To achieve maximum expected line-of-sight range, there must be no blockage except the curve of the earth, mountains, high hills or buildings. It is also assumed that no RFI is present at the ground control station (GCS) site from industrial sources such as fluorescent lighting, high-power machinery, arc welding operations or conveyor belts. If RFI is detected at a GCS location, the site should be moved to an area without these sources to prevent inaccurate data.

In the two-way radio relay business, radio sites on high hilltops have historically been shared among multiple users. Site owners often upgrade or add repeaters to their own racks and towers. New subsystems must not degrade existing equipment. Transmitters often operate at 50 watts or more, with antennas staggered to reduce interference. High-performance RF filters are used to ensure each transmitter and receiver has dedicated pre- and post-filters. Without these, nearby high-power transmitters can cause frequency mixing, creating interference that jams other systems.

Uncrewed aerial vehicles (UAVs) and uncrewed aerial systems (UAS) face similar risks. Most RF data link equipment does not provide highly effective bandpass filters in transmitter or receiver chains. Instead, weak broadband filters are often used for a single RF band, such as 2.3–2.5 gigahertz (GHz) or 3.5–3.9 GHz. These can be bypassed if an RFI source is nearby— ometimes within inches in a UAV or GCS.

UAV and UAS platforms typically house communications, digital electronics and GPS in the same small compartment (e.g., a 6x6-inch space). Nearby components, such as high-definition cameras with hundreds of microprocessors or field-programmable gate array modules, often lack metal shielding and emit digital switching noise. At maximum range, a receiver may be sensitive to -90 dBm, enabling a range of 80 kilometers (km) at 2.5 GHz without RFI. If RFI measures -74 decibel milliwatts (dBm), at the antenna input—just 6 decibels (dB) higher—range may be reduced by 50% or more (to about 40 km at the same 10 megabits per second data rate).

Operators often lack automated systems to monitor UAV-to-GCS link performance. The GCS itself may contribute to interference, as seen with inexpensive digital battery chargers or certain types of fluorescent and LED lighting. Monitoring both UAV/UAS and GCS signal-to-noise ratio is therefore critical.

Preflight checks can identify internal RFI issues on the ground. Software adjustments account for proximity differences between UAV and GCS compared with long-range signal loss. When interference is detected, a variable attenuator may be used to offset problematic subsystems, most often camera CPUs or navigation boards.

Two layers of RF shielding cloth plus absorption sheets can block more than 110 dB of interference. Long term, custom RFI bags with metalized fasteners may provide practical shielding. A quick solution is RF cloth (55 dB shielding) secured with zip ties or staples. Combining this with silicone rubber strips containing carbon composite helps absorb and cancel interference.

Real-time digital spectrum analyzers with small antennas can spot leakage points in shielding. Recommended test equipment includes the Tektronix RSA-503A or Signal Hound analyzers.

New solid-state AC power controllers and dimmers frequently cause interference in personal electronics. For example, adding an inline 110-volt AC speed control to a paint stripper can disrupt a GCS. Electric fences can also create erratic, high RFI when wet grass touches the wire, with effects that vary depending on wind direction.

The best mitigation strategy is avoidance: UAVs should be flown around known RFI areas, especially when operating near maximum range. Advanced UAV downlink boards with direction-finding and directiong finding and signal null steering. Then, you are able to pin down the exact location of the down range RFI source and fly around it.

Author: Ken Andrew
Consultant, NLOS Wireless

Desense: A reduction in receiver sensitivity due to external radio frequency interference (RFI), which limits the system’s operating range.

RFI: Radio frequency interference—any unwanted RF signal, including delayed duplicates, that reduces a receiver’s effectiveness in transmitting data or audio. Most often this is broadband noise, such as interference from fluorescent light bulbs affecting AM broadcast stations in the 550–1750 kilohertz (kHz) range.

Published in the 2025 issue of UAS Magazine

CONTRIBUTION: The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of UAS Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).

Ken Andrew.css-7jxktu{position:absolute;bottom:0px;left:0px;right:0px;height:3px;border-radius:3px;background-color:var(--chakra-colors-brandBlack);}

Latest Articles

Securing the Signal

Ken Andrew