A Circular TE ₀₁ Mode Excitation for Air Drilling Using Novel Inline Rectangular TE ₁₀ Mode to Rectangular TE ₂₀ Mode Transition

ABSTRACT This paper proposes a circular TE 01 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 TE 10 mode using a conventional probe. A novel inline transition structure is subsequently developed, employing a strategically positioned septum to achieve rectangular TE 10 to TE 20 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 TE 20 to circular TE 01 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.