User manual

905U Wireless I/O Module User Manual

105S Serial I/O Module

Page 14 © October 2001

2.2.2 Yagi antennas.

A Yagi antenna provides high gain in the forward direction, but lower gain in other directions. This

may be used to compensate for coaxial cable loss for installations with marginal radio path.

The Yagi gain also acts on the receiver, so adding Yagi antennas at both ends of a link provides a

double improvement.

Yagi antennas are directional. That is, they have positive gain to the front of the antenna, but negative

gain in other directions. Hence Yagi antennas should be installed with the central beam horizontal and

must be pointed exactly in the direction of transmission to benefit from the gain of the antenna. The

Yagi antennas may be installed with the elements in a vertical plane (vertically polarised) or in a

horizontal plane (horizontally polarised). For a two station installation, with both modules using Yagi

antennas, horizontal polarisation is recommended. If there are more than two stations transmitting to a

common station, then the Yagi antennas should have vertical polarisation, and the common (or

“central” station should have a collinear (non-directional) antenna.

Also note that Yagi antennas

normally have a drain hole on the

folded element - the drain hole

should be located on the bottom

of the installed antenna.

905U

Antenna installed

with drain holes

down

Coax feed looped

at connection