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© Copyright 2007 Trio DataCom Pty. Ltd.
O Series Data Radio – User Manual
Part D – System Planning and Design
Effect of Earth Curvature on Long Paths
This path requires greater mast height to offset the earth curvature
experienced at such a distance (73km).
“dish” type antennas such as gridpacks.
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).
Selecting Antennas
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
directional antenna is the yagi, which offers useable beam widths
of 15-40 degrees. Even higher “gain” is available using parabolic
