The errors caused by ignoring the junction inductance in determinations of electrical line length from admittance measurements with the line short-circuited are shown in Figure G. As previously mentioned, corrections can be made for this effect, but its elimination would be preferable. The effect can be eliminated by adding a shunt capacitance of the proper value to form, with the inductance, a short section of artificial transmission line, having the same characteristic impedance as the true line. For reactances small compared to the characteristic impedance, the inductance and capacitance are related by the equation:

The capacitance must be added on the unknown side of the coupling loop so the capacitive current will flow through the line under the loop.

In the new model, the desired additional capacitance is obtained by adding a polystyrene bead to the line, as shown in Figure 7. The electrical length of line between the unknown and the measur ing point is increased by about 0.7 cm. as a result but, since the corrections or adjustments are always made for the over-all length of 50-ohm line when the actual impedance or admittance is desired, the increase does not complicate the situation, but does eliminate a time-consuming correction. Besides the simplification in correction procedure obtained when admittance is measured, the VSWR is now unaffected by the junction inductance as VSWR is independent of line length and, hence, a greater accuracy is obtained.

The compensation is independent of frequency as long as the length of line added is short compared to a wavelength. Figure 6 shows the results of measurements of electrical line lengths with a compensated instrument.

Another improvement that has been made in the Admittance Meter is the addition of several calibrated points on the multiplying-factor scale as shown in Figure 1. The additional points make greater accuracy possible in many cases by permitting the use of a lower multiplier setting.

A new susceptance standard has been designed for use at the lower frequencies to replace the long adjustable stub. The new standard is a small, shielded, variable air capacitor, shown in Figure 1,

Figure 7. Sketch of modified junction showing the location of one of the compensating capacitances.

junction from

Figure 7. Sketch of modified junction showing the location of one of the compensating capacitances.

junction from

which is calibrated directly in frequency ience, and accuracy in measurements of from 41 to 150 Mc. Measurements at low VSWR impedance and admittance in frequencies will be facilitated by the the U-H-F range. The improvements extended frequency range and small size discussed above increase still further its of the new standard. utility and acceptability for these meas-

The Admittance Meter has earned a urements. deserved reputation for speed, conven- — R. A. Soderman

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