Yep, Tommy's right. The diode is a one-way-street for the flow of electricity so it will allow the transmitter circuit to "suck down" the voltage on the module input but won't allow that voltage to go any higher than the 3V the module uses.
The schottky diode goes in series with the signal to block current from flowing in one direction (from the transmitter to the module where excessive voltage could damage stuff).
A Zener diode effectively does nothing until the voltage across it exceeds the rated voltage -- then it begins to conduct excessive voltage. You could use a zener between the PPM in put and ground but that's not the best solution for a number of reasons -- not the least of which because it would also require an extra resistor to avoid the excess current which might flow from overheating the zener.
So the Shottky diode is like a one-way valve and the zener is like a bypass valve.