Radial System and Grounding
When mounting the antenna more than three feet above ground, a resonant radial system
must be used, such as Hy-Gain's 14RMQ Radial System Kit. If the antenna is roof
mounted and the roof space is too small for a radial system, you can droop the radials
over the edge of the roof at almost any angle without seriously changing the performance
of the antenna. The radial system must be insulated from the roof and connected to a
good ground or lightning protection. See Figure 5. For best performance, the 18AVQII
should be ground-mounted clear of building and other structures. When the antenna is
ground-mounted, a radial system is sometimes not needed. In most areas, where soil
surface conductivity is poor and a good ground plane is not possible, lay out ground
radials to improve the efficiency of your antenna.
Installation of Radials
There is no need to make radials exactly ¼ wavelength long for the 18AVQII. In fact, the
only case where you should have ¼ wavelength radials would be for approximately 90
radials. This differs rather dramatically from the case of the Ground Plane antenna where
resonant radials are installed above ground. Since the radials of a ground-mounted
vertical are actually on, if not in, the ground, they are coupled by capacitance or
conduction to the ground, thus resonance effects are not important.
Basically, the function of radials is to provide a low-loss return path for ground currents.
The reason that short radials are sufficient when few are used, is that at the perimeter of
the circle to which the ground system extends, the radials are sufficiently spread apart.
Most of the return currents are already in the ground between the radials rather then in the
radials themselves. As more radials are added, the spaces between them are reduced and
longer lengths help to provide a path for currents still farther out. Since the 18AVQII is a
multi-band, vertical antenna, the radial system should be optimized on the lowest
frequency you plan to use. Higher frequencies will benefit equally from the ground
system, while lower frequencies will not show as much improvement.
To determine the optimum radial installation for your 18AVQII, you must first decide
what the limiting factor for your installation will be. The list below includes factors that
need to be considered.
1. Cost of radial wires
2. Area available for radials
3. Efficiency of your antenna
Use figure 1 below to design a radial system to the optimum length for your preferred
operating band.
