Abstract: An RF waveguide signal transition apparatus for minimizing leakage at RF frequency is described which consists of providing a plurality of plated through-holes (16) of blind holes in a circuit board (10). The plated holes (16) are disposed on the circumference of a circle around the RF waveguide to microstrip transition of probe (14) and each of the plated holes (16) is connected to a ground plane (24). The size of distribution of the plated holes provides a barrier to signal leakage at RF frequencies in the range 10.95-11.7 GHz and they also provide a transition impedance similar to the 50 ohm characteristic impedance providing satisfactory matching and minimizing leakage into the board. The apparatus can also be used to minimise leakage from microstrip to microstrip transitions passing through the circuit board.