WX16 User Manual | 63
Wavelength in meters = 300 / frequency in MHz
Wavelength in feet = 1000 / frequency in MHz
Therefore, 900 MHz antennas require at least 2/3 meter (2 feet) and 2.4GHz 15 cm (6
inches). Antennas may be mounted with less clearance, but radiation will be reduced.
If the radio path is short, this won’t matter. It is important the antenna mounting bracket
be well connected to “earth” or “ground” for good lightning surge protection.
7.5.2 Yagi
Antennas
Yagi antennas are directional along the central beam of the antenna. The folded
element is toward the back, and the antenna should be “pointed” in the direction of the
transmission. Yagis should also be mounted with at least 1 to 2 wavelengths of
clearance from other objects. The polarity of the antenna is the same as the direction
of the orthogonal elements. For example, if the elements are vertical, the Yagi
transmits with vertical polarity.
In networks spread over wide areas, it is common for a central unit to have an omni-
directional antenna and the remote units to have Yagi antennas. In this case, as the
omni-directional antenna will be mounted with vertical polarity, then the Yagi’s must
also have vertical polarity. Care needs to be taken to ensure the Yagi is aligned
correctly to achieve optimum performance.
Two Yagis can be used for a point-to-point link. In this case they can be mounted with
the elements horizontally to give horizontal polarity. There is a large degree of RF
isolation between horizontal and vertical polarity (approx –30dB), so this installation
method is a good idea if there is a large amount of interference from another system
close by transmitting vertical polarity.
An important mounting tip: If a Yagi has drainage holes in the dipole element, do not
mount the antenna with the drainage.
7.5.3 Mounting Near Other Antennas
Avoid mounting your network’s antenna near any other antenna even when the other
antenna is transmitting on a different radio band. High RF energy of the transmission
from a close antenna can “deafen” a receiver. This is a common cause of problems
with wireless systems.
Because antennas are designed to transmit parallel to the ground rather than up or
down, vertical separation between antennas is a lot more effective than horizontal
separation. If mounting near another antenna cannot be avoided, mounting it beneath
or above the other antenna is better than mounting beside it. Using different polarity to
the other antenna (if possible) will also help to isolate the RF coupling.
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