FAQs - Radio Propagation

What Effect does Terrain or Water have on Propagation?

WLAN signal paths on 2.4 and 5.8 GHz must be line-of-sight.  There must not be any hills, mountains, large buildings or obstructions for the signal to pass through.  Visual line-of-sight is sometimes not enough. The University of Kansas Wireless Network Visualization Project can help you visualize coverage areas. The radio path should also allow for Fresnel-zone clearance.  (See Reference 1) A few trees (0.3 - 0.5 dB/meter) are not usually a problem, however a forest will block the signal (300 dB/km).  You can check topographic maps of your area at Topo.Com or Topozone.Com.  Also, there a few cool 3D tools such as Google's Keyhole. You can find your exact latitude & longitude for any address at Geocode. Find distance and direction between two points at Indo. Find the elevation at any lat & long from Widders. Path profiles may be created using TopoUSA or Terrain Navigator. There are also several companies who market propagation modeling software or services. We recommend WirelessMapping, Wireless Valley, EDX Signal Pro, MicroPath 2001 , Pathloss.  Free terrain modeling software may be obtained at Radio Mobile or MicroDEM. Help for using Radio Mobile can be found here. WLAN paths over water or extremely flat ground may require optimization of antenna height at one end of the path.  This is due to specular reflections adding in-phase or out-of-phase.  Adjustment of antenna height by 1 to 3 meters should move the signal from a null to a peak.  Antenna diversity (with height separation) at both ends of the path should work great. Hint: Place one antenna in a peak and the other in a null. CP antennas have also shown to work well over water. Also, with vertical polarization, you may use the Pseudo-Brewster Angle to eliminate all reflections.

What is the Brewster Angle?

The Pseudo-Brewster Angle (PBA) is the angle at which the reflected TM wave (from a flat earth or water surface) is 90 degrees out of phase and minimum amplitude with respect to the direct wave. "Pseudo" is used here because the RF effect is similar to the optical effect from which the term gets its name. Above this angle, the reflected signal is in-phase with the direct signal. Below this angle, the reflected wave is between 90 and 180 degrees out of phase with the direct wave. Some degree of cancellation takes place in either condition, depending upon the difference between the lengths of the direct path and the reflected path. The largest amount of cancellation occurs near zero degrees, and steadily less cancellation occurs as the PBA is approached from below.

The factors that determine the PBA for a particular location are not related to the antenna itself, but to the ground or water surface around it. Surface conductivity, dielectric constant and operating frequency all affect the PBA of a particular system. The PBA increases with increasing frequency, all other conditions being equal.

At 2400 MHz, over fresh water, the PBA is approximately 6 degrees. At 2400 MHz, over land, the PBA is approximately 17-20 degrees. The signal cancellation effect is more noticeable over water than land because foliage and buildings normally attenuate and scatter the reflected signal over land.

There are several ways to reduce the effect of signal cancellation. The best way is to adjust the height of one antenna, either up or down until the signal moves from the null to a peak. At 2400 MHz, an adjustment of 1 - 3 meters in height should be enough. Another good method is to place the path midpoint on a rough area of land by moving the path endpoints. Changing the antenna polarization from vertical to horizontal may help some of the time. If the PBA can be determined, then placement of the antennas at prescribed heights for a given distance can minimize the reflected signal amplitude.

How can I get my signals through trees?

Trees are a BIG problem in Fixed-Wireless systems. They absorb and scatter RF energy and can prevent a WISP/FWA system from functioning. Check out this cool University of Texas reference on tree attenuation.

* 900 MHz systems can usually penetrate trees better than either 2.4 or 5.8 GHz systems.
* High-power systems and FHSS work better than lower power systems and DSSS.
* Placing the both the AP and CPE antenna above the tree-tops works the best.
* If there is a small LOS hole through the trees, 5.8 GHz signals may pass through, due to the smaller Fresnel distance required.
* Horizontal and 45 degree polarization has shown to have a slight advantage over vertical polarization at 2.4 GHz.
* Using an Access Point at extreme height (>500 feet) with mechanical or electrical beamtilt also helps clients within 5 miles because the signals pass through fewer trees.
* Wet trees are worse than dry trees.
* Pine trees are worse than leafy trees

What effect does rain or fog have on performance?

2.4 GHz signals may be attenuated by up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 inches/hr). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile). Even though rain itself does not cause major propagation problems, rain will collect on the leaves of trees and will produce attenuation until it evaporates.

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Technical FAQ Categories

Basics, Access Point, Antenna Properties, Canadian RSS-210, Client Equipment, FCC Part 15, Point to Multi-Point, Point to Point, Pre-installation and Site Preparation, Radio Propagation, Reference Material, Troubleshooting.