Common Sources of Interference

This post is the second in a series on understanding, finding, and mitigating interference for operators of wireless audio equipment. 

There is no way to cover every source of interference in a single blog post, or even an entire book. There are far too many things that can and do produce RFI (radio frequency interference) and EMI (electromagnetic interference). Nearly anything that uses electricity can produce RFI—and even some things that don’t!

Interference can, however, be separated into three distinct categories, each of which contains a broad range of devices and phenomena. 

• Natural radiators: Stuff in the natural world that causes the emission of electromagnetic energy through various means. Natural radiators don’t usually extend above low frequencies, but there are exceptions.  
• Unintentional radiators: Manmade technology of any type that emits electromagnetic energy as an unnecessary or accidental aspect of its operation. Sometimes unintentional radiators are truly unintentional—like electrical contacts on a breaker corroding over time. While sometimes EMI coming from a device is known by both its manufacturer and the FCC, but considered too small or trivial to be filtered or controlled. 
• Intentional radiators: Manmade technology that harnesses radio energy to send communication signals.

These three categories can be further divided into endless subcategories, which we have partially done in the attached infographic. We also are not discussing constructive or destructive interference, which refers to signal problems caused by waves themselves moving through and interacting with their physical environment.

Luckily, some sources of interference are more common than others—much more common—and wireless microphones only need to worry about interference that occurs in three frequency bands: the broadcast band (54-698 MHz in the United States), 900 MHz, and 2.4 GHz. There are some microphones and wireless audio devices that use other bands, but they are uncommon and not discussed here. 

It is important to note that intentional radiators in the TV broadcast band are placed into hierarchies by the government to prevent interference. There are primary users—those who own or lease spectrum as a form of property from the government, who have special privileges (like high transmission power and broader channel widths), and the most protection from interference. There are secondary users—those who have licenses to use spectrum but fewer rights than primary users, and some protection from interference. And there are tertiary users who do not hold or need a license to operate and have no protection from interference. Tertiary users are indirectly regulated by route of the devices they use having undergone certification by the FCC or an FCC certified testing facility.  

The TV broadcast band is unique. It hosts a variety of services, both licensed and unlicensed. It stands in contrast to other licensed bands that contain only a few or in some cases a single occupant that claim exclusive property rights to the frequencies there, as well as in contrast to unlicensed (ISM) bands, like 2.4 GHz and 900 MHz, that resemble a "spectrum commons,” where an unlimited number of entrants may use spectrum as long as they all follow a set of standards.

But comparing and contrasting spectrum management models is an interesting topic for another day. In the meantime, stay tuned for another installment of our series on interference. Next up is one on finding sources of interference using tools you already have.

Leading photo courtesy Richard Gould.