9
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Length ft
Frequencyin MHz
( )
=
240
NOTES ON GROUND/RADIAL SYSTEMS FOR VERTICAL ANTENNAS
GROUND MOUNTING
A vertical antenna in its simplest form is electrically equivalent to one-half of a dipole antenna stood on end. When the
antenna is mounted close to the ground, the earth below takes the place of the "missing" half of the dipole. If ground
conductivity is fair to good, a short metal stake or rod may provide a sufficiently good ground connection for resonant and
low SWR operation on the bands for which the antenna is designed. This basic arrangement is shown in figure one. In
most every case, however, the efficiency of a vertical antenna will be greater if radial wires are used to improve ground
conductivity as in figure two. Wire size is unimportant, and, in most ground-mounted installations, the radials may be any
convenient length. They need not all be the same length, nor do they all need to be laid in a straight line. It is generally
more convenient to slit the sod and to push the radials into the slits to a depth of several inches, but they may be left on the
surface of the ground if they do not constitute a hazard. A large number of long radials will naturally be more effective in
reducing ground losses than a small number of shorter ones, but one should remember that the greatest loss will occur in
the earth near the base of the antenna where current flow is greatest. For this reason, it is generally better to use a larger
number of radials of shorter length than a smaller number of longer ones for a given amount of wire.
In some cases wire mesh (e.g., "chicken wire") may be used as a substitute for radial wires and/or a ground connection, the
mesh or screen acting as one plate of a capacitor to provide coupling to the earth beneath the antenna.
It should be noted that A GROUND ROD IS USEFUL ONLY AS A DC GROUND OR AS A TIE POINT FOR
RADIALS. IT DOES NOTHING TO REDUCE RF GROUND LOSSES, REGARDLESS OF HOW FAR IT GOES INTO
THE GROUND!
ABOVE GROUND MOUNTING
It is possible to operate a vertical antenna at any height above ground provided that something is done to supply the
equivalent of a ground connection. It is NOT ENOUGH TO RUN A LONG LEAD TO A GROUND ROD OR COLD
WATER PIPE, for current will flow in the lead, making it part of the overall antenna length and detuning the antenna.
The usual approach to this problem is to install three or four resonant quarter-wave radials at the base of an elevated
vertical and to connect them to the braid of the coaxial feedline as well as to the supporting mast or tower (if metal).
Unlike the ground-mounted case, length of above ground radials is important, and the proper length(s) for any band may
be found from the formula:
Four such radials, equally spaced, would be the equivalent of a highly conductive ground plane for any band at antenna
base heights of one-half wavelength or more. In addition to providing a ground plane, radials of the proper length act as
decoupling stubs to choke off current flow on any DC ground lead or grounded mast or tower, thus eliminating the
detuning effect noted above. The basic ground plane system is shown in figure three. Radials may be run parallel to the
earth or sloped downward any convenient amount without seriously affecting antenna feed point impedance or
performance. Figure four shows a multi-band system using four separate sets of radials for 40, 20 and 10 meters.
Inasmuch as the 40 meter radials are also resonant on 15 meters, a separate set is not required for that band. This same
system will provide enough capacitive coupling to ground for operation on 75/80 meters at antenna base heights below
about 40 feet in some cases, but since ground conditions vary widely, it is advisable to use at least one resonant 80 meter
radial in an above-ground installation.
Figure five shows a simpler system which uses a single resonant radial for 80/75, 40,
30, 20, 17, 12 and 10 meters. With this arrangement, the antenna resembles a bent
dipole on any one band, and the radiation pattern contains both horizontal and
vertical components. The system in figure four, however, produces omnidirectional
vertically polarized radiation. (Note: 30, 17 and 12 meter radials are not shown, but
may be included in this type of system.)
Figure six illustrates the construction of a multi-band radial which is resonant on 40, 20, 15 and 10 meters. Good quality 300 ohm TV ribbon
lead should be used (velocity factor is critical), and the conductors should employ at least on strand of steel wire to support the weight of the
radial. Four such radials will be the practical equivalent of the system shown in figure four for operation on 80 through 10 meters.
Regardless of the specific system used, radials should be well insulated at their far ends and kept clear of large masses of metal or other
conductors that could cause detuning.
OTHER MOUNTING SCHEMES
In cases where a resonant vertical antenna may be neither ground mounted nor used with an elevated ground plane, operation may still be
possible if connection may be made to a large mass of metal that is directly or capacitively coupled to ground, e.g., central air conditioning
systems or structural steel frames of apartment buildings. Some amateurs have reported good results with vertical antennas extended
horizontally or semi-vertically from metal terraces which serve as the ground connection. Alternatively, a quarter-wave vertical may be
window mounted if a short ground lead to a cold water pipe or radiator can be used. If a long lead must be used, tuned radials will be
required for resonance on one or more bands. GREAT CARE SHOULD BE EXERCISED IN SUCH INSTALLATIONS TO AVOID
POWER LINES AND TO KEEP THE ANTENNA FROM FALLING ONTO PERSONS OR PROPERTY.
The metal shells of camper trailers, vans and mobile homes may also be used as ground systems for vertical antennas. Whenever possible,
the antenna should be mounted with its base close to the top of the roof, and the shortest possible ground lead should be used. Even so,
tuned radials may be required for low SWR on one or more bands.
