the
surface where the cable enters the waveguide, and only a short cable part, modeled as
a perfectly matched layer (PML), is necessary.
In 3D, the electromagnetic waves application mode uses vector elements, and these
elements are very sensitive to incompatible meshes. The master and slave side of an
assembly pair usually get incompatible meshes in 3D, so the identity coupling must be
stabilized with the divergence condition. The divergence condition adds an extra
equation that complicates the use of iterative solvers, so if it is possible to create
compatible (or identical) meshes it is definitely worth the effort. It is possible to copy
a mesh from one boundary in an identity pair to the other side using interactive
meshing and script programming. This approach is used in this model to avoid having
to turn on the divergence condition.
Results and Discussion
For cable feeds it is interesting to compare the reflected wave going back into the
coaxial cable with the wave propagating in the rectangular waveguide. You can make
such a comparison by evaluating the S-parameter on the coaxial port and compare it
with the S-parameter on the rectangular port. There is also a coaxial port on the
204 | CHAPTER 3: RF AND MICROWAVE MODELS