The paper presents overmoded circular waveguide mode converters (TE01 into HE11 with TE11 as an intermediate mode) used in a 42 ± 0.2 GHz, 200 kW gyrotron transmission line system for plasma heatin...

Millimeter-wave TE01–TE11–HE11 mode converter using overmoded circular waveguide

To accurately measure the performance of overmoded devices used in high-power microwave systems, a novel structure that effectively converts the TE10 mode in a rectangular waveguide to a high-purity TM01 mode in a circular waveguide is proposed. The circular TM01 mode is excited by a hybrid rectangular E-T waveguide structure, which uses a circular waveguide as the E element. Convex slots loaded at the rectangular waveguide, axially symmetric about the cylindrical waveguide, are introduced to suppress the competing lower order TE11 mode. A tuning stub loaded at the rectangular short wall is used to increase the frequency tuning range. The scattering matrix technique was used to study the mode converter along with the microwave network analysis method using some generalizations of the input port reflection and conversion efficiency, which were verified by the numerical full-wave simulation. Based on the theoretical analysis, an X-band TM01 mode generator is designed and measured. Low-power tests show that, when the length of the tuner increases from 5.7 to 11.7 mm, a wide frequency tuning range of ~500 MHz is obtained with a conversion efficiency of ~99%. Measured results and the theoretical analysis are in good agreement.

High-Efficiency, Broadband Converter From A Rectangular Waveguide TE 10 Mode to A Circular Waveguide TM 01 Mode for Overmoded Device Measurement

This study proposed a long-range non-destructive pipe inspection approach using linearly-polarized circular TE11 mode microwaves, enabling rapid detection of both axial and circumferential cracks on the inner surface of a metallic pipe. The applicability of the TE11 mode microwaves for crack detection was discussed in the light of the currents induced on the inner surface of a pipe due to the propagation of the TE11 mode microwaves. A dual-bend structured mode converter was designed to propagate linearly-polarized TE11 mode microwaves inside a pipe with a certain inner diameter, based on the theoretical and numerical analysis of mode conversion due to a bend. A 3.5 GHz working bandwidth of the mode converter was achieved with a conversion efficiency of the TE11 mode of over 90%. Experimental verification was conducted using stainless-steel pipes with an inner diameter of 23 mm and a total length of 7 m. An axial and a circumferential slit were introduced to simulate cracks. The experimental results confirmed reflections from both the axial and circumferential slits, indicating that the TE11 mode microwaves are applicable to crack detection. The results also confirmed the correlation between TE11 mode polarization and its detection sensitivity toward slits with different orientations along the circumferential direction. Furthermore, the axial and circumferential slits situated at different longitudinal and circumferential locations in the pipe were successfully detected by orthogonally deploying the mode converter to change the polarization of the TE11 mode by 90°.

Detection of cracks with arbitrary orientations in a metal pipe using linearly-polarized circular TE11 mode microwaves

Based on the mode coupling theory, a TE01—TM11 mode converter was designed and optimal results were obtained. In this paper, bandwidth of mode converter with axis exponential structure is the largest, and mode converter with axis sinusoidal structure is the most compact in the case of the center frequency is 28GHz and the waveguide radius is 16mm. If the bending angle between input port and output port of mode converter was demanded for 90°, mode converter with axis parabola structure had the characteristics of compact structure, higher mode purity and larger relative bandwidth. Meanwhile, we found that the converter could be also used as TE11 mode transition with transmission efficiency above 99%. But due to bending angle of this structure was restricted, its center frequency and waveguide radius had the obvious corresponding relation.

High-power Mode Converter of Overmoded Bent Circular Waveguide

This article proposes a novel method to design a 90° oversized TE01-to-TM11 mode converter for high-power transmission line. In order to obtain a full power conversion at 90° bend angle rather than an acute angle, the mode coupling process inside the oversized circular waveguide bend is analyzed, and a phase rematching method is deduced. Based on the analysis results, a phase rematching structure, consisting of two identical circular-to-elliptical transitions and a uniform circular waveguide which can transform the phase difference from $-\pi $ /2 into $\pi $ /2, is introduced. Two mode converters with the input/output radius of 14 mm are designed at the center frequency of 34 GHz to validate the proposed method. The results show that the mode converter with a limited height has a bandwidth of 4.6 GHz, while the mode converter with a limited width has 4.3 GHz with conversion efficiency over 95%. The mode converter with a limited height is manufactured and measured, and the high-purity outputs are validated by the obtained radiation field patterns at the output port of the mode converter from 32 to 36 GHz. The proposed mode converter provides a new approach for developing a compact, high-efficiency, and broadband high-power millimeter-wave transmission line.

A 90° Oversized Broadband TE 01 -to-TM 11 Mode Converter for High-Power Transmission Line

Based on the bent waveguide mode coupling theory, a combined TM01-TE11 mode converter with parallel input and output ports is designed by numerical calculation with the phase rematch technology employed. A numerical calculation comparison between the converter and another double-arc TM01-TE11 mode converter with nonparallel input and output ports shows that the former has obvious advantages over the latter in conversion, bandwidth and compact. Good agreement between tests and calculations shows the converter meets the application well.

Xian-Neng Zhu

Papers

High-power Mode Converter of Overmoded Bent Circular Waveguide

Study of Ka-band TM01 to TE11 Mode Converters with Parallel Input and Output Waveguides