B&B Electronics ZXT9-MB2A-KIT - Manual - Manual (Page 162)

Document Number: ZXTxRM-5110m
Page
162
Beware 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.
D.8 Path Loss Rule 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.
D.9 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.
D.10
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 for two
popular cable types are listed below.
Cable Types
Frequency RG-58U* LMR-400*
900 MHz 1.6 dB 0.4 dB
2.4 GHz 2.8 dB 0.7 dB
*Loss per 10 feet (3 meters) of cable length