This paper proposes switchable radial stub resonator for isolation improvement of Single Pole Double Throw (SPDT) discrete switch for Time Division Duplex (TDD) switching of wireless communications. A commercialized discrete PIN diode is used to switch the radial stub resonator between bandstop to allpass response where an analytical modeling ofthe switchable resonator is presented and analyzed. In this analysis, correlation between inner radial and angle radial stub with the characteristic impedance and attenuation pole is determined. Isolation improvement is analyzed with two-port network of single shunt PIN diode with switchable radial stub resonator where it is found that an additional isolation can be obtained with the switchable radial stub resonator. In measurement result, the SPDT switch with switchable radial stub resonator has shown more than 30 dB of transmit-receive (Tx-Rx) isolation at centre frequency of 3.5 GHz giving better isolationcompared to conventional SPDT switch design. The potential application of this SPDT switch is TDD switching for WiMAX and LTE communication system.