How to maximize S21 for monopoles?

I am trying to simulate two monoples by using MWS. First monopole is the transmitter and the second one is the receiver. Monopoles are fed by a coaxial cable. and there is no ground plane. I wonder if there is a rule of thumb for maximizing S21 given the inner and outer dimensions of the coaxial cable. for instance, is there a rule for the length of the protruding conducter?

What are you trying to do? Inductive or capacitive coupling is much more efficient than free space transmission.

these antennas will be used for another purpose. The idea is this: to create a source as close as possible to a point source. And to create a receiver which does not disturb the local field strongly when measurements are performed.