Appendix G: Radio Frequency Basics
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Radio manufacturers advertise “line of sight” range figures. Line of sight means that, from antenna A, you can see antenna B.
Being able to see the building that antenna B is in does not count as line of sight. For every obstacle in the path, de-rate the
“line of sight” figure specified for each obstacle in the path. The type of obstacle, the location of the obstacle, and the number
of obstacles will all play a role in path loss.
Visualize the connection between antennas, picturing lines radiating in an elliptical path between the antennas in the shape of
a football. Directly in the center of the two antennas the RF path is wide with many pathways. A single obstacle here will have
minimal impact on path loss. As you approach each antenna, the meaningful RF field is concentrated on the antenna itself.
Obstructions located close to the antennas cause dramatic path loss.
Be sure you know the distance between antennas. This is often underestimated. If it’s a short-range application, pace it off. If
it’s a long-range application, establish the actual distance with a GPS or Google Maps.
The most effective way to reduce path loss is to elevate the antennas. At approximately 6 feet high (2 m), line of sight due to
the Earth’s curvature is about 3 miles (5 km), so anything taller than a well-manicured lawn becomes an obstacle.
Weather conditions also play a large role. Increased moisture in the air increases path loss. The higher the frequency, the
higher the path loss.
Beware of leafy greens. While a few saplings mid-path are tolerable, it’s very difficult for RF to penetrate significant
woodlands. If you’re crossing a wooded area you must elevate your antennas over the treetops.
Industrial installations often include many reflective obstacles leading to numerous paths between the antennas. The received
signal is the vector sum of each of these paths. Depending on the phase of each signal, they can be added or subtracted. In
multiple path environments, simply moving the antenna slightly can significantly change the signal strength.
Some obstacles are mobile. More than one wireless application has been stymied by temporary obstacles such as a stack of
containers, a parked truck or material handling equipment. Remember, metal is not your friend. An antenna will not transmit
out from inside a metal box or through a storage tank.
Path Loss Rules of Thumb
To ensure basic fade margin in a perfect line of sight application, never exceed 50% of the manufacturer’s rated line of sight
distance. This in itself yields a theoretical 6dB fade margin – still short of the required 10dB.
De-rate more aggressively if you have obstacles between the two antennas, but not near the antennas.
De-rate to 10% of the manufacture’s line of sight ratings if you have multiple obstacles, obstacles located near the antennas,
or the antennas are located indoors.
Antennas
Antennas increase the effective power by focusing the radiated energy in the desired direction. Using the correct antenna not
only focuses power into the desired area but it also reduces the amount of power broadcast into areas where it is
not
needed.
Wireless applications have exploded in popularity with everyone seeking out the highest convenient point to mount their
antenna. It’s not uncommon to arrive at a job site to find other antennas sprouting from your installation point. Assuming
these systems are spread spectrum and potentially in other ISM or licensed frequency bands, you still want to maximize the
distance from the antennas as much as possible. Most antennas broadcast in a horizontal pattern, so vertical separation is
more meaningful than horizontal separation. Try to separate antennas with like-polarization by a minimum of two
wavelengths, which is about 26 inches (0.66 m) at 900 MHz, or 10 inches (0.25 m) at 2.4 GHz.
Cable Loss
Those high frequencies you are piping to your antennas don’t propagate particularly well through cable and connectors. Use
high quality RF cable between the antenna connector and your antenna and ensure that all connectors are high quality and
carefully installed. Factor in a 0.2 dB loss per coaxial connector in addition to the cable attenuation itself. Typical attenuation
figures per 10 feet (3 meters) for two popular cable types are listed below.
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