Comments on "Measurement of Microwave Loss Tangent by Means of Microwave Resonator Bridge" [Letters]

In the above paper, Kneppo and Weis claim that the microwave resonator bridge (MRB) is a new and highly sensitive method for measuring dielectric losses of materials. Contrary to their belief, the principles of measurement are not new but have already been reported in [1] and [2]. Especially, the system arrangement described in [2] is the same as that of MRB, except that the former uses a shutter in tandem with a matched termination as a reference load of the bridge, while the latter uses a reference resonator. Thus the point in question is whether or not this replacement of the reference load improves the sensitivity of the bridge.