EMI/RFI Filtering

Overview EMI/RFI Filtering
By Defense Advancement Last updated: April 12th, 2023

The radio segment of the electromagnetic spectrum encompasses frequencies of nine kilohertz/KHz up to 300 gigahertz/GHz. Frequencies within this threshold are used for a host of activities like television and radio broadcasting, radar, satellite navigation and wireless communications. Any technology dependent on radio waves depends on this part of the spectrum. This includes applications as diverse as radio astronomy, remote control systems and runway instrument landing systems.

The myriad of applications for radio makes filtering imperative. Any apparatus using radio signals depends on an antenna which may receive (Rx) signals, transmit (Tx) them, or do both. A basic rule says all antennas must be one half or one quarter the wavelength of the Tx/Rx signals they handle. For example, the wavelength of a 120 megahertz/MHz signal is 2.49 metres/m. Wavelength is the measurement between two peaks or two troughs in a single radio wave. Hence a Tx/Rx antenna handling 120MHz signals would need to be either 1.24m or 0.6225m long. However, there is a risk that the antenna may inadvertently receive signals on other frequencies. For example, 240MHz signals have a 1.24m wavelength. Half this wavelength is 0.62m. Thus, the 120MHz antenna could also receive spurious 240MHz transmissions. This could result in the device experiencing interference. Interference is caused by human activity like broadcasting or cellular communications. It can also be naturally occurring. Both must be filtered out to avoid interference.

This interference was discovered during the early days of radio broadcasting when it was realised that radio signals could inadvertently disturb other electrical systems. A modern-day example of this is the potential for interference caused by cellphones on aircraft. These may cause interference to cellphone towers on the ground as the aircraft passes overhead and/or interfere with the aircraft’s navigation. This is why cabin crew ask passengers to switch their devices to flight mode.


Some interference can be avoided through national and international regulation. The United Nations’ International Telecommunications Union (ITU) is the global custodian of the radio frequency spectrum as radio is a global resource. The ITU mandates which frequencies can be used for which purposes internationally to help minimise interference. For example, X-band frequencies of 7.9 gigahertz/GHz to 8.4GHz and 7.25GHz to 7.75GHz are reserved globally for military satellite communications. National governments may also legislate that specific frequencies are allotted for specific tasks domestically. Nonetheless, such regulations cannot prevent all human-made and naturally occurring radiation causing interference.

Filter Design

One way to mitigate this is to use filtering to block out radio interference known as EMI (Electromagnetic Interference) or RFI (Radio Frequency Interference). EMI and RFI filters perform a simple task. They allow some radio signals to pass through a device while stopping others. This helps reduce the interference that would be experienced otherwise. Four basic filter designs are used, Band Pass, Band Stop, Low Pass and High Pass.

An RF Band Pass will permit all signals through one continuous band between two frequency points, for example all signals between one and two megahertz are allowed. All signals outside this threshold are stopped. RF Band Stop filters prevent all signals in one continuous band from passing but permit others. In this case, the filter would prevent all signals between one and two megahertz but permit all signals outside these two points. Band Stop filters perform the opposite task of a Band Pass filter.

RF Low Pass filters take a specific frequency point, 1.255MHz for example, and allow all signals below this to pass, but prevent all frequencies above 1.255MHz doing the same. Finally, RF High Pass filters allow signals above a specific frequency to pass preventing those below doing so. It is possible to design and deploy filters with high degrees of precision. This ensures only the precise signals a device needs are allowed to pass through it.