Feeder Characteristics

Author: Edmund A. Laport

Figure 3.21 shows a typical arrangement for a center-fed horizontal dipole having a length of the order of one-half wavelength, where the design is for one frequency only. However, it will be shown in Sec. 3.10 that such an antenna can be used for a considerable frequency range if adequate measures with respect to impedance matching are taken. In such cases the computation of antenna voltages would proceed from different considerations, which are more involved than the foregoing example.

FIG. 3.21. Horizontal dipole antenna center-fed by a balanced feeder.

In general, a half-wave dipole antenna need not be cut to one precise length but may be of the order of one-half wavelength. Small deviations in length will not modify the radiation pattern appreciably, and because in most practical applications the antenna impedance may be considerably different from that of the balanced two-wire feeder that will ordinarily be employed, we can ignore its actual value and prepare for making a proper impedance match in the feeder, as near to the antenna as practicable, by means of a stub section, a coupled section, or a lumped reactance of the proper value at the proper place on the feeder. Standing waves will exist on the feeder on the antenna side of the matching device, and substantially traveling waves only will exist between this device and the transmitter.

A satisfactory impedance match can often be made by shunt-feeding the antenna, using the Y match which is so well known (see Fig. 3.19). This method is an approximate match in impedance only but can be sufficiently exact to have a low standing-wave ratio on the feeder. The theoretical adjustment for a single half-wave horizontal dipole in terms of its height above ground is shown in Fig. 3.18 (upper curve). Actual conditions cause the exact adjustment to depart slightly from these values in each case, for optimum impedance match.

The calculation of the impedance at the center of a horizontal half-wave dipole cannot usually be accomplished with an accuracy of better than 10 percent. because of the empirical factors of end loading, conductor diameter, and the complex dielectric constant of the earth underneath. Theoretical computations can be made from the usual idealized assumptions for an infinitely thin wire, together with the mutual impedance with its image in the case of a perfectly conducting ground. Mutual impedances can be read from Fig. 3.61. Idealized values are the starting point for practical adjustment.

In the foregoing remarks it has been assumed that, for transmitting, a two-wire balanced feeder would be used. The common characteristic impedance value of 600 ohms for such a feeder is a matter of convenience only. Values of characteristic impedance as low as 450 ohms may be used with two-wire feeders, and lower values still by using four-wire feeders.¹ The choice is usually determined by mechanical and cost considerations.

Precautions must be taken to prevent coupling between the antenna and the feeder. This requires exact symmetry of arrangement of the two. The feeder must be perpendicular to the antenna for a distance of several wavelengths, and the feeder connections at the antenna must be symmetrical. When there is radiation coupling between antenna and feeder, currents of the same phase are induced into both sides of the feeder circuit. These induced currents add vectorially at all points along the feeder with the normal feeder currents, causing an unbalance which manifests itself as unequal standing waves on the two sides of the feeder and a displacement of the minimums in the standing-wave patterns on the twro sides. The induced currents exist as parallel currents in the two sides of the feeder to ground. This effect makes impedance matching difficult and also causes radiation from the feeder. Parallel currents can be eliminated by using parallel-wave drains, described in Chap. 4. It is better to avoid the condition by proper initial design.

1)	For receiving antennas, various kinds of twisted or flat pairs of much lower impedance can be used for feeders.