User's Manual

UtiliNet® Endpoint User Guide 1-5-2007
Page 22 of 30
© Cellnet 2005
When a ground plane is required, but is not large enough, effective gain reduction results. If applicable, a
reduction of 1 to 2 dB on antenna gain would be in order for ground planes smaller than about 6 to 8 inches
along the narrowest axis.
Transmitter Transmission Line Loss
This is the equivalent cable loss of any transmission line that might be used to mount an antenna remotely
from the radio assembly. Many times the purpose for remote mounting the antenna is to get it up higher and
more in the clear. This functions to reduce the Path Loss Adder term by more than the amount of this
transmission line loss term, hence a net gain is realized. The maximum tolerable cable loss is a subjective
call. It is recommended that the cable type and length be selected such that this loss term does not exceed 3
dB. A cable loss of 1.5 dB would be a reasonable goal. For this example, assume a good quality cable is
used, and the loss at 915 MHz is 1.5 dB.
Total Path Loss
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