The problems of mode purity and the interaction effect generally necessitate a large insertion loss in precision waveguide-below-cutoff (piston) attenuators to obtain a linear output. An attenuator excitation system, used on a precision attenuation measuring system, that reduces these problems is described. Experimental work as well as theoretical considerations are discussed and designs are presented that have produced out-put voltages of the piston attenuator greater than 1 V while still maintaining a linearity of 0.001 dB as a function of piston position. This has been accomplished by 1) operating with large input power (100-200 W), 2) development of new exciting units with improved geometry for better field excitation and utilizing water-cooled elements for high-power operation, 3) special impedance matching networks having exceptional stability with high power input, 4) use of current feedback to achieve effectively constant current excitation, and 5) a mode filter that attenuates the undesired TM01mode 98 dB while attenuating the desired TE11mode only 0.2 dB.
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Cook et al. (1971) studied this question.