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This paper describes a multiport, biconical antenna that can be used as a wide-band direction finder or multiplexer. When the antenna is used for reception, incident plane waves excite in the feeding coaxial line both the TEM mode and the orthogonal TE11 mode whose azimuthal orientation depends on the direction of arrival of the signal. Directional information can be obtained by measuring the sum of these two modes at four fixed detectors arranged at 90-degree intervals around the coaxial lines. Alternatively, the linearly-polarized TE11 mode can be resolved, with appropriate circuitry, into right- and left-hand circularly-polarized TE11 modes. Then directional information can be obtained by measuring the phase difference between either of the circularly-polarized modes and the TEM mode. This latter form of direction finder is well suited to multiplexing applications, because each of its three ports is well isolated from the others. Furthermore, this antenna has the unique property that a signal fed into any one of these three ports will cause the antenna to radiate omnidirectionally in azimuth. A waveguide version of this antenna has been built and tested and found to operate well over the frequency range of 8.2 to 12.4 kmc.