If the lines get even closer, then they no longer work as two independent coplanar lines since the mode fields interact with each other and the waveguide becomes a multipin waveguide. Please refer to the Multipin Port Overview for this type of waveguide.
If no symmetry condition is defined in the middle of the coplanar line, both even and odd modes can exist and therefore need to be taken into account. If an electric symmetry condition is specified, only the odd mode can exist. On the other hand, setting a magnetic symmetry condition will consider the even mode only.
If the microstrip line is grounded, another parasitic microstrip mode will exist too as long as no electric symmetry is defined. All this leads us to the following table for the number of modes normally used for the simulation at the coplanar port:
Symmetry Ungrounded Coplanar None Electric Magnetic 2 modes 1 modes 1 modes Grounded Coplanar 3 modes 1 modes 2 modes In case you are interested in the fundamental odd mode only, we recommend to use electric shielding around the port.
The corresponding number can be specified in the Number of modes field in the port properties dialog box.
Note that the order in which the port modes are calculated may vary depending on the dimensions of the structure as well as the frequency. Therefore we highly recommend always inspecting the port mode results in order to avoid misinterpreting the S-parameter data.
Another important aspect in the simulation of coplanar lines is the fact that the mode pattern is frequency dependent (unlike the mode patterns in empty guides or coaxial lines).
The frequency domain solvers automatically recalculate the mode patterns for every frequency point so that this frequency dependency does not constitute a difficulty for analysis.
In contrast, the time domain solver uses the same mode pattern for the entire frequency band which may cause port mode mismatches at frequencies other than the mode calculation frequency. The error increases with increasing distance to the mode calculation frequency.
By default, the transient solver computes the mode pattern at the center frequency of the frequency band, but this behavior can be changed by specifying the Mode calculation frequency in the solver specials dialog box on the Waveguide page.
Despite this small mismatch at the ports, the broadband simulation results are still sufficiently accurate in most cases. However, very high accuracy requirements or very large bandwidths may require you to activate the inhomogeneous port accuracy enhancement option in the transient solver dialog. box. This feature will improve the accuracy of the S-parameters but on the other hand will also slow down the simulation. up
Inhomogeneous Waveguide Ports (Special Treatment)
A waveguide is called inhomogeneous whenever more than one different dielectric material is present in the cross-section of the waveguide. As mentioned before this is the case for microstrip lines and coplanar lines as well as for dielectrically loaded waveguides, representing QTEM and no QTEM modes respectively.
The most important fact of inhomogeneous waveguide ports is that the mode pattern is frequency dependent. As an example in the pictures below the basic TE mode of a dielectrically loaded waveguide is presented for three different frequency points. The higher the frequency (from left to right), the more the field is concentrated in the material with the higher dielectric value (colored in light brown).
The frequency domain solvers automatically recalculate the mode patterns for every frequency point so that this frequency dependent behavior does not cause a difficulty for the analysis.
In contrast, the time domain solver uses the same mode pattern for the entire frequency band which may cause port mode mismatches at frequencies other than the mode calculation frequency. The error increases with increasing distance to the mode calculation frequency.
By default, the transient solver computes the mode pattern at the center frequency of the frequency band, but this behavior can be changed by specifying the Mode calculation frequency in the solver specials dialog box on the Waveguide page.
Despite this small mismatch at the ports, broadband simulation results will still be sufficiently accurate in most cases. However, possibilities to achieve very high accuracy requirements or very large bandwidths are discussed in the following for microstrip lines / coplanar lines (QTEM Modes) and dielectrically loaded waveguides (No QTEM Modes). up
Microstrip Lines / Coplanar Lines (QTEM Modes)
Because of the inhomogeneous port region, the waveguide port operator works less accurately with increasing distance from the mode calculation frequency. However, the occurring broadband error may be prevented and the accuracy of the S-parameters improved with help of the inhomogeneous port accuracy enhancement feature, achievable in the transient solver dialog. This feature requires the excitation of all port modes and thus will slow down the simulation - it is therefore advisable to activate this functionality carefully and if possible together with the usage of S-parameter symmetries. Dielectric losses in the port region can not be directly considered during the time domain calculation. However, using this feature, it is possible to consider their influence in a post processing step.
A single microstrip line is calculated with two normal waveguide ports as demonstrated in the following picture:
Due to the chosen mode calculation frequency, the reflection is accurate only in the range about 10 GHz. In contrast to this, the right picture shows the result of the calculation applying the inhomogeneous port accuracy enhancement. Here, the reflection is less than -60 dB over the complete frequency range as to be expected.
Without inhomogeneous port accuracy
enhancement
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Dielectrically Loaded Waveguides (No QTEM Modes)
With inhomogeneous port accuracy
enhancement
The following explanations therefore exclude these types of waveguides and focus on dielectrically loaded waveguides. A typical example is shown in the following picture:
In terms of assigning ports to these waveguides, the procedure is very similar to the Empty waveguide case explained earlier in this section. The main difference here is that the port mode pattern is no longer frequency independent.
The frequency domain solvers automatically recalculate the mode patterns for every frequency point so that this frequency dependency does not constitute a difficulty for the analysis.
In contrast, the time domain solver uses the same mode pattern for the entire frequency band which may cause port mode mismatches at frequencies other than the mode calculation frequency. The error increases with increasing distance to the mode calculation frequency.
If you are interested in only very small frequency ranges, a viable solution may be to set the mode calculation frequency equal to the center frequency of this frequency band. This setting can be made by specifying the Mode calculation frequency in the solver specials dialog box on the Waveguide page.
However, if you are interested in broadband results within larger frequency ranges, the transient solver needs to be advised to make a special (and computationally expensive) treatment for the inhomogeneous ports. The Broadband for inhomogeneous waveguide ports (no QTEM modes) option can also be activated on the Waveguide page of the solver’s specials dialog box:
This option will provide much more accurate broadband results for this type of port. Unfortunately, it is limited to the fundamental (propagating) mode only. up
Generalized port mode solver
The generalized port mode solver can be used for any kind of homogeneous or inhomogeneous waveguides.
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