A Circular TE01 Mode Excitation for Air Drilling Using Novel Inline Rectangular TE10 Mode to Rectangular TE20 Mode Transition

This paper proposes a circular TE₀₁ mode excitation for downhole microwave communication during air drilling, aiming to meet the critical requirements of preserving sufficient cross-sectional airflow area while maintaining a relatively short structural length. The excitation approach initiates by generating a rectangular TE₁₀ mode using a conventional probe. A novel inline transition structure is subsequently developed, employing a strategically positioned septum to achieve rectangular TE₁₀ to TE₂₀ mode conversion via field bisection into opposite phase components, while preserving maximum airflow capacity. The excitation further incorporates a compact three-section transition to facilitate efficient rectangular TE₂₀ to circular TE₀₁ mode conversion, reducing longitudinal dimension to mitigate drill pipe deformation effect. A cross-radial metal sheet (CRMS) configuration is implemented to effectively suppress spurious modes and enhance mode purity. Finally, a back-to-back prototype was fabricated and measured, showing a return loss better than 15 dB and an insertion loss lower than 1.3 dB from 6.13 to 6.27 GHz, demonstrating the performance of proposed method.