Noise is unwanted random radio energy that adds to your signal. Whether you can copy a station depends less on how strong it is than on how far it stands above the noise: the signal-to-noise ratio (SNR), normally quoted in decibels. A weak signal over a quiet background can be perfectly readable; a strong signal on a noisy band may not be.
What SNR looks like
Same signal every time; only the noise level changes. Dashed line: the signal on its own.
Signal power ÷ noise power10×
Slide it down and the signal sinks into the hash. Your ears and brain, or a decoder, can still dig a tone out of noise well below the point where you can see it on a screen, and some digital modes decode signals around 20 dB below the noise in a 2.5 kHz bandwidth. The limit is set by the SNR in the bandwidth you are listening to; see Decibels for the dB arithmetic.
Where noise comes from
Schematic, not measured data: real levels vary with time, place and weather.
Thermal noise. Every warm object has randomly jiggling electrons, which produce noise in any circuit or antenna. Its power is k × T × bandwidth, which at room temperature is −174 dBm in every hertz. It sets the floor that no receiver can beat.
Atmospheric noise. Lightning storms around the world produce static that travels by skywave. It is worst on the low HF bands, at night and in summer, and fades with rising frequency.
Man-made noise. Power lines, switching power supplies, motors, LED lighting and computers radiate hash. In towns it often dominates on HF and into VHF. See Power-line and switching noise.
Galactic noise. The sky itself hisses, most noticeable on the upper HF and VHF bands when man-made noise is low.
Bandwidth and the noise floor
Noise power is proportional to the bandwidth you let in. Every doubling adds 3 dB of noise, so a narrow filter that passes only the signal cuts noise without cutting signal:
Receiver noise floor−174 dBm/Hz + 10 log₁₀(bandwidth in Hz) + noise figure in dB
A lower noise floor means weaker signals stand out. Narrow the bandwidth or lower the noise figure.
Noise floor-132.2dBm
At 2.4 kHz, the SSB voice bandwidth, thermal noise alone is about −140 dBm. A CW filter 500 Hz wide is about 7 dB quieter, which is why CW can get through when SSB cannot.
What to do about it
Find out which noise is limiting you, because the fix differs.
Noise arriving from outside (HF, and any noisy location): a quieter receiver won't help, because the noise comes in with the signal. Raise the signal with a better antenna, find and fix local noise sources, move the antenna away from them, and use a narrower filter or noise cancelling.
Noise from the receiver itself (VHF, UHF and microwaves, with weak signals): put a low-noise preamplifier at the antenna, and keep feed-line loss before it small; see Sensitivity, selectivity and dynamic range.
Everywhere: a more directional antenna hears less noise from other directions as well as more signal.