So i have 2 antennas on my car, one cb antenna and one for the vhf/uhf ham bands. How far apart do they really need to be if i only ever transmit on the cb antenna? As i understand it rx isn't necessary effected by distance between the antennas just tx. Im mainly trying to optimize my cb antenna location.
Well, as with many things, it depends on a number of variables. There is no one measurement that will guarantee protection.
The amount of space is what ever is needed to keep too much RF from getting into the other receiver. Since radio manufacturers usually don't list a maximum rf input on the receivers, you sort of have to do some trial and error. 1/4 watt has been tossed around. So, make sure that when you transmit with one radio, less than 1/4 watt makes it to the other receiver. Less is better.
A good way to check that is to have an accurate watt meter that will work on the correct frequencies and will measure down to a 1/4 watt or less. Terminate the watt meter with a 50Ω load, and connect the other side to the receiving antenna. Transmit with the transmit radio and read what the meter says.
The amount of power will vary depending on spacing of the antennas, coax cable length (all coax has loss), coupling between the antennas, etc.
But, also keep in mind that you want a proper ground plane under the antenna. Don't put the antennas out on the very edge of the roof. For the dual band ham radio antenna, you want at least 19" of metal around the base of the antenna to give it a proper ground plane.
On CB, a 1/4 wavelength is around 9 feet, and you won't get 9 feet of metal all around the underside of the antenna on any road legal vehicle, so just give it as much as you can.
