This paper proposes an equivalent circuit of a bidirectional antenna using a probe excited rectangular ring. The field expressions are derived by using Green's function. The impedance of the antenna is realized by using an induced EMF method. The input impedance at the probe of the antenna for various probe positions and probe lengths is considered. The appropriate parameter that accomplishes the matching condition at the probe is clarified. Therefore, all of these conditions have to be considered in the design for a probe excited rectangular ring antenna. From the results, it is obvious that the probe position of s = 0.05/spl lambda/ and s = 0.07/spl lambda/ provides optimum matching condition when 0.29/spl lambda/ < l < 0.35/spl lambda/ and 0.2/spl lambda/ < l < 0.29/spl lambda/, respectively. For the input impedance at the probe, the probe position of (x = 0.05/spl lambda/, y = -b/2, z = 0) is chosen because of the impedance matching together with symmetrical radiation pattern. Moreover, it is found that SWR is lower than 2:1 when the probe length is longer than 0.21/spl lambda/ for any case and the lowest SWR occurs when the probe length is around 0.28/spl lambda/.