Recently a novel printed antenna was proposed consisting of an annular ring coupled to a shorted microstrip patch [1]. The resulting configuration is smaller than a conventional microstrip patch antenna, with enhanced impedance bandwidth. This latter feature can be attributed to the fact that the antenna consists of two coupled radiators, similar in concept to a conventional parasitically coupled configuration, although considerably smaller.

Despite the previously mentioned advantages of the annular ring coupled to a shorted patch, unfortunately there are several drawbacks. First, the cross-polarization level is typically higher than a conventional microstrip patch antenna, typically -10 dB. The higher cross-polarization can be attributed to the presence of the shorting pin, which is required to reduce the size of the driven patch. In some applications the levels obtained are still acceptable, however, for generation of circular polarization, this higher cross-polarization will degrade the achievable axial ratio. The other drawback is the sensitivity of the impedance of the antenna to the relative position of the shorting pin with respect to the probe feed. The loading effect of the annular ring on the shorted patch does allow the shorting pin to be moved further away from the probe feed, easing manufacturing tolerances. However, a reasonable degree of positional accuracy is still required.

In this paper, the above mentioned drawbacks are addressed and means of alleviating these seemingly inherent problems discussed. Also, techniques are presented for reducing the size of the antenna so that it can be accommodated in typical mobile communication handsets. One such technique includes using high dielectric constant materials. The merits and disadvantages of this method are examined, and comparisons of theory with experiment for input impedance and radiation patterns are presented.

[1] D. M. Kokotoff, R. B. Waterhouse, and J. T. Aberle, ``An annular ring coupled to a shorted patch,'' to appear in IEEE Trans. Antennas Propagat., 1997.