add asymmetric cpw example

[vvahidd]

https://tentzeris.ece.gatech.edu/TMTT_2005_Asym_CPW.pdf

[mdmaas]

I was just taking a look at the paper. We could simulated the 2d asymmetric CPW modes using the 2d eigensolver in #150, and then do 3D simulations including the airgaps to check if the airgaps effectively reject the problematic mode. The 3d sim can be done with a waveport lumped ports, and with/without airgaps.

[mdmaas]

We've been thinking more about this paper and the corresponding simulations.

Due to its asymmetry, the asymmetric CPW hosts 2D eigenmodes that deviate from those of a classic CPW, though they retain similar field profiles. Consequently, the traditional even (common / "slotline") and odd (differential / coplanar) modes become hybridized asymmetric modes.

In particular, when exciting an asymmetric CPW using standard differential techniques (as one would for an ordinary CPW), both asymmetric modes are inadvertently driven—meaning the parasitic slotline mode gets excited. This introduces two primary issues:

1. Radiation Losses: The slotline mode is inherently leaky.
2. Signal Integrity Degradation: The two modes propagate at different phase velocities ($v_p = 2\pi f/ k_n$), leading to modal dispersion and severe signal degradation.

To systematically analyze how this effect occurs and is mitigated by the inclusion of airbridges, we propose the following simulation plan:

• 2D Modal Analysis: Compare the classic CPW against the asymmetric CPW to track how the pure even/odd modes transition into hybridized slotline and CPW modes.
• 3D Full-Wave Simulations: Utilize lumped port excitations with and without airbridges. This will allow us to quantify how much slotline mode is excited in the absence of airbridges and directly measure the airbridges' effectiveness in suppressing the unwanted mode.