Below cut-off piston attenuator as a calculable signal vernier for microwaves up to 15 GHz

Simple circular cylindrical piston attenuators have been constructed which, unlike commercially available below cut-off attenuators, are operated in the aperiodic TM 01 mode. The instruments provide broad-band measurements up to 15 GHz over an extended range of attenuation. The accuracy offered by these TM 01 attenuators is +or-0.02 dB. Calibration and correction procedures are not needed. The TM 01 mode attenuator is therefore an attractive alternative to the conventional TE 11 attenuator, since the latter, matched to a nominal frequency, requires extensive calibration measurements if applied over a broad frequency band. The construction of the TM 01 piston attenuator is presented and test measurements are described to verify the favourable properties of the device.