User manual

Appendix A: Path Evaluation Information A-5
Moseley SL9003Q 602-12016 Revision G
Where CF is the Fresnel zone clearance and F1 is the first Fresnel zone radius.
A.1.5 Path Profiles
Using ground elevation information obtained from the topographical map, a path profile should
be prepared using either true earth or 4/3 earth's radius graph paper. To obtain a clear path, all
obstacles in the path of the rays must be cleared by a distance of 0.6 of the first Fresnel zone
radius. Be sure to include recently erected structures, such as buildings, towers, water tanks,
and so forth, that may not appear on the map. Draw 0a straight line on the path profile clearing
any obstacle in the path by the distance determined above. This line will then indicate the
required antenna and/or tower height necessary at each end. If it is impossible to provide the
necessary clearance for a clear path, a minimum clearance of 30 feet should be provided. Any
path with less than 0.6 first Fresnel zone clearance, but more than 30 feet can generally be
considered a grazing path.
A.2. Path Analysis
A.2.1 Overview
Path analysis is the means of determining the system performance as a function of the desired
path length, required equipment configuration, prevailing terrain, climate, and characteristics of
the area under consideration. The path analysis takes into account these parameters and
yields the net system performance, referred to as path availability (or path reliability).
Performing a path analysis allows you to specify the antenna sizes required to achieve the
required path availability.
A path analysis is often the first thing done in a feasibility study. The general evaluation can be
performed before expending resources on a more detailed investigation.
The first order of business for performing a path analysis is to complete a balance sheet of
gains and losses of the radio signal as it travels from the transmitter to the receiver. "Gain"
refers to an increase in output signal power relative to input signal power, while "loss" refers to
signal attenuation, or a reduction in power level ("loss" does not refer to total interruption of the
signal). Both gains and losses are measured in decibels (dB and dBm), the standard unit of
signal power.
The purpose of completing the balance sheet is to determine the power level of the received
signal as it enters the receiver electronics—in the absence of multipath and rain fading; this is
referred to as the unfaded received signal level. Once this is known, the fade margin of the
system can be determined. The fade margin is the difference between the unfaded received
signal level and the receiver sensitivity (the minimum signal level required for proper receiver
operation).
The fade margin is the measure of how much signal attenuation due to multipath and rain fading
can be accommodated by the radio system while still achieving a minimum level of
performance. In other words, the fade margin is the safety margin against loss of transmission,
or transmission outage.