Outrun your rear camera?

Radio signals (RF) are nothing more than electro-magnetic disturbances cause by placing any varying electronic signal on a piece of wire. That electronic signal generates an electro-magnetic field by conducting through the air to a ground or ground plane. That path is commonly referred to as a ground lobe. An antenna can be made directional by simply placing a ground plane in one direction. It is made omni directional by providing equal ground lobes in all directions. Land based transmitters use the earth as a ground plane, thus the term. Mobile systems use a metalic surface to simulate a ground. If it's length isn't appropriate to the frequency transmitted, it becomes an inefficient ground plane. Ideally, mobile antennas have an active antenna element with a wavelenght appropriate for the frequency transmitted and a ground plane of equal dimensions. In reality, the ground plane for mobile applications is the skin of the vehicle. It is not ideal because it is abnormal in shape and conductivity.

EDIT Since most vehicles are longer front to back than side to side, the ground plane for vehicle mounted transmitters is usually more efficient front to back than side to side, thus my initial BS comment regarding ground plane issues when transmitting from trailer to cab.

EDIT On the receiving end, that same electro-magnetic field passes through a wire (the antenna) and causes a current to flow which creates voltage that is the recieved signal. Receiving antennas are also directional because they are designed to reflect/direct the signal or to shield it from being received.