The invention is a class of planar unidirectional traveling-wave (TW) antenna comprising a planar four-arm TW radiator ensemble, such as a 4-arm spiral, which is fed medially with a twin-lead feed connected with only a pair of opposite arms of the TW radiator, with the other two arms parasitically excited. The use of a mode suppressor enhances the purity of single-mode TW propagation and radiation. The twin-lead feed is connected with the balanced side of a balun, and is impedance matched with the TW radiator on one side and the balun on the other side. This simple feed structure using a single balun is generally smaller and much simpler, and thus much less costly than the conventional feed for a 4-arm spiral, which is a complex one-to-four power divider that contains hybrids, power dividers, couplers, matrices, etc.
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