Antenna Positioning

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The proper positioning or orientation of an antenna helps to ensure maximum coverage area. Antennae should generally be mounted as high and as clear of obstructions as is practically possible. Best performance is attained when both the transmit and receive antennae are located at the same height and within direct line of sight of each other.

ANTENNA RADIATION PATTERNS

Antenna radiation patterns are affected by polarization, free space loss, and propagation in solids. The transmission loss between a transmitting and receiving antenna is a function of the antenna gain, the distance, and the frequency. For best performance, the transmitting and receiving antennae must have similar polarization alignments.

Additional "free space loss" occurs because of signal spreading: as a signal radiates outward from an antenna, its radiated power is spread across an expanding spherical surface, with the power level inversely proportional to the distance from the source antenna. This antenna radiation must pass by and through solid objects, so it will be subject to losses from reflection and absorption. For example, oxygen atoms in the atmosphere cause a prominent peak in the attenuation effect of the atmosphere. Clandestine inter-satellite communications are performed on this frequency, so that the signals will not reach earth. The atmosphere's attenuation almost vanishes at 94 GHz (the W-band), which is why many radar systems operate around this frequency.

Water, however, is a very good signal absorber above 2 GHz. Fog, rain, the leaves of trees, people, etc. can rob energy from a microwave signal. This is why microwave ovens produce electromagnetic waves in the 2.4 GHz range; 2.4 GHz penetrates food very well, but the water molecules cannot vibrate as fast as the microwaves are pushing them, so the molecules absorb the microwave energy and release it as heat.

Reflections from objects (metal objects in particular) give rise to multipath distortion (fading). Some paths can converge and become constructive (adding to signal strength) or destructive (fading). Elevation pattern shapes are controlled, to keep the maximum response at or slightly below the horizon for best far and near field coverage. The dimensions and height of a communication sector will determine an antenna's azimuth and elevation beam width requirements. These can be derived from the established beam area formula for the approximate gain needed:

G(dBi) - 10 log 10 / 29,000 / antenna azimuth * the antenna elevation

Spacing in excess of .75 wavelengths from a large conducting surface leads to deep nulls. Antennae placed in the middle of a roof are prone to diffraction loss if they are not high enough to clear the roof edges.

In most instances, access point antennae should be positioned on or close to the ceiling. Also, omni-directional antennas should be placed in the center of the coverage area whenever possible. If, for some reason, it is necessary to mount an omni-directional antenna below the coverage area (e.g. on a desk, table, window seal, etc.), point the top of the antenna up. But if the antenna is mounted above the coverage area (e.g. in the ceiling), the top of the antenna must be pointing down. On the other hand, directional antennas always should be pointed in the direction of the coverage area.

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