Last Updated on May 10, 2026 by John Berry
Radio noise consists of unwanted signals that degrade your receiving experience. These signals originate from many different sources. Engineers divide these into two categories: system noise and environmental noise. System noise comes from the components within your radio equipment. Conversely, environmental noise includes all external sources that degrade the receiver threshold.
This page focuses on the characteristics of environmental radio noise.
Radio noise sources
External noise sources are either natural or man-made. The Sun is a primary source of natural radio noise. Its impact varies depending on solar activity and your antenna system.
For example, Earth-Moon-Earth (EME) operators avoid the Moon when it sits near the Sun. Solar noise can degrade a modest receiver threshold by 6dB. High-gain systems suffer even greater degradation.
Natural noise also interacts with your antenna design. An antenna’s noise figure includes noise picked up by its side lobes. These lobes often point at the warm ground or other natural sources. I consider this when calculating system noise in another page.
Radio noise is probabilistic. You must think of it in terms of exceedance. This describes the time for which and the locations where a specific noise power is exceeded.
Man-made noise
Man-made radio noise comes from two distinct source types. First, multiple emitters in a local area create a general noise background. This defines the noise profile for a typical town-based station. Second, noise can come from a single, specific emitter or a small number of emitters. Examples include a nearby cellular site, radar, or a noisy computer monitor.
Recommendation ITU-R P.372-17 provides a standard model for man-made noise degradation. Generally, noise follows this equation:
F{am} = c – d log f
In this model, F{am} is the noise figure in dB. The frequency is f in MHz. The constants c and d change based on your environment. These categories include City, Residential, or Rural areas.
Essentially, noise reduces as frequency increases. Therefore, radio noise impacts HF operators more than UHF operators. Environmental noise adds directly to your system noise figure. It degrades your receiver threshold decibel for decibel.
Practical impact
Consider a station in a residential town operating in the 40m band. ITU data suggests an environmental noise figure of approximately 50dB. A bench test might show a sensitivity of 0.3 microvolts p.d. (-148dBW). However, 50dB of noise raises this threshold to -98dBW (90microvolts).
Noise figures are shown against frequency in the graph below for different environments.
for a short vertical lossless grounded monopole antenna – from
Rec. ITU-R P.372-17 Radio Noise Part 6: Man Made Noise
This change in noise figure equals roughly 8 S-points. Your S-meter would for much of the time read S7 or S8. You would only hear signals stronger than this background level. Moving to a quiet rural site reduces degradation to 30dB. This lowers the noise floor to S5, which is a massive improvement – but you’d need to move QTH.
Variations in radio noise
Noise levels depend on the time of day and your specific location. The ionosphere reflects noise from distant sources at certain times. The ITU Recommendation provides tables showing these variations. At certain times, residential noise might rise by 10dB (about two S-points). It could even drop by 5dB. This could make the 40m band unusable sometimes and very usable at others.
As society demands more consumer devices that make radio noise, such as those operating within the Internet of Things, noise levels are almost certainly set to rise over the coming years.
Mitigation against radio noise
You can often mitigate specific noise sources. Use the RSGB guidance to trace local interference. Sometimes, you must add ferrites or filters directly to the offending electronic devices.
Antenna choice also matters. An HF vertical antenna with a high angle response may be much less susceptible to local noise than a horizontal wire. A folded dipole may be less susceptible than an open wire. And directional VHF Yagis can “null out” noise by pointing away from the source.
Summary
Radio noise originates from the Earth, the atmosphere, and the galaxy. It also comes from every electronic device in our modern world. While man-made noise is pervasive, proper antenna engineering helps. Understanding these facts allows you to optimise your station for the best possible receiver threshold.