AvMesh
®
RF Communications Systems
Installation & Service Manual
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13
The AvMesh
TM
Communications system uses the Avlite wireless remote controller as well
as the lights themselves as transmitters. It is therefore essential to consider the impact of
the following on the RF propagation:
•
Range of the lights
•
The distance between each light (node) in the system
•
The distance from the control tower to the first light (node) on the runway (or
applicable area)
It is important to note that this type of propagation requires the transmitter and receiver
to be in view of each other without any form of obstacle (e.g. terrain, vegetation, buildings,
transmitter towers etc.) between them.
However, even if a direct line of sight does exist between the transmitter and receiver
(i.e. clear of obstacles), the signal strength may still not be strong enough. Geographical
obstacles
near
the direct path between the transmitter and receiver as well as the
curvature of the earth can also have an effect on the ability of the signal to propagate.
Hence, the area of clearance required for good connection is not uniform, but rather an
elliptical
region between the transmitting and receiving antennas known as the Fresnel
zone. (See section
3.6.1 Fresnel zones
for more information).
Interference can be defined as any external factor (such as physical obstacles, noise or
other radio signals) that degrade or reduce the clarity of the desired radio signal.
The conditions that result in interference are unique to each individual environment and
therefore, there is no standard level of interference nor is there a single formula to calculate
or quantify it.
In point to point wireless communication systems, it is important for the line of sight
between the transmitter and receiver to be free of any obstructions to enable ideal RF
propagation. However, it is important to note that an obstruction-free line of sight will not
always provide a perfect connection. There is also a requirement for the clearance of what
is known as the Fresnel zone, which is a series of concentric ellipsoidal regions between
the transmitting and receiving antennas that have constructive and destructive effects on
the waves that are reflected, refracted or diffracted within them.
The size of the Fresnel zone is governed by the frequency of operation and the distance
between the transmitting and receiving antennas.
There are an infinite number of Fresnel zones, however, the first three zones usually have
a notable effect on RF propagation, with the first Fresnel zone being the most significant.
This is because the strongest signals lie closest to the direct line of sight path between the
transmitter and receiver.
3.6 Interference
3.6.1 Fresnel zones
