Antenna Ground Plane - Motorola F2265A Owner's Manual

2.4.2 ANTENNA GROUND PLANE

2.4.2.1 General
The ground plane provides an RF current return path
for the antenna. For efficient operation, the loss
resistance in the ground must be small in comparison to
the antenna radiation resistance. Furthermore, the
effective length of the ground system largely affects the
shape of the antenna radiation pattern. A poor ground
will cause an otherwise good radio to perform poorly.
This section provides a description of a ground plane
and explains its installation.
2.4.2.2 Location
A good ground plane must be spread out with the
antenna located normally in the center.
2.4.2.2.1 Ground Plane for Whip Antennas
(23 to 35 feet, 7 to 11 meters, non-roof
mounted)
12 radials, 35 to 60 feet long, form an adequate ground
plane (see Figure 2-7). The 60 feet radials are used for
the longer communication ranges. A ground rod should
be driven into a ground near the base of the antenna to
provide a lightning discharge pass.
Figure 2-7. Practical Ground Plane for a Whip Antenna
2.4.2.2.2 Ground Plane for Wire Antennas
(non-roof mounted)
Twelve radials, 35 to 60 feet long, will form a good
ground plane. A ground rod should be driven into the
ground close to the antenna base to provide a lightning
discharge pass.
The ground plane shape for an Inverted "L" wire
antenna should be distorted with most of the surface
area placed under the antenna (see Figure 2-8). The
sloping wire antenna should have a circular ground
plane shape with the antenna feed point at the center.
a. Side View
b. Top View
Figure 2-8. Ground Plan for Inverted "L" Wire
Antenna
2.4.2.2.3 Ground Plane for Roof Mounted Whips
and Wire Antennas
A fairly effective ground can also be achieved by
grounding to the building structures or metal roof (see
Figure 2-9), provided that the roof pieces are
electrically bound together. Antennas far from earth on
insulated structures require a ground plane (or radial
system).
Two or four radials cut to a quarter or half wavelength
for each frequency used (3/8 x wavelength is a fair
optimum) form an effective ground plane above 7
MHz. Below 7 MHz, the required length gets
unreasonably long; however, more than four radials 30
to 70 feet (9.1 to 21.3 m) long are recommended. A
direct connection to a ground rod driven into the earth,
Installation
2-5