Basically, when you mount a transmit antenna on a vehicle, the metal mass of the vehicle becomes what's called the "ground plane". The signal reflects off the ground plane of any antenna, therefore the signal is stronger in the direction of the offset ground plane. Back in the 80s, before the advent of the internet, we got our jollies and met people on the CB and many of us would have what's called a T-hunt. 1 person would park somewhere hidden, like a back parking lot of a business and transmit while the rest of the group with antennas mounted on the rear of the vehicles would use thier recieve meter and thier RF Gain dial to pinpoint direction from 2 different points to triangulate where the hiding person was. It was a lot of fun and shows ya how bored we were before the advent of the net.