Theoretically, the loss between vertical and horizontal polarization is infinite. However, in the "real world", 20 dB is the generally accepted figure. In specific cases, the loss can be at least 30 dB and occasionally greater.
Where circular polarization is concerned the same theoretical infinite loss happens when right hand is used for one signal and left hand is used for the other signal. Again, in the "real world", 20 dB is the generally accepted figure.
Now, when the cross polarization is between circular polarization and vertical, horizontal, or any other linear polarization, the loss is 3 dB. Glen, K9STH
For "line of sight" communications where can I look up the signal loss due to polarization difference for a given receiving antenna polarization facing a transmitting antenna with a known different polarization? Al-N7EQF