User's Manual

UtiliNet® Endpoint User Guide 11-1-2006
Page 22 of 30
© Cellnet 2005
The total path loss if highly variable and can be estimated by:
• Field propagation measurements—This can provide the most valuable data, but these results can still vary
from one time to another time.
• CAD tools based on various models—Some models tend to overestimate path loss; some models tend to
underestimate path loss.
If a high gain or directional type antenna were used, a ground
plane would probably not be required because these types of
antennas are usually designed to be ground plane independent.
As a basis, first calculate free-space path attenuation. Then add a correction factor of 0 to 45 dB (or more
for very extreme conditions), depending on variables such as antenna height, terrain, foliage, or obstacles.
Free Space Path Loss
Free space assumes no obstacles blocking the path, no obstructions or significant field perturbations of any
kind present. This is the best you can hope for, but is seldom achieved unless the path is perfectly line-of-
sight. This is a useful starting point, however, for estimating path loss since it is the best achievable. Free
space attenuation at 915 MHz, for isotropic antennas at each end, is 96 dB at one mile and increases 6 dB
when distance doubles. Sometimes free space path loss is calculated differently, using dipole antennas
instead of isotropic antennas. Such is not the case here, but if it were, the 2 dB factors for dBd to dBi
antenna gain conversions would not be included.
Additional Path Loss Attenuation
For non-line-of-sight paths, an additional factor must be included that corrects for variables such as terrain,
obstacles (including buildings), foliage, people, or antenna height. This is a factor to take into account for
the installation and the path, as it is today. This factor can be derived from the works of many individuals
who have studied radio propagation and derived mathematical models: Okumura/Hata, Longley/Rice, Lee,
and others. There are various CAD tools available that are based on these measurements or algorithms.
Note that some of these CAD tools will only return the total path loss factor. Other CAD tools perform the
entire link budget analysis. For our discussion, let’s continue to consider the RF path as a sum of the
freespace path loss term and an additional path loss term. Height of both antennas, the type of environment
along the path, distance, and other parameters can determine this factor. Typically, this factor ranges from 5
to 40 dB.
This factor, plus the free space path loss, gives total median path loss.
Example: Assume a path that is 5 miles long. One antenna is on a hill or water tank 300 ft. higher than the
surrounding area. The other antenna is on a 30 ft. support structure, for example on the rooftop of a 1 story
building. Except for the
Table 3.1 Free-Space Path Loss