Figure 5d. Equivalent circuit showing the location of the compensating capacitance.
The approximate equivalent circuit of this arrangement is shown in Figure 5a. The series inductance of the line between the actual junction and the center of each loop is Lj. The total capacitance of the section of line is Cy. Inair equivalent of the very short section of line, half of this capacitance can be placed at either end of the series inductance. The capacitance appearing directly across the voltage source has no effect on the accuracy and, hence, can be neglected.
The current flowing through the capacitance on the load side of the junction line does not induce a voltage directly into the loop since it does not flow in the line under the loop. This current does, however, produce a voltage drop when it flows through the junction inductance and, hence, has an effect on the voltage applied to the unknown circuit. In the actual instrument, the capacitive reactance is so large compared to the inductive reactance that the voltage drop caused by the capacitive current can be neglected and, hence, Cy can be eliminated from the circuit.
The current flowing to the unknown circuit passes through the junction inductance and causes a voltage drop which can have an appreciable effect on the measurements when the measured admittance is large compared to 20 millimhos.
Since the pickup loop responds only to the current flowing in the line under the loop, the junction inductance can be shifted to the unknown side of the coupling loop without affecting the performance as shown in Figure 5c.
The same junction inductance appears in each branch, and, hence, when the unknown impedance is equal to the characteristic impedance of the line, its effects cancel. The magnitude of the effect of the inductance increases with frequency and with the magnitude of the unknown admittance. This junction inductance is minimized by bringing the outer conductor as close as is practicable to the inner conductor as shown in Figure 7. The junction inductance under these conditions is approximately 1.2 millimicrohenries. Correction can be made for its effect on the measurements, and correction charts for the older Type 1602-A Admittance Meters are contained in the instruction book.
Figure 5b. Approximate equivalent circuit of each branch with junction capacitance neglected.
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