The Problem

Radio Frequency Interference is growing at a rapid rate due to the increasing use of wireless networks, cell phones, digital pagers, hand-held controllers and other wireless devices. RFI can disrupt virtually any electrical or electronically controlled device, with potentially harmful or even disastrous results.

Interference from Radio Frequency Fields

Cell phone tower

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.

Electronic equipment will produce low level narrow and broadband signals at frequencies up to and above 1 GHz. The U.S. Federal Communications Commission (FCC) specifies the maximum level 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 RF communications and broadcast systems.

The Basics of Radio Frequency Fields

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

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.