Noise Floor
The noise floor is the line in the sand below which signals become undetectable. It is the total noise power present at the output of your receiver referenced back to its input — the combination of thermal noise from the antenna and feedline, plus the noise added by every stage in the receiver itself. Any signal arriving at the antenna with less power than the noise floor will be buried in the noise and cannot be recovered by amplitude detection.
Knowing how to calculate the noise floor of a real receiver is not just academic. It tells you the absolute best-case sensitivity your station can achieve, allows you to compare receivers objectively, helps you decide whether an investment in a better preamplifier or a lower-loss feedline will actually improve reception, and explains why narrowing the IF bandwidth is one of the most effective tools for pulling weak signals out of the noise.
The noise floor is the sum of three terms: the thermal noise density at room temperature (−174 dBm/Hz), the bandwidth factor (10·log₁₀ of the bandwidth in Hz), and the receiver's noise figure in dB.
View LargerThe Noise Floor Formula
The noise floor of a receiver is the power of the noise present at the receiver's input, expressed in dBm. It is the signal power that would produce an output equal to the noise output — the level below which the signal is indistinguishable from noise. The formula combines everything from the previous two lessons:
Where:
N = noise floor in dBm
−174 = thermal noise density at 290 K in dBm/Hz
BW = receiver noise bandwidth in Hz
NF = receiver noise figure in dB
Let's unpack each term:
−174 dBm/Hz is the thermal noise power density at room temperature (290 K), as derived in the previous lesson. This is the floor that exists even before any receiver hardware is considered — it is the noise generated by the thermal motion of electrons in the 50 Ω source impedance at the antenna terminals.
10 × log10(BW) is the bandwidth factor. The total noise power in bandwidth BW is BW times the noise density per Hz. In dB, multiplying by BW becomes adding 10 × log10(BW). For a 2.4 kHz SSB bandwidth: 10 × log10(2400) = 10 × 3.38 = 33.8 dB. For a 500 Hz CW filter: 10 × log10(500) = 10 × 2.70 = 27.0 dB.
NF is the receiver's noise figure in dB, from the previous lesson. Every real receiver adds noise beyond the thermal floor. A 6 dB NF receiver raises the noise floor by 6 dB compared to a perfectly noiseless receiver.
Worked Examples
Receiver NF = 8 dB, Bandwidth = 2400 Hz (SSB filter)
N = −174 + 10 × log10(2400) + 8
N = −174 + 33.8 + 8
N = −132.2 dBm
This means any SSB signal arriving at the antenna input with power below −132.2 dBm cannot be heard.
Receiver NF = 8 dB, Bandwidth = 500 Hz (narrow CW filter)
N = −174 + 10 × log10(500) + 8
N = −174 + 27.0 + 8
N = −139.0 dBm
Switching from 2.4 kHz SSB to 500 Hz CW improved the noise floor by 6.8 dB — at the same transmit power, more contacts become possible. This is purely a bandwidth effect; the receiver hardware did not change.
System NF (LNA + cable + receiver combined) = 2 dB, Bandwidth = 2700 Hz (2m SSB)
N = −174 + 10 × log10(2700) + 2
N = −174 + 34.3 + 2
N = −137.7 dBm
This is the noise floor of a well-optimized 2m station. EME signals can be as weak as −150 dBm, which is 12 dB below this noise floor — which is why EME uses digital modes that can work 15–25 dB below the instantaneous noise floor by accumulating data over many seconds.
Noise Floor Calculator
Receiver Noise Floor
Calculate the noise floor from bandwidth and noise figure using N = −174 + 10·log₁₀(BW) + NF. You can also select a common mode from the dropdown to auto-fill the bandwidth.
Why Bandwidth Is the Key Variable
The bandwidth term in the noise floor formula is something you can actually control — the noise figure is usually fixed by the hardware you own. Every time you halve the bandwidth, you reduce the noise floor by 3 dB. Every time you reduce the bandwidth by a factor of 10, the noise floor improves by 10 dB. This is why narrow-bandwidth modes and filters are so powerful for weak-signal work.
Consider what happens when you select your radio's narrowest CW filter for a DX contact:
| Filter Bandwidth | Mode | BW Factor | Noise Floor (NF = 8 dB) | Improvement over 2.4 kHz |
|---|---|---|---|---|
| 2400 Hz | SSB | +33.8 dB | −132.2 dBm | reference |
| 1800 Hz | SSB narrow | +32.6 dB | −133.4 dBm | 1.2 dB |
| 500 Hz | CW wide | +27.0 dB | −139.0 dBm | 6.8 dB |
| 250 Hz | CW medium | +24.0 dB | −142.0 dBm | 9.8 dB |
| 100 Hz | CW narrow | +20.0 dB | −146.0 dBm | 13.8 dB |
| 50 Hz | CW very narrow | +17.0 dB | −149.0 dBm | 16.8 dB |
Moving from 2.4 kHz SSB to 50 Hz CW improves the noise floor by 16.8 dB. This is equivalent to increasing transmit power by 48 times — from 100 W to nearly 5,000 W — without spending a penny on amplifiers. This is the fundamental reason why CW is the most efficient voice-equivalent mode for working DX on a budget.
Digital modes take this even further. WSPR uses a 6 Hz bandwidth, giving a noise floor improvement of 10 × log10(2400/6) = 26 dB compared to SSB. Combined with coherent detection algorithms that accumulate signal over 2 minutes, WSPR can detect signals that are genuinely 30–40 dB below what you can hear on SSB.
Minimum Discernible Signal (MDS)
The noise floor gives the level at which the signal power equals the noise power — a 0 dB SNR point. But a 0 dB SNR is not a usable signal for most purposes. A CW operator needs at least 3–6 dB SNR to copy code. An SSB voice contact needs 10–15 dB SNR for comfortable intelligibility. A digital mode like FT8 needs only about −10 dB SNR (the signal is 10 dB below the noise floor) because the algorithm accumulates data over 15 seconds.
The minimum discernible signal (MDS) is the input signal power that produces a specified SNR at the output. For practical receiver specifications, MDS is often defined as the signal that produces a 3 dB increase in output power when switched from antenna disconnected to antenna connected — meaning the signal power equals the noise power (0 dB SNR). This is essentially the noise floor itself.
To convert MDS to a useful operating sensitivity for a specific mode, add the required SNR for that mode:
Example: 500 Hz CW, NF = 8 dB, noise floor = −139 dBm
Required SNR for comfortable CW copy = 6 dB
Practical sensitivity = −139 + 6 = −133 dBm
Comparing Receivers by Noise Floor
When comparing two receivers, the noise floor calculation immediately reveals which will perform better for a given application. A receiver with 6 dB NF in a 2.4 kHz bandwidth has a noise floor of −174 + 33.8 + 6 = −134.2 dBm. A receiver with 15 dB NF in the same bandwidth has a noise floor of −174 + 33.8 + 15 = −125.2 dBm. The 6 dB NF receiver is 9 dB more sensitive.
However, this comparison only holds when the receiver is noise-limited (no strong interfering signals). A receiver with 15 dB NF might have better large-signal performance (higher dynamic range, better IP3) than the 6 dB NF unit. The choice depends on whether your operating environment is dominated by weak-signal sensitivity or large-signal interference problems.
Practical Noise Floor Examples
Here are noise floor values for typical ham radio equipment to give you a sense of real-world numbers:
| Equipment | Typical NF | Mode/BW | Noise Floor |
|---|---|---|---|
| Icom IC-7300 HF transceiver | ~10 dB | SSB, 2.4 kHz | −130 dBm |
| Elecraft K3 HF transceiver | ~8 dB | SSB, 2.4 kHz | −132 dBm |
| RTL-SDR dongle (no LNA) | ~15 dB | SSB, 2.4 kHz | −125 dBm |
| RTL-SDR with 20 dB/2 dB LNA | ~2.5 dB | SSB, 2.4 kHz | −138.3 dBm |
| Typical 2m FM transceiver | ~8 dB | FM, 15 kHz | −124.8 dBm |
| 2m EME station (mast LNA) | ~1.5 dB | SSB, 2.7 kHz | −138 dBm |
These noise floor values assume the external noise arriving at the antenna is below the receiver's noise floor. On HF, this often is not the case in populated areas — atmospheric noise, power line noise, and man-made noise commonly raise the effective noise floor by 10–30 dB above the receiver's internal noise floor, making receiver sensitivity improvements worthless until the external noise is addressed.
Frequently Asked Questions
Why does my S-meter show S0 when the receiver's noise floor is −130 dBm? That seems loud.
The S-meter reads the signal level at a point inside the receiver that includes considerable gain. The noise floor of −130 dBm refers to the antenna input port — the level is genuinely −130 dBm there. By the time the signal reaches the S-meter drive point, the receiver has amplified it by perhaps 100–130 dB, producing audio. What you hear as "noise" is the thermal noise amplified to an audible level. The −130 dBm figure is the equivalent signal level at the antenna input that would produce the same audio output level as the noise.
Can I improve the noise floor by using a narrower roofing filter?
Yes — a roofing filter placed before the first IF amplifier reduces the noise bandwidth of the receiver, directly improving the noise floor. This is one of the main benefits of high-quality transceivers with narrow (500 Hz or 3 kHz) roofing filters. However, the roofing filter must be placed before the IF amplifier for full benefit; a filter placed after a high-gain stage cannot reduce the noise that the IF amplifier itself introduced.
Why does switching to digital modes like FT8 seem to work so much better than SSB?
FT8 uses a very narrow transmission bandwidth of about 50 Hz and processes the signal over 15 seconds, giving it an effective processing gain of many dB compared to SSB. The receiver still has the same noise floor in the same bandwidth, but the digital decoder acts as an extremely narrow matched filter for the FT8 signal. The result is that FT8 can decode signals that are 10–15 dB below what the human ear can detect on SSB — equivalent to running 10–30 times more transmitter power.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.