The Fundamental Limit to the Balance of a Microwave Bridge Containing a Dispersive Element

An expression is derived for the maximum balance obtainable with a magic-T microwave bridge as a function of the variance of the frequency modulation (FM) noise pedestal of the microwave power source when one arm of the bridge is terminated by a dispersive component. This expression is verified experimentally and the long-term stability of the balance with/without a dispersive component is studied.