Special Techniques for Measuring Low and High Values of Attenuation with a Modulated Subcarrier System

Various shortcomings of modulated subcarrier attenuator calibrators are discussed. Very precise attenuation measurements on short lengths of waveguide have been obtained by inserting into the subcarrier channel a tuned reflectometer system which contains a waveguide switch for setting the zero. Using this technique, total spreads of less than 0.0013 dB have been obtained at 4 GHz in 10 completely independent measurements on the same unknown. The full theory of this method is given in the Appendix. The total range of a modulated subcarrier system has been extended to over 100 dB by using a 1-W microwave oscillator and placing in the subcarrier channel a rotary attenuator with very good resettability and a fully matched two-path configuration that contains two waveguide switches and the unknown.