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    Influence of wave port dimensions on reflection characteristics - HFSS

    Hi Edaboard community,

    I am trying to simulate a homogeneous CPW structure using HFSS and for exciting the quasi TEM mode of this structure, I am defining wave ports at the ends of the structure.
    I have computed the Poyiting vector at the surface of the wave ports; and I notice that the value is not always the same and that for certain frequencies I cannot get an indicent power larger than 0.80 Watts. (From what I understand HFSS excites every mode with a 1 Watt across the defined wave port area)

    I would like to know what is the influence of the wave port dimensions on the reflection between the exciting wave port and my structure.

    Both wave ports (ends of my structure) are identical and I would expect for this type of simulation, S11 parameters in the range of -20dB and that the transmission characteristics of my structure (CPW with finite ground planes) would be primarily influenced by the losses of the medium.

    I would appreciate if anyone could help me to verify the reflection effects between the exciting structure (wave port) and my 3D CPW structure.

    Any comment or feedback, will be appreciated

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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    Quote Originally Posted by kalosu View Post
    I have computed the Poyiting vector at the surface of the wave ports; and I notice that the value is not always the same and that for certain frequencies I cannot get an indicent power larger than 0.80 Watts. (From what I understand HFSS excites every mode with a 1 Watt across the defined wave port area)
    It sounds like your line is not (impedance) matched to the port. For sure, the size of the port will affect its impedance. Ensure that the wave port is sufficiently large to encompass the bulk of the modal fields. You can also specify a port mesh refinement under the analysis setup options -- I think the default is 2%, but I usually set it to 0.1%.


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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    Thanks PlanarMetamaterials,

    I increased the size of the excitation wave port and adjusted the convergence criteria and now I got the desired result for the S11 parameters (-20dB for all of my frequency simulation range).

    I am now trying to simulate the same but for a CPW that has discontinuities along the propagation direction. Nevertheless I cannot get to have the same performance as for the homogeneous cross section case.

    I have read that for designs that include discontinuities along the propagation direction, it is needed to include a sufficiently large section of the wave port homogeneous cross section additionally to the ends of the structure. In this case, all of the higher order modes that could be excited at the discontinuity interface and that are reflected back to the excitation waveport could decay and do not affect the S11 characteristics of my design.

    Nevertheless, my design "length" is fixed. It is not clear for me if this "addition of a homogeneous cross section length" is needed just for the consistency of the simulation results or if in deed they mean that such extension should be added to the design.

    Do you have any idea about this issue?



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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    I don't think a wave port is the right tool to deal with evanescent modes, they're best used for propagating modes; more likely you should just add extra lengths of unperturbed TLs in the propagation direction between the ports and the discontinuity to allow these modes to decay sufficiently that they do not impact the results.



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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    Thanks PlanarMetamaterials for your comment.

    I tried adding straight and uniform sections of my waveguide at the ends of my device, but it seems that the length of these "extensions" was not enough. I still cannot have similar performance in comparison to the simulation of a straight and homogeneous CPW case. (S11 parameter)

    After this I have tried using a lumped port excitation scheme (with a vertical, normal to the structure, type of lumped port; similar to the type of excitation using when probing the device with GSG probes). For this case, I have better performance in terms of both reflection(S11 below -20dB) and transmission but how can I explain these differences?

    I know that the lumped port excitation uses a voltage difference applied between the terminal and the reference conductors (ground electrodes for a CPW design), which tells me that the simulation is actually not solving in terms of the modal parameters (S modal parameters) but for the voltage-current defined S parameters.

    How could I reconcile the concept of mode propagation through the structure with the results obtained using lumped ports excitation? Could I assume that the applied voltage results in a linear combination of "modes" (propagating, leaky and evanescent) ? And if so, why would the S parameters for both simulation schemes (wave port and lumped port) differ in their performance?

    If you have any idea, thanks for sharing it.



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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    That's not a bad description, but I would lean towards saying that the wave port does not adequately excite the correct mode(s). Whereas, the lumped ports do.



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    Re: Influence of wave port dimensions on reflection characteristics - HFSS

    Coplanar waveguides are often difficult to simulate as they are mode rich to say the least. This is a subject that has been addressed here (search and review earlier posts). There are several useful books on the subject that you might want to run down (search is your friend). Simon and Waddel come to mind. There are a number of different transmission line configurations within the CPW family. That can add more confusion. GCPW is most common and usually operated in a "micro strip" like mode with the strongest fields between the ground plane and center conductor. The presence of vias is also common and adds additional complexity.



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