Control of Characteristic Impedance

Author: Edmund A. Laport

There are many common applications where the feeder characteristic impedance may be of any convenient though arbitrary value. But in certain types of application one may require a feeder of a specific characteristic impedance.

To obtain a lower characteristic impedance the following general conditions apply:

1. The conductor sizes can be increased while maintaining the same center-to-center distances.

2. For given wire sizes, the distances between conductors can be decreased.

3. The number of wires used in each side of the feeder (if balanced) or the high-potential side (if unbalanced) can be increased.

4. Two or more feeders may be used in parallel.

5. Lumped shunt capacitors may be connected across the line at equal distances to produce the effect of smooth loading at the working frequencies, with lower characteristic impedance.

To increase characteristic impedance, opposite methods are used. Instead of increasing the capacitance per unit length as in item 5, inductance may be uniformly distributed in series with the line.

The formulas for the characteristic impedance of several practical forms of transmission lines for radio applications are given in the following section. Formulas for other configurations can be developed by applying the principles of logarithmic potentials from Chap. 6.