Waveguide Port Overview
Waveguide ports represent a special kind of boundary condition of the calculation domain, enabling the stimulation as well as the absorption of energy. This kind of port simulates an infinitely long waveguide connected to the structure. The waveguide modes travel out of the structure toward the boundary planes thus leaving the computation domain with very low levels of reflections.
Very low reflections can be achieved when the waveguide mode patterns in the port match perfectly with the mode patterns from the waveguides inside the structure. CST MICROWAVE STUDIO uses a 2D eigenmode solver to calculate the waveguide port modes. This procedure can provide very low levels of reflection below -100dB in some cases.
In general, the definition of a waveguide port requires enclosing the entire field filled domain in the cross section of the transmission line with the port area. The eigenmode solver can then calculate the exact port modes within these boundaries. The number of modes to be considered in the solver calculation can be defined in the waveguide port dialog. The strategies to properly define waveguide ports depend a bit on the type of the transmission line. In the following we focus on the most commonly used applications. Please note that the input signal of an excited waveguide port is normalized to 1 sqrt(Watt) peak power.
Contents
Empty Waveguides
Port Creation Port Modes Coaxial Waveguides
Port Creation Port Modes Microstrip Lines
Port Modes / Port Dimensions Port Creation Coplanar Lines
Port Modes / Port Dimensions Port Creation
Inhomogeneous Waveguide Ports (Special Treatment)
Microstrip Lines / Coplanar Lines (QTEM Modes) Dielectrically Loaded Waveguides (No QTEM Modes) Multipin Waveguide Ports
Homogeneous Multipin Ports Inhomogeneous Multipin Ports
Periodic Waveguide Ports General Information
Losses in Waveguide Ports Impedance Definition Mode Calibration Mode Polarization
Waveguide Ports in Hexahedral Mesh View See also
Empty Waveguides
The most frequently used waveguide type of this category is the rectangular waveguide shown below:
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Port Creation
The port assignment for this type of waveguide is quite simple. However, it is important to make sure that the port covers the entire waveguide cross-section. The easiest way to define a port’s face is to pick structure elements such as points / edges / faces before opening the port definition dialog box. The port dimensions will then automatically adjust to the bounding box of the picked elements. If the interior of the waveguide is modeled by a dielectric material block, you can simply pick the end face of the waveguide as shown in the picture below:
The bounding box of the picked end face will then exactly define the dimension of the port.
When the background material is not made of PEC material, it is often required to model the wall of the waveguide, too. For a construction like the one shown in the first picture above, the end face of the rectangular waveguide has not been modeled directly.
We therefore recommend to pick the edges on the circumference of the waveguide or to pick two opposite points on the corners:
In both cases, the bounding box of the picked elements will be large enough to cover the entire field filled cross-section of the waveguide. The resulting port definition will then look as follows:
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Port Modes
CST MICROWAVE STUDIO solvers generally allow using more than just the fundamental mode in the waveguide port. This becomes especially important when higher order modes need to be taken into account.
In the pictures below, the first three modes of an empty waveguide are shown, sorted by their respective cutoff frequency. The number of propagating modes vary due to the chosen frequency range. As a rule, the number of modes to be considered at a waveguide port should at least be the number of propagating modes, because unconsidered modes will be reflected by the port operator.
If an evanescent port mode result is selected, a box will appear visualizing the distance from the port where the port mode has decayed by -40dB:
Multiple waveguide modes can convert energy into each other at the structure’s discontinuities. Due to these phenomena, the S-parameters of the propagating modes can also be affected by the evanescent modes. Therefore a certain number of evanescent modes needs to be taken into account. We recommend that the number of modes used for the simulation be chosen such that the -40dB distance of the last considered mode is shorter than the distance to the next discontinuity. This setting will ensure that all modes not considered for the simulation will make only very small contributions to the mode conversion. Mode Polarization
In the case of quadratic or circular waveguides, the mode polarization is an issue. Please find a detailed discussion in the section Mode Polarization. up
Coaxial Waveguides
The most frequently used waveguide type of this category is the coaxial cable shown in the picture below:
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Port Creation
The port assignment for this type of waveguide is very simple. However, it is important to make sure that the port covers the entire cross-section of the coaxial cable. The easiest way to define the port
face is to pick structure elements such as points / edges / faces before opening the port definition dialog box. The port dimensions will then automatically adjust to the bounding box of the picked elements.
In most cases, the outer conductor part of the coaxial cable will be modeled with a solid cylinder. If this is the case, simply pick the end face of the conductor as shown below:
The bounding box of the picked end face will then define the dimension of the port. Note that the waveguide port is still shown as a rectangular face. However, the computation of the waveguide mode will be automatically restricted to the interior of the coaxial cable.
If the end face of the outer conductor can not be picked for some reason, you can also pick edges or points on the circumference of the coaxial cable as described for the empty waveguides.
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Port Modes
In most cases, you will need to consider the fundamental mode of the coaxial cable so that you don’t need to worry about the number of modes for the port. The mode is automatically polarized so that the electric fields point from the inner conductor to the outer conductor as shown in the following picture: