Truck drivers' made co-phase antenna set-ups popular back in the early 70's. There are several good reasons for their use. First of all, co-phased antennas create a directional pattern that favors communications in front and in back of the vehicle. This is ideal for truckers and RV's that use their radios to speak to those on the same road/highway that they are traveling on. If you are speaking with someone in front or behind your vehicle and either one of you take a turn and leave the more powerful RF lobe, a definite change in clarity will be noted.
The second advantage of co-phased antennas is there ability to perform where there is little reflective ground plane for the single antenna to radiate its energy from. For instance, on fiberglass vehicles or those with light aluminum sheeting over a non-metallic frame. In those situations a co-phase set-up allows the antennas to use each other's radiation field to direct the combined energy across the horizon. In order to be effective at least 2/3's of each antenna must have unobstructed line-of-sight to the other antenna.
The third justification for dual antennas involves situations where some or the entire signal would be blocked if a single antenna was used. For instances, if you were pulling a large trailer. In such cases, having an antenna on both sides of the vehicle prevents signal blockage. Note: Drivers that pull large trailers and choose to use one antenna will (should) mount the antenna on the drivers side of the vehicle in order to maintain contact with vehicles traveling in the opposite direction.
One additional reason has to do with power. Stock CB's leave the factory with the transmitter power limited to 4 watts but there is an underground market of linear amplifiers in existence. Inasmuch as power will eventually turn into heat, and the antenna will be required to dissipate that heat, two antennas will more than double the heat dissipation abilities of a single antenna.
For general communication, with an Omni-directional radiation pattern, a single antenna system is all you probably need.
Single antenna installations for 2-Way Radio requires 50-ohm coaxial cables such as RG-58A/U or RG-8X. Dual antenna installations require 75-ohm coaxial cable (co-phase harness) to perform properly. Note: From time to time we hear from people in the field that have made co-phase harnesses from 50-ohm coax and report better than average SWR. In tests performed at our facility we found that the lower SWR readings were a results of power losses that often exceeded 50% of the radio output when compared to RG-59A/U coax and higher SWR readings. Accordingly, do not use 50-ohm coax for co-phase antenna installations.
Spacing of the antennas in a dual antenna installation will have an affect on the radiation pattern of the antennas. Technically, spacing between the antennas should be nine feet (2.7m) to have the most balanced pattern. If that distance cannot be achieved, that alone should not discourage you from using co-phased antennas as you will still receive most of the benefits of a dual antenna set-up. However, spacing less than five feet (1.5m) may require some special tuning skills in order to achieve satisfactory SWR readings.
An article that deals specifically with dual NGP antennas is available by clicking here.