Recently a small printed circular antenna was proposed which incorporated a single shorting post to significantly reduce the overall size of the antenna [1]. Due to the inherent small physical dimensions of the radiator (with a radius, R = 0.025 l_g compared to R = 0.076 l_g for a conventional circular microstrip patch mounted on the same substrate), the antenna is very suited to situations where minimum antenna real estate is a premium. Such scenarios occur for mobile communication hand-sets, for both present day and next generation mobile systems where the operating frequency is still relatively low. As is expected for electrically small antennas, the price paid for reduced antenna real estate is unfortunately reduced impedance bandwidth, as well as lower gain. However, it has been shown that by using techniques usually employed for conventional microstrip patch antennas (for example, the use of low dielectric constant substrate), these parameters can be improved [2].

A fundamental problem with the single shorting post microstrip patch antenna is the dependence of the input impedance of the antenna on the relative positioning of the probe feed with respect to the short circuited pin. This feature in itself is not unexpected; however, to achieve a 50 W match at the resonant frequency, typically the relative position is a small fraction of a wavelength (0.002 l_g), thus requiring very accurate fabrication processes. This is primarily due to the small radiator relying on the physical discontinuities associated with the close proximity of the probe to the post to achieve the necessary currents on the printed antenna.

In this presentation, techniques are proposed and investigated which can alleviate the fabrication difficulties associated with shorted printed antennas. One such method incorporates the use of multiple shorting pins aligned in the same plane. Simplistically, as the number of shorting pins along this plane increases, a `stronger' shorting plane is established on the radiator, and thus the performance of the printed antenna approaches that of a conventional quarter-wave patch. Importantly, as the number of shorting pins is increased the relative position of the probe feed with respect to the shorting plane is enlarged. This ease in fabrication tolerances is achieved at the expense of increased antenna real estate. Thus a compromise between fabrication tolerances and overall size must be considered. For example, using two shorting posts in the previously mentioned case increases the separation of the probe feed and shorting plane to 0.004 l _g and increases the radius of the patch conductor to 0.029 l _g. A rigorous analysis of this configuration and other shorted patch variations was achieved by generalising the analysis presented in [3], such that a multiple number of pins can be included. A thorough investigation of these shorted patches will be presented, including an investigation of the impedance performance and the far-field radiation properties.

REFERENCES

[1] R.B. Waterhouse, ``Small microstrip patch antenna," Electron. Lett., vol. 31, pp. 604-605, Apr. 1995.

[2] R.B. Waterhouse and S.D. Targonski, "Performance of microstrip patches incorporating a single shorting post", to be presented at IEEE Antenn. Propagat. Symp., Baltimore, 1996.

[3] J.T. Aberle, D.M. Pozar and C.R. Birtcher, ``Evaluation of input impedance and radar cross section of probe fed microstrip patch elements using an accurate feed model'', IEEE Trans. Antennas & Prop., AP-39, pp. 1691-1697, Dec. 1991.