User's Manual

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