User's Manual

Page 15
M Series Data Radio – User Manual
© Copyright 2004 Trio DataCom Pty. Ltd.
Part D – System Planning and Design
Antenna Gain
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 stablilise 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,
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.
Selecting Antennas
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 a
“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.