Page 20
E Series Ethernet Radio – User Manual
Part D – System Planning and Design
By compressing the transmission energy into a disc or beam, the
antenna provides more energy (a stronger signal) in that direction,
and thus is said to have a performance “gain” over a basic omni
antenna. Gain is usually expressed in dBd, which is referenced
to a standard folded dipole. Gain can also be expressed in dBi,
which is referenced to a theoretical “isotropic” radiator. Either way,
if you intend to send and receive signals from a single direction,
there is advantage in using a directional antenna - both due to
the increased signal in the wanted direction, and the relatively
decreased signal in the unwanted direction (i.e. “Interference
rejection” properties).
Tuning the Antenna
Many antennas are manufactured for use over a wide frequency
range. Typical fixed use antennas such as folded dipoles and yagis
are generally supplied with the quoted gain available over the
entire specified band range, and do not require tuning. Co-linear
antennas are normally built to a specific frequency specified when
ordering.
With mobile “whip” type antennas, it is sometimes necessary
to “tune” the antenna for the best performance on the required
frequency. This is usually done by trimming an antenna element
whilst measuring VSWR, or simply trimming to a manufacturer
supplied chart showing length vs frequency. These antennas would
normally be supplied with the tuning information provided.
Antenna Placement
When mounting the antenna, it is necessary to consider the
following criteria:
The mounting structure will need to be solid enough to withstand
additional loading on the antenna mount due to extreme wind, ice
or snow (and in some cases, large birds).
For omni directional antennas, it is necessary to consider the
effect of the mounting structure (tower mast or building) on the
radiation pattern. Close in structures, particularly steel structures,
can alter the radiation pattern of the antenna. Where possible,
omni antennas should always be mounted on the top of the mast
or pole to minimise this effect. If this is not possible, mount the
antenna on a horizontal outrigger to get it at least 1-2m away from
the structure. When mounting on buildings, a small mast or pole
(2-4m) can significantly improve the radiation pattern by providing
clearance from the building structure.
For directional antennas, it is generally only necessary to consider
the structure in relation to the forward radiation pattern of the
antenna, unless the structure is metallic, and of a solid nature.
In this case it is also prudent to position the antenna as far away
from the structure as is practical. With directional antennas, it is
also necessary to ensure that the antenna cannot move in such
a way that the directional beamwidth will be affected. For long
yagi antennas, it is often necessary to install a fibreglass strut to
stabilise the antenna under windy conditions.
Alignment of Directional Antennas
This is generally performed by altering the alignment of the
antenna whilst measuring the received signal strength. If the signal
is weak, it may be necessary to pre-align the antenna using a
compass, GPS, or visual or map guidance in order to “find” the
wanted signal. Yagi antennas have a number of lower gain “lobes”
centred around the primary lobe. When aligning for best signal
strength, it is important to scan the antenna through at least 90
degrees, to ensure that the centre (strongest) lobe is identified.
When aligning a directional antenna, avoid placing your hands or
body in the vicinity of the radiating element or the forward beam
pattern, as this will affect the performance of the antenna.
There are basically two types of antennas – omni-directional and
directional.
Omnidirectional antennas are designed to radiate signal in a 360
degrees segment around the antenna. Basic short range antennas
such as folded dipoles and ground independent whips are used
to radiate the signal in a “ball” shaped pattern. High gain omni
antennas such as the “co-linear” compress the sphere of energy
into the horizontal plane, providing a relatively flat “disc” shaped
pattern which goes further because all of the energy is radiated in
the horizontal plane.
Directional antennas are designed to concentrate the signal into
“beam” of energy for transmission in a single direction (i.e. For
point-to-point or remote to base applications).
Beamwidths vary according to the antenna type, and so can be
selected to suit design requirements. The most common UHF
directional antenna is the yagi, which offers useable beam widths
of 30-50 degrees. Even higher “gain” is available using parabolic
“dish” type antennas such as gridpacks.
Antenna Gain
