Broadband, robust, low profile monopole incorporating top loading, dielectric loading, and a distributed capacitive feed mechanism

Various impedance transformation techniques have been used to implement broadband, electrically-small top-loaded monopole antennas. While these impedance transformation schemes are quite useful, the bandwidth of such an antenna is still limited by the high radiation Q of the top-loaded monopole at its series resonance. The radiation Q is a very strong function of electrical size and hence moving the operating point slightly above the series resonance provides a very substantial decrease in radiation Q. We present a new design which is designed to exploit the region above the series resonance where the radiation Q is lower but compact size is still obtained. Because the radiation resistance is larger in this region, no broadband transforming schemes are necessary. To tune the antenna in this region, it is necessary to provide a series capacitive reactance in order to cancel the inductive reactance of the antenna. This can be done with an external tuning capacitor. However, it is desirable that this capacitance be integral to the antenna assembly to enhance reliability and facilitate low-cost manufacture. In particular, it has been found very useful to use a distributed capacitive sleeve in order to exploit the attendant frequency variation in reactance slope to effect a broadband impedance match.