User's Manual

905U-D Radio Modem Module User Manual
Page 14 © May 2000
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 aerial.
Yagi aerials should not be used where a module is receiving messages from more than one
other module such as repeater of “base-station” situations. An omni-directional aerials such
as a dipole or a collinear aerial should be used.
2.2.3 Collinear (3dB) aerial.
A 3dB collinear aerial may be used in the same way as a 3 element Yagi to compensate for
the losses in long lengths of coaxial cable. This type of aerial is generally used at a central
site with more than one remote site. The collinear aerial looks similar to the dipole, except
that it is longer.