Considerations of a Rectangular Waveguide below Cutoff Piston Attenuator as a Calculable Broad-Band Attenuation Standard between 1 MHz and 2.6 GHz

Theory, essential construction data, and results for the systematic uncertainties are presented for a precision rectangular waveguide below cutoff (WBCO) serving as a broad-band attenuation standard for the frequency range between 1 MHz and 1 GHz, and with a slight increase in the total uncertainty to 2.6 GHz. The contributions to the systematic uncertainty resulting from the mechanical tolerances, the uncertainties in length measurement, frequency stability, surface conductivity, ambient conditions (air refractive index), and higher mode disturbances are discussed. It is shown that under optimum conditions the lowest limit of systematical uncertainty is about 0.0001 dB/10 dB between 1 MHz and 1 GHz for an evacuated WBCO; for an air-filled waveguide under adequate laboratory conditions a total uncertainty ≦0.0005 dB/10 dB should be achieved between 50 MHz and 1.5 GHz.