This is interesting. From the Firestik page about the dual setup:
Co-phased (or dual) antennas are normally chosen for use by vehicle owners that spend much of their time on the interstate highways. Dual antenna systems create a transmission pattern that has increased field strength directly in front and behind the vehicle. This is especially helpful on vehicles pulling large trailers that tend to shield the antennas radiated energy in one or more directions. Co-phase antenna systems are also useful on vehicles that are lacking in ground plane (composite materials, etc.) as they use the magnetic field from one another as the RF field counterpoise. However, to work properly the antenna mounts MUST have a good chassis ground and the antennas MUST have unobstructed line-of-site between the top 50% or more of the antennas overall length.
There is also much talk regarding the spacing between the antennas. Many people believe that dual antennas will not work unless they are spaced 1/4 wave length (approx 9ft) apart. While this may be true to the nth degree, it does not mean that they will not work with lesser spacing. The spacing will have some effect on the radiation pattern but short of testing every installation and weighing the results against a 9ft spaced installation on the same vehicle, it cannot be assumed that the spacing is better or worse for performance on that particular vehicle. We have seen satisfactory results with antennas spaced as close as 5ft apart.
And don't forget ... ALL antennas, regardless of style or manufacturer, MUST be tuned after installation on the vehicle. Transmitting antennas are not "plug-n-play" devices.
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