A novel approach for the design of a compact multiband monopole antenna for improving radiation characteristics at higher resonant frequencies is presented. The proposed structure consists of a conventional printed monopole loaded with spiral ring resonators and fed by microstrip. When at higher resonant frequency, the electrical length of the conventional monopole antennas is relatively large and the surface currents distribute on the patch periodically which will degrade the omnidirectional property. To achieve good radiation characteristics at the upper bands, two spiral ring strips are inserted into the microstrip line on the different layer and connected through via hole. Thus, the direction of surface currents on the spiral ring strips changes with the alteration of spiral structure and their effects on the radiation pattern are reduced. The radiation pattern is mainly contributed by the surface current on the microstrip line and very good stable radiation pattern can be obtained within all the operating bands. In comparison to the previous printed strip monopole structures, the miniaturized antenna dimension is only about 28 mm×20 mm×1 mm. The experimental results show that the proposed antenna can provide operating bands which meet the required bandwidths specification of 2.4/5.2 GHz WLAN and 3.5 GHz WiMAX standard. Detailed design considerations of the proposed antenna are described, and both the simulated and measured results of the proposed antenna are also presented and discussed.
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