| 摘要 |
A dielectric-loaded loop antenna for operation at frequencies above 200 MHz has an elongate cylindrical core with a relative dielectric constant greater than 5, a pair of co-extensive helical antenna elements, a coaxial feeder structure extending through the core from a proximal end to a distal end where it is coupled to the antenna elements, and a balun formed on the core cylindrical surface and connected to the feeder structure at the proximal end of the core. Each helical antenna element is bifurcated at an intermediate position so that proximally, it is formed of two generally parallel branches each of which is coupled to a respective linking path around the core to meet a corresponding branch of the other elongate element therefore forming a conductive loop between the two conductors of the feeder structure. The two conductive loops have different electrical lengths as a result of, for example, the branches being of different lengths. In a preferred embodiment, the linking paths around the core are formed by the rim of a split conductive sleeve constituting the balun. The sleeve is formed in two parts separated by a pair of longitudinally extending diametrically opposed quarter wave slits each of which extends from the space between the branches of a respective helical antenna element to a short circuited end adjacent the proximal end of the core.
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