Also known as: noise floor
The noise floor is the constant background level of random energy present in any receiver — thermal noise in the electronics plus environmental RF picked up by the antenna.1 It is measured in dBm and sets the bar every signal must clear to be usable.
How it works
The floor has a hard physical bottom set by thermal noise. Any resistor — or antenna radiation resistance — at temperature T generates noise power kTB, where k is Boltzmann’s constant, T is temperature in kelvin, and B is bandwidth.2 At room temperature this works out to about −174 dBm per hertz. Multiply by bandwidth (add 10·log₁₀(B) in dB) and you get the floor for a perfect receiver: roughly −121 dBm in a 12.5 kHz channel, −114 dBm in a 100 kHz span. No receiver can hear below this.
Two things push the real floor above that ideal. First, the receiver adds its own noise, quantified by noise figure: a 5 dB noise figure raises the floor 5 dB above kTB. This is why a low-noise amplifier placed first in the chain matters — it sets the noise figure of everything after it. Second, the environment contributes external noise: atmospheric, galactic, and especially man-made interference from switching power supplies, USB buses, LED lighting, Ethernet, and solar inverters. In a noisy urban install this environmental noise, not the receiver, dominates the floor.
Because the thermal contribution scales with bandwidth, the floor moves with the measurement. Halving the receive bandwidth lowers the noise power by 3 dB while keeping a narrowband signal intact — which is exactly why matched filtering and narrow channel filters improve SNR. Conversely, a wideband capture shows a higher floor simply because it integrates noise over more spectrum.
In practice
The margin between a signal and the floor is the SNR, and the floor’s height directly sets receiver sensitivity — the weakest signal that still decodes. Lowering the effective floor is therefore the cheapest way to hear more:
- Put a low-noise preamp at the antenna, before cable attenuation degrades noise figure.
- Reduce environmental pickup: ferrite chokes, quality shielded cable, and moving the SDR away from computers and chargers can drop the floor 10 dB or more.
- Filter out strong out-of-band signals that would otherwise raise the floor through front-end intermodulation.
- Match the receive bandwidth to the signal so noise is not admitted needlessly.
A common diagnostic: if the displayed floor drops several dB when you disconnect the antenna, external environmental noise is dominating and the fix is at the install, not in the software.
Relevance to SDR
On an SDR, the floor you see on the waterfall is the sum of thermal noise, the front-end and ADC noise figure, and whatever the antenna is collecting. GopherTrunk cannot decode a signal that sits at or below the floor, so getting the floor as low as possible — quiet install, LNA at the antenna, right bandwidth — is the highest- leverage thing an operator can do before touching any DSP setting.
Sources
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Noise floor — Wikipedia, the background noise level in a measurement system and its constituents. ↩
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Johnson–Nyquist noise — Wikipedia, the kTB thermal-noise law that sets the physical floor. ↩