What is the purpose of the following configuration of diodes on an RF line?
In this application, the forward voltage drop of the diodes means that they have a relatively high impedance for voltages less than about &plusmn;0.65V, which effectively disconnects the source impedance of the pulse driver from the transducer when receiving, preventing the driver from "loading down" the high impedance of the sensor and reducing the overall sensitivity.

When transmitting, the drop is negligible.