Tower Radiator

Author: Edmund A. Laport

The next significant step in the progression of improvement was the advent, in 1930, of the guyed cantilever steel tower of a height in conformance with Ballantine's optimum-height formula, which was five-eighths wavelength. At the same time, the use of an extensive system of long radial ground wires centered about the base of the radiator was introduced. This was also an important step forward.

Unfortunately, the original form of cantilever radiator was not of uniform cross section. It tapered from a point resting on the base insulator to a maximum thickness just below mid-height and then tapered again to a point at the top. The guys were attached at the waist, or maximum cross section point. This structure was a success mechanically but did not yield full expected performance, because its double taper modified the current distribution in a way that reduced its vertical directivity. Nevertheless, this type of radiator performed sufficiently near to expectations to provide a satisfactory proof of the antifading properties of such systems. The deficiency of the double-taper tower was finally verified by field-strength measurements in aircraft and by scale-model measurements of current distribution.

Tower design then evolved to the form now prevalent, using either very slender self-supporting towers, or guyed towers of uniform cross section.

The advent of the tower radiator was novel in yet another respect - it used the steel structure as the antenna directly. The use of a steel mast as a radiator had been tried as far back as 1906 at Brant Rock but was never adopted as a design for an antenna. The economic advantages of the tower radiator are quite apparent. A tower radiator is less costly than two towers of similar height supporting a wire antenna. Furthermore, supporting towers, being in the strong field of the antenna, often had large currents induced in them, which made them secondary radiators and produced directive effects in the horizontal pattern which were often undesirable. The tower radiator became essentially an ideal radiator with electrical and mechanical requirements satisfied by a single structure. The tower radiator could also be adapted to use in directive arrays.

Further study of the optimum-height antenna disclosed eventually that the conditions of maximum ground-wave field gain and best anti-fading characteristics were not obtained with the same height. The height that gave the highest field strength (225 degrees) had a rather large secondary lobe of high-angle radiation. This lobe could be reduced to a point where its effect was negligible at some small sacrifice in horizontal field-strength gain. The optimum choice for antifading over land was experimentally established at about 190 degrees, or slightly over one-half wavelength in height. Over salt water the 225-degree radiator has certain advantages, as will be explained later.

FIG. 2.1. Relative field strength versus height of a uniform straight vertical radiator with sinusoidal current distribution.

Subsequent special modifications were introduced to obtain optimum-height operation characteristics with shorter structures, as often necessitated by airways near the transmitter site. One form was the top-loaded vertical radiator which employed a horizontal circular steel capacitance area at the top to substitute for a certain amount of missing vertical height. The amount of top loading was limited structurally to values equivalent to 15 to 30 degrees of electrical height. When the top-loading structure was insulated from the tower and its reactance reduced by means of a series inductance, its electrical effect could be somewhat enhanced. Another approach was to sectionalize the tower with insulators at a point somewhat above three-tenths wavelength from ground and insert a series inductance to reduce the reactance of the upper section. The whole tower could then be physically shorter than optimum height and still perform as an optimum-height antenna.

By about 1934, the modern broadcast radiator had evolved to its present state. The tower radiator was the essential element for the directive broadcast antenna, which is presently of utmost importance in the development of ever-increasing broadcast services within the limited frequency spectrum.