In the last unit, we
discussed the ground wave, and the necessity that the ground wave have vertical
polarization. A vertical antenna is used to launch a vertically polarized RF
wave. Vertical antennas are most often used in two areas:
1. Low frequency communications – at frequencies below 2 MHz, it
is difficult to use dipole antennas because of their length and the requirement
that they be mounted at least a half wavelength above ground. For example:
a 2 MHz dipole antenna is approximately 234 ft long and needs to be
approximately 234 feet above ground. Also, most communications at frequencies
below 2 MHz is via ground wave, which requires vertical polarization.
2. Mobile communications – it is difficult to mount a
horizontally polarized dipole on a vehicle. A vertical antenna only has one
mounting point and less wind resistance.
The
most common vertical antenna is the Marconi antenna. It is a vertical conductor l/4
high, fed at the end near ground. It is essentially a vertical dipole, in which
one side of the dipole is the RF image of the antenna in the ground.
This may sound strange, but remember that ground reflects RF as a mirror
reflects light
The image antenna formed in the ground under a Marconi antenna
This
type of antenna, unlike the dipole, is an unbalanced antenna, and should be fed
directly with coaxial cable. The shield of the coax is connected to the ground at the
base of the antenna and the center lead of the coax is connected to the vertical
radiator.
Because
the ground under a vertical antenna is actually part of the antenna, it is
necessary that ground losses be minimized. To minimize the losses, the
electrical conductivity of the ground must be made as high as possible, or an
artificial low loss ground must be provided.
Ground
conductivity can be improved by using ground radial wires. These are wires
buried just under the earth’s surface or laid on the surface that provide a
low resistance path for RF currents flowing in the ground. The ground currents
are greatest in the vicinity of the feed point of a Marconi antenna, so the
radials run out from the feed point, up to a distance of l/4
from the antenna, if possible. The ground radials do not have to be any specific
length and the general rule is that a large number of short radials is
preferable to a few long radials. The diagram below shows how current flows
through the ground to the feed point of the Marconi antenna.
When
a Marconi antenna cannot be mounted on the ground, an artificial ground system,
called a counterpoise, is used. The counterpoise consists of l/4
wires emanating radially from the antenna feed point as shown below. The shield
of the coax is connected to the counterpoise at the feed point. The counterpoise
is not connected to ground.
Ground
losses affect the feed point impedance and antenna efficiency. A Marconi
antenna mounted on a perfectly conducting ground would have an input
Ground loss reduces the antenna's efficiency, because part of the power being delivered to the antenna is being dissipated in the ground rather than being radiated. The efficiency can be computed from the measured value of input resistance by using the following formula:
The radiation pattern of the Marconi antenna is a half doughnut as shown in the figure below. There is no radiation straight up in the direction of the wire. The bulk of the radiation occurs at a low elevation angle, which is what is needed to launch a ground wave.
