Mett R R, Sidabras J W, Anderson J R, Hyde J S
Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226-0509, USA.
Rev Sci Instrum. 2011 Jul;82(7):074704. doi: 10.1063/1.3607432.
The two-way insertion loss of a 1 m length of waveguide was reduced by nearly 5 dB over a 4% bandwidth at W-band (94 GHz) for an electron paramagnetic resonance (EPR) spectrometer relative to WR10 waveguide. The waveguide has an oversize section of commercially available rectangular WR28 and a novel pair of tapers that vary in cross section with axial position according to a hyperbolic-cosine (HC) function. The tapers connect conventional rectangular WR10 waveguide to the WR28. For minimum loss, the main mode electric field is parallel to the long side of the WR28. Using mode coupling theory, the position of maximum flare (inflection point) in the taper was optimized with respect to the coupling to higher order modes and the reflection of the main mode. The optimum inflection point position is about one-tenth of the taper length from the small end of the taper. Reflection and coupling were reduced by about 20 dB relative to a pyramidal (linear) taper of the same length. Comb-like dips in the transmission coefficient produced by resonances of the higher order modes in the oversize section were about 0.03 dB. Specially designed high-precision, adjustable WR28 flanges with alignment to about 5 μm were required to keep higher order mode amplitudes arising from the flanges comparable to those from the HC tapers. Minimum return loss was about 30 dB. This paper provides a foundation for further optimization, if needed. Methods are not specific to EPR or the microwave frequency band.
对于电子顺磁共振（EPR）光谱仪，在W波段（94 GHz）的4%带宽内，相对于WR10波导，1米长的这种波导的双向插入损耗降低了近5 dB。该波导具有市售矩形WR28的超大截面以及一对新颖的渐变段，其横截面根据双曲余弦（HC）函数随轴向位置变化。这些渐变段将传统的矩形WR10波导与WR28连接起来。为了实现最小损耗，主模电场与WR28的长边平行。利用模式耦合理论，针对与高阶模式的耦合以及主模的反射，对渐变段中最大 flare（拐点）的位置进行了优化。最佳拐点位置约为渐变段小端起渐变段长度的十分之一。相对于相同长度的锥形（线性）渐变段，反射和耦合降低了约20 dB。超大截面中高阶模式共振产生的传输系数中的梳状凹陷约为0.03 dB。需要专门设计的高精度、可调节的WR28法兰，其对准精度约为5μm，以使法兰产生的高阶模式幅度与HC渐变段产生的幅度相当。最小回波损耗约为30 dB。本文为进一步优化（如有需要）提供了基础。方法并非特定于EPR或微波频段。
