When designing a two-port antenna, S11 is quite good compared to other antennas' performance like gain, efficiency, etc. However, S21 is around between -6 and -10 dB.
If we consider that a commercial coupler will be used, what would be the effect of this S21?
If I already got the good antenna performance (S11, gain, efficiency, etc.), is S21 still important?
S21 presents how much there's interconnection between the ports. If we assume you have in port 1 a radio transmitter and the receiver is connected to port 2 you receiver can have hard times if S21 is only minus few decibels.
If both ports have a receiver some problems can arise from high S21, at least if one of the receivers is not well matched. You can get frequency dependent attenuation. I think -6...-10dB is meaningless.
If both ports have a transmitter and there's near zero S21 some malfunction can be caused. Leaked strong signal can make the output stage of the other transmitter to a mixer which operates at high signal levels.
If your S21 is high, you're transferring energy from port 1 to port 2 and it's not actually radiating. Therefore your radiation efficiency will be low (and you could potentially damage a device connected to port 2).
If you're using a commercially available 3D EM tool (like HFSS or CST) be sure to check realized gain not just directivity and IEEE gain.
