How can we realize the previously discussed switch modes? Here we illustrate how to construct a Shunt SPDT switch using an ordinary PIN diode. The SPDT switch is used when high isolation is required. Lets have a look how this circuit functions in transmit state. D1 is reverse biased, open across Tx, low impedance at the antenna at a ¼ length away. D2 is forward biased, shorts Rx, and presents open-circuit to the antenna at a ¼ length away. The transmit insertion loss (IL) depends on both rD and Cd of the PIN diode. Isolation, however, depends on the forward impedance of the diode, that can be made very low, and consequently a high impedance at a ¼ length away. Remember that because of the ¼ length sections Insertion Loss (IL) and Isolation are frequency dependent.