A Modal Analysis of Electromagnetic Fields Coupling into an Open-Ended Waveguide Mounted on a Finite Flange: Evaluation of a Rectangular Waveguide with a Square Flange

TL;DR

This work develops a reciprocity-based modal framework to quantify electromagnetic penetration into an open-ended rectangular waveguide mounted on a finite PEC flange. By expressing penetrated mode amplitudes $C_m$ in terms of the aperture's far-field pattern $\bar{G}_a$ and including both aperture reflections and edge-diffraction via GTD, the authors obtain accurate radiated-field components and thus penetrated fields for all waveguide modes. The method is validated against 3D full-wave simulations and measurements, showing excellent agreement and a runtime speedup of about 60× over conventional 3D simulations. The findings highlight the importance of finite-flange effects and diffraction in shielding and waveguide-coupling analyses, enabling fast, high-fidelity design insights for EMC and microwave applications.

Abstract

This paper presents a modal analysis based on the reciprocity theorem to calculate the coupled/penetrated fields into an open-ended waveguide mounted on a finite flange. Although there is no limitation on the geometry and type of the external source, an infinitesimal dipole is chosen to produce a plane wave incident to the waveguide aperture. The proposed method relates the amplitude of each penetrated mode into the waveguide to the far-field radiation components of that mode from the waveguide aperture. The accuracy of the final result depends on the accuracy of the calculated radiated fields. The radiated field components are calculated considering the reflected fields due to the aperture and the diffracted fields due to the flange edges. For the first time, the impact of a finite flange on the penetrated fields into a waveguide is discussed comprehensively. The geometry of a rectangular waveguide mounted on a thick square flange is selected to be evaluated. The effects of changing the main parameters of the geometry on the penetrated fields are discussed rigorously in various examples. Our results are compared with 3D full-wave simulations, and an excellent agreement was found between the results while our analytical approach showing a 60 times faster performance. In addition, we compared our results with the measurement reported in a previous study.

Figures (11)

• Figure 1: An open-ended waveguide with an arbitrary cross-section mounted on a finite, arbitrarily shaped PEC flange radiating into free space by a surface current of $K_a$ on the waveguide cross-section surface $S$ at $z=z_0$, and an infinitesimal dipole with a current of $I_b$ in the direction of $\hat{p}$ on the path $C$, placed far from the waveguide aperture at a distance $r$.
• Figure 2: Geometry of an open-ended rectangular waveguide mounted on a thick square PEC flange and an external plane wave incident to its aperture.
• Figure 3: Geometry and boundary conditions of the implemented CST model.
• Figure 4: (a) Magnitude and (b) Phase of NPR$^h_{10}$ in terms of frequency changes.
• Figure 5: Magnitude of NPR$^h_{10}$ in terms of frequency changes for 11cm $\times$ 10cm flange.