Evaluation of input impedance of a bidirectional antenna using a probe excited rectangular ring

This paper presents the impedance characteristics of a bidirectional antenna using a probe excited rectangular ring. The field at the aperture is derived from the field inside the ring by using Green´s functions. The impedance of this antenna is realized by using the induced EMF method. The impedance of a probe with optimum ring length that provides the single mode is firstly determined. Then, the resultant input impedance of the antenna is the reciprocal of the combination of the probe admittance and both apertures admittance. Experimental results, verifying the impedance of the antenna, agree very well with the theoretical predictions. This antenna is suitable for a bidirectional base-station antenna in a street cell.