I decided to film a video showing the strong effects of the polarized radar emanating from microwave door openers. I used my 9500i for this video. You will see that orientating a modern RD (my 9500i) vertically versus horizontally greatly reduces the received signal strength from the door opener. Here is the video:
Microwave Door Opener Polarization Demonstration
Before you think of mounting your RD vertically to greatly reduce false alerts from modern K band microwave door openers, consider the fact that the off-axis sensitivity of your vertically mounted RD will be different compared to when it is mounted horizontally in its "normal" orientation. I haven't yet taken the time to either test or make videos of the differences in off-axis sensitivity when a RD is mounted vertically versus horizontally. Also consider the fact that very few RDs use neutrally flared horn designs. Most RD horn designs feature a predominately upward or downward flare. This means that, if you orient your RD vertically, off-axis sensitivity will be different towards the left of your car versus towards the right of your car.