Measurement of Radio-Frequency Impedance with Networks Simulating Lines

The complex value of an impedance may be measured at radio frequencies by measuring voltage ratios on a transmission line terminated in the unknown impedance. The line may often be replaced advantageously by an equivalent network. Several new methods of carrying out the measurement have been developed and a study of the factors affecting the accuracy and the technique of manipulation has been made. Optimum accuracy obtains when the characteristic impedance of the network and the impedance to be measured are equal. Only two quantities need be accurately known; viz., (1) the capacitance of a variable condenser, and (2) the frequency of the applied voltage; this is a decided advantage at radio frequencies. The conditions imposed on the voltmeter are easily satisfied in practice. A common lead connecting generator, voltmeter, network, and unknown impedance makes grounding and shielding simple. Experimental results at frequencies from 50 to 1500 kilocycles have agreed well with the computed performance of the device and they demonstrate the practicability of the method.