CB Co-Phase Antennas (Dual Whips)

The Engineering Web

Ever wonder if tractor trailers know something you don't about CB antennas when you see them with not one, but two CB antennas? Is there an advantage to having two? Let's find out.

Image showing placement of twin CB antennas

You've seen them. Two antennas on big trucks. Usually one on each rear view mirror. Do they really help provide better CB Radio coverage? Are their any pitfalls to avoid? Let's apply some engineering and see what we can find out.

First just how do two antennas fed in phase with a co-phase harness interact? Using an antenna simulation software package called EZNEC we can generate some answers on a computer.

To start we will keep things very simple and simulate two 1/4 wave vertical antennas on average ground seperated by 1/8, 1/4, 3/8 and 1/2 wavelength.

Two antennas seperated by 1/8 wavelength or about 4.5 feet.

Simulation of two antennas eighth wavelength apart.

This reveals there is not much difference between two antennas close together and a single antenna.

Two antennas seperated by 1/4 wavelength or about 9 feet.

Simulation of two antennas quarter wavelength apart.

Here we begin to see how the antennas increase their gain forward and aft of the truck at the expense of some power to the sides... the signal to the side is about 1/2 an S unit lower.

Two antennas seperated by 3/8 wavelength or about 13.5 feet.

Simulation of two antennas three eighths wavelength apart.

Going further, we continue to lose gain to the sides and add gain fore and aft.

Two antennas seperated by 1/2 wavelength or about 18 feet.

Simulation of two antennas half wavelength apart.

Complete cancellation of the signals to the sides results when seperated by exactly 180 degrees. Gain fore and aft is about 6dB or one S unit higher than a signal antenna.

Dual antennas can work, but try to keep them within 9 feet of each other

One final note:

Ensure you use a good co-phase harness designed for 27MHz CB Radio. These are usually two lengths of 75 ohm coax joined to form a single 50 ohm connection to your transciever. Keeping the two feeder lengths exactly the same length is very important if you want to maintain some kind of symmetry of your CB Radio antenna pattern around your truck. Here is an example of when one of the antenna cables is shortened by 2 feet:

Simulation of two antennas fed out of phase.

Here it is again with about 3 feet shorter:

Simulation of two antennas fed out of phase.

This is exactly the technique radio stations and ham radio operators use to design in gain in line with the two antennas. This is probably not what you want for your truck CB Radio installation so make sure your co-phase harness is left the way you bought it. If you have excess cable, just bundle it up somewhere.


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