User's Manual

42 Document Number: 0100SM1401 Issue: 10-14
Radio Accessories
Antennas
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 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 (3-6 ft) away from the structure. When mounting
on buildings, a small mast or pole, 2-4m (6-12ft) 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 stabilize the antenna under windy conditions.
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 - due to the increased signal
in the wanted direction, and the relatively decreased
signal in the unwanted direction (i.e. “interference
rejection” properties).
There are basically two types of antennas –
omnidirectional 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
directional antenna is the YAGI, which offers useable beam
widths of 15-40 degrees. Higher “gain” is available using
parabolic “dish” type antennas such as gridpacks.
Connecting Antennas and RF Feeders
The RF antenna system should be installed in
accordance with the manufacturer’s notes. Consider
the antenna gain when setting transmit power on the
radio. EIRP limits will vary depending on your geographic
location. Check with your local authority to ensure you
are abiding to the local laws.
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 optimal signal strength, the antenna should be
scanned through at least 90 degrees, to help 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.
WARNING
HAZARD OF RADIO FREQUENCY (RF) BURNS
Ensure that a matching load or antenna is attached to the RF port prior to
applying power to the device.
Failure to follow these instructions can result in death or serious injury.
Part E – Radio Planning and Design