Weidmuller WI-I/O-9-U2: Wireless Mesh I/O & Gateway User Manual V1.2.2 - Manual (Page 26)

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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), however both antenna must
be in the same plane for maximum signal. If the antenna are mounted in different planes
the receive signal level will be reduced by around 30dB.
Figure 8 - Yagi Antenna Mounting
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.
Note that Yagi antennas normally has a drain hole on the folded element -
the drain hole should be located on the bottom of the installed antenna.