Increasing Bandwidth of Vertical Radiators

Author: Edmund A. Laport

The multiple-down-lead technique offers excellent possibilities for low-frequency broadcast antennas where bandwidth is a special problem. The requirement of a very large diameter vertical radiator as a means of increasing the bandwidth can be met by using a central steel mast with an outrigger at the top to support a cage of vertical wires having a substantial diameter without excessive weight or cost. Let us take, for example, a wide-band vertical radiator for broadcast purposes for a carrier frequency of 218 kilocycles. A steel mast 200 meters high (52.3 degrees at 218 kilocycles) must have a cylindrical height-to-diameter ratio of 20 in order to have a response within 2.5 decibels for the upper and lower 10-kilocycle side frequencies. This information can be computed from the impedance data of Figs. 2.15 and 2.16, taking into account that the electrical height of the antenna will vary from 49.9 degrees at 208 kilocycles to 54.7 at 228 kilocycles. To obtain a ratio of height to diameter of 20, the requisite diameter of 10 meters can be obtained by using an outrigger at the top of the mast to support at least eight vertical wires in the form of a cage enclosing the mast. A larger number of wires would more nearly approximate a complete cylinder.

The self-impedance of a radiator of these dimensions at 218 kilocycles is of the order of 11 ohms resistance and 80 ohms reactance. Ground resistance and other loss resistances must be added to this. The antenna current will be equally divided, by symmetry, among the eight vertical wires, and a residual portion of the total current will flow in the steel central supporting mast. The exact proportion of the total antenna current flowing in the mast itself can be computed by means of logarithmic-potential theory, but we shall assume for the present that it is the same as that in one of the vertical wires. The system therefore is equivalent to a nine-wire antenna with equal current division. We may choose to use a double tuning system, by including anywhere from one to eight wires in the fed portion, the remainder being tuned directly to ground. There is therefore a range of input impedances available for feed purposes, as shown in Table 1.4.

In the resistances given in the table the ground and other loss components have been omitted for simplicity. It is seen immediately that a wide range of input resistances is available according to the number of wires (with the supporting mast counted as a wire) included in the fed portion of the system, and with the remaining wires multiple-tuned in such a way as to maintain equal currents in all wires. It is interesting that, with two wires in the fed portion, the resistance is of a value that would permit direct matching of convenient types of open-wire feeders.

FIG. 1.11. Multiple-tuned low-frequency broadcast antenna.

With four fed wires, the value is suitable for direct matching with coaxial feeders. The antenna-tuning gear consists of two inductors only, and the full bandwidth capabilities of the radiator are utilized by avoiding the use of more complicated networks having additional energy storage. Figure 1.11 illustrates this arrangement when two-wire feed is used to match an open-wire feeder.