Electrical Measurements at Radio Frequencies
S. L. Brown, Malcolm Young Colby · Physical Review · 1927
Methods of measuring resistance, inductance, capacity, and impedance at radio frequencies.---Resistance, inductance, capacity, and impedance are measured at radio frequencies with the aid of a vacuum tube voltmeter. The experiments described and the data presented illustrate methods of measurement at radio frequencies that are comparable to the corresponding measurements at low frequencies with regard to both simplicity and accuracy. The results indicate that the resistance of a circuit may be measured with an accuracy of one percent at a frequency of several million cycles per second when the value of the resistance is of the order of 0.01 ohm. Much lower values can be measured with a reasonable degree of accuracy. The calculated value of the high frequency resistance of No. 22 copper wire differs from the measured value by less than one percent. The inductance of a portion of a circuit may be measured by a voltmeter-ammeter method with an accuracy of one percent, even though the value of the inductance be only a small fraction of a micro-henry. The calculated values of the inductance of either a circular coil of one turn or two parallel wires forming a return circuit agree with the measured values. Capacities of the same order of magnitude as the smallest readable variation of the variable standard condenser which is used to tune the circuit may be very quickly and easily determined. The measured and the calculated values of the capacity of two parallel wires are in very close agreement. Certain rolled plate types of telephone condensers have sufficient internal inductance to cause them to resonate at low radio frequencies.The methods of making many radio frequency measurements have been improved: (a) By using such a low resistance circuit that the coupling to the source of power could be made very loose. The coefficient of coupling was frequently as low as 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. (b) By using a negligible amount of power from the oscillator, with the result that the e.m.f. induced in the tuned circuit remained constant. The power drawn seldom exceeded 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ watts. (c) By employing a sensitive and accurate voltmeter that is independent of frequency. Voltage changes of 0.2 millivolts could be detected by this instrument.