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Page 16

E Series Data 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

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, 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