Interference from Radio Frequency Fields



Microwave tower
The majority of EMI concerns are centered on radio frequency (RF) emission sources due to the massive global increase in personal computers, digital pagers, hand-held radios, cellular phones, wireless devices, etc. Although most RF transmissions are achieved under controlled conditions, it is this transmitted energy that may create an interference producing phenomenon, a phenomenon that may affect the performance of many electronic devices.

In office areas these fields will be predominately caused by two way radio or mobile phones and general EMI from neighboring equipment. Depending on the type of equipment, radio frequency fields are normally considered to present a hazard to standard office electronic equipment when the field strength exceeds 1 to 3 V/m.


Radio transmission tower
Electronic equipment will produce low level narrow and broadband signals at frequencies up to and above 1 GHz. The U.S. Federal Communications Commission specifies the maximum amounts of radio frequency interference (RFI) that computer equipment is permitted to generate in the FCC Rules and Regulations. These limits are set in order to prevent computer equipment from interfering with the operation of radio and television receivers.

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. 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.


Two-way radio
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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