The basis for a good antenna system is a good ground, a good feedline, and sufficient ground plane.
Two antenna systems does not require much planning, but if you are going to extend the system for shootout, then you need to map the system out before you start. The following text assumes previous antenna experience. The reflector antenna in all of the described systems will be grounded through a capacitor, although grounding the rear antenna will give close results.
If you are planning on using only 2 antennas, a convenient spacing may be used between the two antennas anywhere from 6 foot to 86 inches. 6 foot will give the most gain for 2 straight sticks but cannot be expanded into a competitive 3,4 or 5 antenna system. 86 inches will give the most gain for 2 coils and can be expanded into any shootout system.
Regardless of spacing, in the 2-antenna system, you want some ground plane area in front of the 'HOT' antenna. Placement of the rear antenna farther back can also help in two ways, It closes the back door more since there is less ground behind the antenna: and it gives the 'HOT' more room in front of it.
To quickly tune 2 antennas, place only one antenna in the 'HOT' position. Tune it for the lowest reflect. Next move this antenna to the rear. Put the second antenna in the 'HOT' position and start with it 5 inches shorter than the first one ended up at. Tune until the lowest reflect is reached. Very little fine-tuning of the rear antenna may be necessary. If the antennas don't tune very quickly in this manner, then the antennas may not be resonant and other antennas should be tried.
The performance of the two-antenna system can be compared to a single antenna by changing from one system to the other. A station a few miles away can give the results for both receive and transmit. Another test with the two antennas is to transmit to the other station while turning your truck around. The station should report a large change in signal from the point at which you were facing the station, and the point at which you were facing away.
Another test that you can perform alone is to use a field strength meter and walk around the truck while your radio is keyed. Keep the same distance from the 'HOT' antenna as you walk around the truck and you should get a good idea of the shape of your radiating pattern.
Adding a third antenna requires bracket fabrication and usually is part of an expansion to four antennas. The bracket is usually made of 4 inch C-channel. It is grounded to the roof at the mounting point closes to the 'Hot' antenna, and is isolated by the insulators closest to the windshield. The insulators also serve as leveling adjustments. The antenna mount on the third bracket is isolated for RF. It should also be movable so the mount can be moved forward and backward on the bracket. The proper distance for the third antenna varies with the vehicle ground plane. Recommended values are from 40 to 50 inches.
The length of the antenna will also vary with systems. It should not be longer than the 'Hot' antenna, and not shorter than 6 inches below the 'Hot' antenna. NOTE: when changing from a 3 antenna to a 4-antenna system, the third antenna will have to be adjusted both ways (height and distance), as you are adjusting number 4. The 3-antenna system does not show a large amount of gain on the field strength meter or on a receiving station several miles away. In competition, it provides the antenna system with more "Push Away" power when keying next to another truck.
The four-antenna system can be very different from one truck to another. The two basic types of brackets make the systems very different. One bracket system extends the bracket from the roof out to accept a fourth antenna about six feet from the number 3 antenna. The antenna mount is insulated and the antenna ends up pretty close to the same length as the third. The bracket is grounded at both ends. The other bracket system mounts on the front bumper and extends out a distance that changes with hood length and grill material type. Some grills are made of plastic and some are metal so the capacitance changes. If the bracket is metal, two choices of mounting are isolated and grounded. In the grounded system, the fourth antenna is very short, around 90 inches. In the Isolated mount, the antenna is much longer, 140 inches or more. If the bracket material is PVC, then you have an isolated system with a longer antenna, around 140 inches or more. Both antenna systems work well but to decide which will work on your vehicle, you must spend some time tuning antennas and measuring gain. Keep notes on changes that you have made so you can find the best gain for your system and can revert back if things change to the bad. Good luck, and happy tuning.
The above paper was taken from '55's website a few years ago and I have taken the time to pass it along. As stated above this has been reproduced with 55's knowledge and permission.
Click Here to Visit Skater229.com