Good info in this thread. Ground plane and polarization of antennas theory is very interesting and gets into the nittygritty of rf electrical engineering.
But it's not usually found in the advertisements for even quality cameras, and for most laymen it's really not a "need to know".
The main thing to look for in a wireless camera is interference elimination in the design and reliability, both of which are found in WIRED camera systems. More difficult to install, but if you count on seeing that car behind you changing lanes and possibly presenting a hazard, then the wired system is for you. It's not to say that wired systems are totally reliable, because there's always the connection at the tow vehicle that can become intermittent. But that connection is easily repaired.
I have a wired camera, and I have been tempted to get a wireless one, but I still come back to the reliability quotient of the wired camera.