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The velocity of RF energy is slower in dielectric materials that are denser than
dry air. The result is refraction, or the change of direction of propagation as radio
waves pass through these materials. The time spent in the denser material
controls the degree of refraction and the resulting direction of the emerging wave.
2.1.2.3 Diffraction
Perfect shadows that are cast by RF-opaque structures or objects are rare at
wireless system frequencies. Because of diffraction, energy scatters at the edges
of the obstruction. Diffraction is more pronounced in sharp or knife edged
corners. According to field theory, edges cause secondary radiation when
illuminated. This is generally independent of polarization when the scatterer is
much longer than the impinging signal’s wavelength.
Diffraction forms a fuzzy signal source at the edges, which tends to fill in the
shadowed regions. The signal levels are low but often usable. These edges can
be corners of a building, window frames or large vehicles. Diffraction also figures
significantly in scattering at hilltops lying in the propagation path.
2.1.2.4 Locating the Antenna
Free space radiation patterns are the baseline performance criteria before
installation in the real world environment. Since sites are located on rooftops, or
on the side of buildings; many opportunities exist for interference from chimneys,
walls, masts, towers and other antennas. Although it is difficult to predict level
changes you can take steps to minimize pattern distortion.
Obstructions are either conducting or non-conducting. Conducting objects create
the most severe disturbances. Those near the resonant length of the antennas,
such as other antennas, behave as the parasitic elements of an uncontrolled
array, producing random nulls and lobes. Larger conducting structures causes
severe shadowing. The closer the spacing, the larger the shadow.
Locate the antenna as far from other conducting objects as feasible. In
particular, avoid placing the antenna where obstructions are within the view of
the antenna main beam. For highly directional antennas, the presence of
conductive objects in the rear lobe of the antenna is not critical.
