RADIO PATH PLANNING

Although the wireless bridge uses IEEE 802.11a radio technology, which is capable of reducing the effect of multipath signals due to obstructions, the wireless bridge link requires a “radio line-of-sight” between the two antennas for optimum performance.

The concept of radio line-of-sight involves the area along a radio link path through which the bulk of the radio signal power travels. This area is known as the first Fresnel Zone of the radio link. For a radio link not to be affected by obstacles along its path, no object, including the ground, must intrude within 60% of the first Fresnel Zone.

The following figure illustrates the concept of a good radio line-of-sight.

Visual Line of Sight

 

Radio Line of Sight

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If there are obstacles in the radio path, there may still be a radio link but the quality and strength of the signal will be affected. Calculating the maximum clearance from objects on a path is important as it directly affects the decision on antenna placement and height. It is especially critical for long-distance links, where the radio signal could easily be lost.

When planning the radio path for a wireless bridge link, consider these factors:

Avoid any partial line-of-sight between the antennas.

Be cautious of trees or other foliage that may be near the path, or may grow and obstruct the path.

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3Com WL-575, 3CRWEASYA73 manual Radio Path Planning