8.8.4 Fixed Microwave Links And Satellite Earth Stations
Point-to-point microwave links and satellite earth stations use highly directional antennas. The assessment should begin by establishing the beam path, antenna height and pointing, aperture size, frequency, transmitter power, gain, and locations that people can access. Conservative calculations may demonstrate that areas outside the main beam are well below the applicable limit, while measurements can verify accessible locations, sidelobes, reflections, leakage, and maintenance positions.
Large apertures can have an extended radiating near field, so a far-field power-density formula should not be applied until its distance criterion is satisfied. Within the radiating near field, field distribution may be nonuniform and systematic spatial measurements or an appropriate computational method may be required. Directional calibrated antennas can help isolate the wanted signal, but their pointing, polarization, antenna factor, and cable corrections must be controlled.
8.8.5 Industrial RF Equipment
Industrial heating, sealing, welding, drying, and processing equipment often produces localized leakage fields in the reactive near field. The field cannot be characterized reliably by assuming a plane wave or by deriving power density from only one field component. Measurements should be made during the operating modes and task cycles that produce the highest credible worker exposure, including normal loading and foreseeable maintenance or setup conditions.
Induction heaters couple energy principally through magnetic fields and usually require a magnetic-field survey around the applicator coil, workpiece, leads, and operator positions. Dielectric heaters couple energy principally through electric fields around electrodes and feed structures and usually require an electric-field survey. The other field component should not be assumed irrelevant, and induced limb current, contact current, or conductor current may also require assessment where large structures or conductive contact paths are present.
The survey should include access openings, shielding joints, cables, operator hand and body positions, and nearby work areas. Interlocks, shielding, grounding, and operating controls should be checked, and measurements repeated after repairs or changes that could alter leakage.