User's Manual
Trailblazer Installation and User Manual version 1.02 5
2.1.3 Line of Sight: Is it enough?
Visual line of sight (LOS) is the straight, unobstructed line between the Base
unit’s antenna and the CPE’s antenna. In other words, when you stand next to
one antenna, you can see the other one. Because RF waves spread out three
dimensionally rather than in a straight line, LOS is important in microwave
transmission but not adequate. As the waves travel out from the transmitting
antenna, the signal density decreases and more three dimensional space around
the visual LOS is necessary to be able to receive a reasonable signal at the
receiving antenna. This three dimensional space is called the RF LOS or
Fresnel zone for the French physicist who first discovered its importance.
2.1.4 Fresnel Zone: What it is in plain English!
The Fresnel zone is the tree dimensional cone around the line-of-sight that radio
waves spread out into after they leave the antenna. The cross section of the first
Fresnel zone is circular; subsequent zones’ cross sections are annular. The
signal strength is strongest in zone 1 and decreases in each successive zone.
Not only trees, buildings and mountains can occupy a Fresnel Zone, the
curvature of the earth, even for some short paths, also has to be taken into
consideration. Because 2.4 GHz is one of the resonant frequencies at which
water absorbs energy, path obstructions such as trees, rivers, lakes or any other
water containing objects are especially detrimental to the microwave signal. A
rule of thumb is that 60% of the first Fresnel zone must be free of any
obstructions for tolerable attenuation of the signal.
In addition to absorbing the RF signal, objects in the RF path can also reflect it,
making the signal out of phase with the original signal, and effectively causing
signal cancellation. In zone 1 the signal will be 0 to 90
o
out of phase in zone 2, 90
to 270
o
in zone 3, 270 to 450
o
and so on. Even numbered zones have the
maximum phase canceling effect and in odd numbered zones the reflected
waves will add to the signal.
Figure 1: Line of Sight is clear, the first Fresnel Zone is NOT clear.