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Analytical Model of the Split-Coaxial Balun and Its Application to a Linearly-Polarized Dipole or a.pdf

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 7, JULY 2007 1909 Analytical Model of the Split-Coaxial Balun and Its Application to a Linearly-Polarized Dipole or a CP Turnstile Sergey N. Makarov, Senior Member, IEEE, and Reinhold Ludwig, Senior Member, IEEE Abstract—A simple analytical transmission line model of a split-coaxial balun is proposed and tested. It is based on the cou- pled symmetric transmission line approach. The model leads to a closed-form analytical expression for the termination impedance/ transfer function of the complete antenna system that includes antenna, balun, and a non-splitted coaxial line of certain length. It accepts the impedance of a center-fed symmetric antenna as input parameter. The model is in excellent agreement with full wave simulations and is confirmed by a series of prototype measurements. It can be applied to a linearly-polarized radiator or to a broadband circularly-polarized turnstile, where the phase quadrature is created by two complex-conjugate impedances. The present model only describes the impedance matching properties of the balun but is not capable of predicting the balance quality of baluns. Index Terms—Antenna feeds, antenna theory, baluns, dipole an- tennas, turnstiles. I. INTRODUCTION AN initial motivation for this work was a necessity to designa low cost mast-conformal linear 20%-wideband UHF wire dipole (400–500 MHz) with the controllable complex antenna factor (CAF) and the controllable phase characteris- tics. Some commercially available dipole antennas are either narrowband (cover the desired band in steps of 20–25 MHz) or have a sufficient bandwidth (300–1000 MHz—TV dipoles or EMI dipoles) but an unknown phase transfer function. The most involved part of the design becomes a balun that simultaneously plays the role of a lossless impedance matching network and an antenna support (a mast). Conventional baluns for symmetric wire dipole antennas are briefly reviewed in several scholarly sources -
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