Radio Frequency Fields

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Radio Frequency Fields

Two-way radio

Sources of Radio Frequency (RF) fields include mobile two-way radio, mobile phones, broadcasting and radar transmitters. In office areas these fields will be predominately caused by two way radio or mobile phones.

Radio transmission tower

Transient Electromagnetic Fields are produced by the switching of inductive loads such as circuit breakers or motors. Lightning will also cause this type of disturbance. A transient signal in a cable produces a radiated emission with a spectral content dependent on the amplitude, rise time and pulse width of the transient. The reception of broadband fields at the lower frequencies is mainly via cables, which are electrically long with respect to the wavelength. A cable longer than one quarter of a wavelength will be an efficient receptor. Broadband radiation from transient sources is rarely found to have significant energy at frequencies exceeding 500 MHz.

Microwave tower

Conducted interference may originate from the coupling of ambient radiated interference or may be capacitively, inductively or galvanically induced in the cable by an emitting source. At audio and lower radio frequencies, EMI is primarily caused by conduction. The impedance presented by power cables, cable screens, etc. is generally low and this type of EMI will be readily propagated. Most cables act as fairly low loss transmission lines up to at least 10 MHz, above which the attenuation increases significantly due to dielectric losses and skin effect. 30 MHz is normally considered the frequency at which EMI becomes a radiated problem rather than conducted.

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