Long-Lines

Author: J.B. Hoag

In this chapter we shall consider systems of conductors whose lengths are great in comparison with the wave-lengths of the radio waves with which they are associated.

Much of the study of " radio " is concerned with the opposite case, where the length of the conducting systems is very short in comparison with the wave-length of the associated wave.

Fig. 35 A. A system of conductors is called a " circuit" when the wires in it are very short compared with the wave-length of the radio waves

In Fig. 35 A, the shaded square represents any one of the conventional amplifiers, oscillators, transmitters, or receivers which have been studied in the preceding chapters. If one of the wires in it is 10 cms. long, while the radio wavelength is 30,000 cms., corresponding to a frequency of 1,000,000 cycles per second, then their ratio is 1 to 3,000. The term circuit is usually used when this ratio is much less than unity, whereas the words long-line are used when the ratio is considerably greater than unity. Later we shall consider the case when the circuits and the waves are of comparable lengths; called short-lines or linear circuits.

Long-lines have two uses: (1) to transmit power with as little loss as possible from one point to another, as from a transmitter to an antenna or from an oscillator to an atom-smashing cyclotron or from a telephone in one city to another in a distant city; (2) to serve as antennas to radiate as much electromagnetic energy as possible. We shall be concerned here more with transmission lines than with antennas.