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The equations for the radiation patterns of directional antenna systems are well known, but the arithmetical work necessary to secure a plot of the radiation pattern is tedious and time-consuming. Several mechanical plotting devices to assist with the problem have been described in the literature. A brief review of a few of these instruments is presented. These mechanical devices yield the radiation pattern for a given choice of configuration and antenna constants. In general, however, the designer of a directional antenna for broadcast-station use knows the pattern required and is faced with the problem of determining the antenna configuration which will yield this pattern. The RCA Antennalyzer was developed to synthesize or to analyze. The instrument is entirely electrical, with no moving parts except the potentiometers which change the various parameters. Developed specifically for the design of directional antennas for broadcast use, the Antennalyzer, as constructed, will yield the radiation pattern of directional antennas which have as many as five towers or sources of radiation. Each source is characterized by four parameters: (1) the distance from a reference point; (2) the azimuth angle with respect to a base line; (3) the amount of current in the antenna; and (4) the phase angle of this antenna current. Thus the Antennalyzer has four potentiometers associated with each antenna, with one exception. One antenna is located at the reference point and carries unit current at zero phase. Hence, no controls are required for this antenna.