The Wave Antenna A New Type of Highly Directive Antenna

Review of the Subject. — A small bungalow in a grove of oak trees just outside of Riverhead, Long Island, with a line of poles along a country road, carrying two copper wires, and ending by a stream nine miles southwest of Riverhead, — this in brief describes the Atlantic coast “ear” of the Radio Corporation of America, where the wireless messages from England, France, Germany and Norway are received, disentangled, amplified, converted into current of telephonic frequency and automatically relayed over telephone circuits to the Broad St. Office in New York, where operators take the messages by ear, or automatic recorders mark the dots and dashes on tape. The present paper deals with the two copper wires on the line of poles, for they constitute the wave antenna which has not only marked a distinct advance in the reduction of interference and “static,” but because of its aperiodic nature and effectiveness as an energy collector, has made possible the sumultaneous reception of a large number of messages by one antenna, and the automatic relaying of the messages over land wires. The use of two wires is not an essential feature of the wave antenna but permits flexibility in the location of the receiving station. In its elementary form the wave antenna consists of a straight horizontal conductor. (See Fig. 10) of the order of a wave length long, parallel to the direction of propagation of the desired signal, with the receiving circuit located at the end farthest from the sending station and with the end nearest the sending station grounded through a resistance of the proper value to practically prevent reflections. Under these conditions the desired signal waves produce comparatively feeble currents at the end nearest the sending station and strong currents at the receiver end, while disturbances coming from the opposite direction cause feeble currents at the receiver end and strong currents at the end farthest from the receiver (nearest the transmitting station) (See Fig. 2). This comparative immunity of the receiving set to disturbances coming from a direction opposite to the desired signal is lost if reflections are permitted to occur at the end farthest from the receiver. The growth of current in the direction of travel of the space wave depends on the velocity of propagation of waves on the antenna in comparison with the velocity of the space waves, the received current being strongest if the two are equal. If the characteristic wave velocity on the antenna is less than that of the space waves (or less than the velocity of light) increasing the length of the antenna increases the received current up to a certain point, after which further increase in antenna length reduces the received current. The length for maximum signal depends on the velocity ratio and wave length. The slower the antenna or the shorter the wave length received the shorter the length for maximum signal. It is very frequently the case, however, that the best directive properties are obtained with an antenna longer than that which gives the strongest signal. The effect of the space wave is to produce in the wire a signal frequency electromotive force which affects the different parts of the antenna progressively as the space wave passes over the line. On this basis the received current can be calculated in terms of the frequency and intensity of the induced electromotive force, the direction of the space waves and the length and electrical constants of the antenna. By assuming the direction to be changed while all other factors remain the same, and calculating the relative value of received current for various directions of signal wave, we can determine the directive properties of the antenna. The result is best shown by means of a polar directive curve. For each assumed direction for which the received current has been calculated, a radius is drawn, with length proportional to the received current. The curve drawn through the ends of these radii is known as the directive curve for the antenna. Directive curves are given (Figs. 35 to 41) which bring out the effects of antenna length, relative to the wave length, velocity of propagation, and line attenuation. The directive curves are for the most part drawn with the maximum radius taken as unit length, since this makes comparisons of directive curves easier. In general, it is found that moderate line losses are not appreciably detrimental to the directive properties of the antennas, while the fact that velocities obtainable with unloaded lines are materially below that of light, results in an actual improvement in directive properties in most cases. As a rule the longer the antenna the sharper its directive curve. While it is possible to obtain fair directive properties with antennas less than a half wave length long, this length is considered about the shortest that can be recommended. By a process of balancing, it is possible to produce a “blind spot” or direction of zero reception, at any angle more than 90 deg. from the signal. One method of obtaining this result is by producing reflections of certain phase and intensity at the end opposite to the receiver. Reflections at the receiver end of the antenna, on the other hand, do not alter the directive properties of the antenna. Experimental work thus far has given a qualitative check on the theory and calculations of the wave antenna, and it is hoped that further observations and measurements will shortly be made. Experimental data on wave front tilt, on which the action of the wave antenna depends, is especially meagre. Data on wave velocity and line losses on an existing antenna can be obtained by means of a radio frequency oscillator and one or two hot-wire milliammeters. Measurements taken by the writers show much higher attenuation and lower velocities for ground return circuits than for metallic circuits. Ground resistance explains this effect. The mean depth of return currents at the longer radio wave lengths appear to be of the order of several hundred feet. The more wires in multiple in the antenna, the lower the velocity and the higher the rate of attenuation (See Fig. 66 and 68). Reduction of atmospheric disturbances or “static”, has probably received more attention from experimenters than any other one phase of radio reception. Of the various lines of attack none has been more fruitful than the employment of directive receiving systems. Every increase in directivity has resulted in an improvement in stray ratio. The wave antenna carries the principle farther than any previous type. Conditions on the eastern coast of North America are especially favorable for taking advantage of differences in direction, for the European stations are to the northeast while the predominating direction of static is from the southwest. Various practical engineering problems in connection with the wave antenna, including its application to short wave reception, are discussed toward the close of the paper.