'55' Antenna Magic
The basis for a good antenna system is a good ground, a good feed line, and
sufficient ground plane.
Two antenna systems do 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
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 foot
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