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If two similar unbalanced transmission lines are coupled together by sharing a common outer conductor for part of their length, so as to form a 3-conductor line, they constitute a symmetrical directional coupler. The output ratio of this varies sinusoidally with frequency, being greatest for lengths equal to an odd number of quarter-wavelengths; the characteristic impedance is independent of frequency, being determined by the cross-section of the 3-conductor line. Two forms of the directional coupler are described. One of these, intended for laboratory power measurement at frequencies up to 1 000Mc/s, consists of two parallel strips enclosed in a square-section outer conductor. The other, a very simple device for monitoring transmitter power, consists of two lead-covered coaxial cables grafted together. Among possible uses suggested are all-pass filters and applications to aerial systems consisting of two parts fed in phase quadrature.