User's Manual

Radio/Serial Telemetry Module User Manual
Page 12 © March 2000
2.2.2 Three element Yagi aerial.
A 3 element Yagi aerial provides approx 4 dB of gain. This may be used to compensate for coaxial
cable loss for installations with marginal radio path. Note that these aerials should not be used if the
coaxial cable lengths are less than the following minimum lengths, otherwise the power transmitted
from the aerial will exceed the power permitted for the unlicensed channel.
RG58 10 metres
RG213 20 metres.
Yagi aerials are directional. That is, they have positive gain to the front of the aerial, but negative
gain in other directions. Hence Yagi aerials 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 aerial. Also
note that Yagi aerials normally have a drain hole on the folded element - the drain hole should be
located on the bottom of the installed aerial.
The Yagi aerials 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 aerials, horizontal polarisation is recommended. If there are more than two stations
transmitting to a common station, then the Yagi aerials should have vertical polarisation, and the
common (or “central” station should have a dipole or collinear (non-directional) aerial.