Development of Broadcast Antennas

Author: Edmund A. Laport

Prior to 1924, almost all engineering experience with antennas was derived from electrically short antennas of the type that had been used for low and very low frequencies since the dawn of radio. It seems remarkable that it required such a long time to develop the principles of the vertical radiator. The recognition and proof of the theoretical aspects of vertical radiators, together with the realization of their practical forms, required several years.

The natural sequence of events was to apply to the broadcast frequencies the same techniques of theory and construction that were common to the low-frequency systems. These antennas usually consisted of two or more towers or masts supporting an aerial system of wires comprising the antenna. Also in conformance with typical low-frequency practice, these antennas were always operated at a frequency equal to or considerably less than their fundamental frequency.

Little was known among practical engineers about radiation patterns. As designed, the radiation resistance of the original broadcast antennas was low, running from about 5 to 35 ohms, the larger of these values being rare. Ground-system design was still in the black-magic stage. With the exception of a few theoretical studies, made mostly by physicists, there was very little thought directed toward antenna development. What little text and reference material existed on the subject was pertinent to the low-frequency applications, the understanding being that as the frequency increased one simply used smaller dimensions.

The publication by Ballantine, of two historic papers led to the development of the modern broadcast antenna. In one of these papers it was shown that for vertical antennas higher than one-quarter wavelength the radiation resistance continued to rise and went to very high values when the height approximated one-half wavelength. This then pointed to a method of increasing radiation efficiency by using antennas having a radiation resistance very large with respect to the ground resistance, the principal loss factor of the antenna system. A way was at hand to make the radiation efficiency about as high as one wished, by employing vertical height sufficient to arrive at some desired large value of radiation resistance.

The second of Ballantine's papers disclosed a hitherto unknown fact: there was an optimum height of vertical radiator for obtaining maximum ground-wave field strength. This resulted from the space wave pattern produced by waves directly radiated above the ground interfering with those reflected from the ground. The result produced a vertical directivity which concentrated the radiant energy normal to the antenna, that is, along the surface of the earth. In a system such as broadcasting, dependent on ground-wave propagation, the existence of an optimum height of antenna from a radiation-effectiveness viewpoint was of great importance.

Still a third important consequence of Ballantine's principle was to appear later. As the power of broadcast stations gradually increased, the situation soon appeared where the ground wave was interfered with by waves reflected from the ionosphere. Interference between these two waves produced serious selective fading at a rapid rate. In the annular regions surrounding a station where both waves were of about equal intensity, destructive interference was maximum, and any coverage in these regions was rendered virtually useless at night. This fading wall became a major obstacle to further increases in power, at least at night. The only hope in sight was to use Ballantine's optimum-height antenna to reduce the amount of energy radiated skyward at high angles and at the same time to increase the radiation along the ground. This should push the fading wall farther from the station. When practical means were found to construct antennas using this principle, this effect was indeed verified.

The theory of the vertical radiator was developed around the condition that the current distribution along the antenna was sinusoidal from its upper end. It was believed, though not proved at that time, that the current distribution was naturally sinusoidal, or very nearly so. At that time, however, the practical realization of optimum-height vertical radiators was not at hand. The first applications of the new principle were made to the T-type wire aerial operating at a frequency above its fundamental frequency and supported in the usual way by two high towers. This gave a worth-while improvement in radiation efficiency but failed to provide sufficient reduction in fading. At this stage, the knowledge of wave propagation probably had not quite developed to the state where the fading-reduction properties of the optimum-height antenna were apparent. From 1925 to about 1930 the T antenna, operating at about 1 1/2 times its fundamental frequency, was the dominant type of broadcast antenna, and many of the systems constructed during this period continued in use many years thereafter.