Pin diodes are special diodes that enable electronic circuit switching at radiofrequency. An excellent reseource on PIN diode theory can be found in chapter 2 of the PIN Diode handbook from Microsemi. This article also contains some circuit suggestions. Also look at AN922
The first thing to note is that there will be at least two PIN diodes in each of the antenna side and LO side switching circuits. (Look at the document that describes the project). Thus you will need at least four PIN diodes. Secondly, when designing a PIN diode control circuit you will need a transistor or op-amp buffer to drive the diodes from the MCU. Thus once you have found a suitable PIN diode type, you should check the current requirements to switch the diode and then design the buffer to interface the MCU to the diode switching circuit.
Ex. 19
Coaxial cables provide a convenient way of transferring impedances
such as the switching of a short circuit. An electronic switch at the
opposite end of a 1/4 length transmission line is opened, thus
producing a short circuit at the input. Look at the following circuit
which allows an antenna to be toggled electronically between a high
power transmitter (TX) and a receiver (RX) using PIN diodes as
switches...
The resistance
values refer to the characteristic impedances of the lines.