USING FIRESTIK
"NO-GROUND-PLANE" CB ANTENNAS
IN DUAL ANTENNA CONFIGURATION
Compliments of Firestik® Antenna Company Technical Support Team
Copyright © 1996 Firestik® Antenna Company
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We had, in the past, advised users that the Firestik NPG antennas could be used in a co-phased (dual) type arrangement. We had conducted many tests and those test led us to believe that this was a viable application for the NGP antennas. During the course of doing business we became aware of some unpredictable circumstances which made us re-evaluate this application. As a result of this awareness, we are no longer promoting the use of NGP antennas systems in a twin antenna configuration. The problem stems from the strong radiation pattern created by the design of our antennas. In a true co-phase setup, each antenna will use the radiated field from the opposing antenna as a reflective element. That condition creates the two primary elements of a co-phase system. First, it will compensate for insufficient ground plane and second, will create the peanut shaped wave pattern that is beneficial to those who need or want maximum field strength to the front and rear of their vehicle. Even though a dual NGP set-up is not a true co-phased system in terms of operational characteristic, the reaction between the two opposing antennas, under certain conditions, upsets the singular balance that was designed into the NGP antenna systems. The spacing of the individual NGP antennas appears to be the key element in our decision to bring awareness to the potential problems that can crop up when using dual NGP antennas. And if it were only a matter of recommending a specific minimum/maximum spacing of the antennas we could simply specify that in our literature. Unfortunately, transmitting antennas will sense and react to practically everything that exists within its near (and strongest)radiation field. Those are the things that we cannot control because the possibilities are unpredictable. That is, one vehicle might be fiberglass over a wooden frame and another may have a metal frame. Likewise, one may have a aluminum roof and the other a rubberized coating over fiberglass. There will also be cases where an air conditioning unit or roof top storage compartment may disturb the magnetic fields that are created during transmission. We just cannot know all of the possibilities. Our technical staff has been challenged by this problem and has worked with several end users to fix their system in order to achieve satisfactory SWR. In every case, each situation required a different fix. One would require a specially wound antenna, another a change in the primary coaxial length and the next, a change in the length of the balancing stub coax. Each became a custom design project that drained the resources of the company. If we were to charge an amount that justified the input, the system would be too expensive for the majority of the market. Even though most of the systems are performing with the satisfactory SWR required, those that do not end up consuming all of the value gained from the successful installations. Not only can we see the loss of resources fixing these few problemic installations, but the process can be very draining on the customer who is trying to work with us to make the system work. So far, every person that we have worked with on these problems has been very understanding and we are very appreciative of that fact. They put in a lot of hours changing parts and rerouting cables to solve the problem. |
