Understanding Noise Figure (NF): When It Matters and When It Doesn’t
What is Noise Figure (NF)?
Noise Figure (NF) quantifies how much noise an amplifier adds to the signal it processes. It is defined as:
NF (dB) = 10 * log10(SNR_in / SNR_out)
An NF of 3 dB means the amplifier degrades the signal-to-noise ratio (SNR) by a factor of 2.
NF is Frequency Dependent
Most datasheets only mention one NF value, but this hides a critical fact:
NF varies with frequency. It's typically low at low frequencies, rises toward the upper HF and VHF range, and often worsens above 500 MHz.
Especially in MMICs and opamps, capacitive coupling, parasitics, and gain roll-off all increase NF at higher frequencies.
Where NF Matters and Where It Doesn't
| Frequency | Dominant Noise Source | Is NF Important? |
|---|---|---|
| < 10 MHz | Atmosphere (QRN), man-made noise | ❌ Mostly irrelevant |
| 10–30 MHz | Mix of atmospheric and system noise | ⚠️ Depends on CMR & site |
| 30–150 MHz | Receiver NF, galactic & man-made noise | ✅ Important for weak signals |
| >150 MHz | Receiver + antenna + losses | ✅ Critical in EME, VHF DX |
Below 30 MHz: It's Not the LNA, It's the World
At LF, MF, and most of HF:
- Atmospheric noise floor is around -100 to -120 dBm/Hz.
- Even a "bad" NF of 10 dB has no real impact on total SNR if your antenna picks up more noise than the LNA adds.
- Improving NF beyond ~6 dB often yields no reception benefit.
What is critical at these frequencies:
- Common-mode rejection ratio (CMRR) to suppress RFI picked up on the feedline
- Proper antenna design to pick up magnetic (H-field) or electric (E-field) energy with good SNR
Above 30 MHz: NF Starts to Matter
As you enter VHF and UHF:
- Atmospheric noise vanishes
- The galactic background becomes dominant until about 300 MHz
- Above that, your receiver’s NF sets the limit — especially in quiet rural sites or for EME, SAT, or weak signal DX
Myth: "Lower NF always means better reception"
Nope. That’s marketing nonsense.
Unless your system noise floor (antenna + sky + RFI + losses) is below your LNA noise floor, improving NF gives you nothing.
What does matter:
- Low NF in VHF/UHF/Microwave
- High CMRR and shielding in HF and below
- Proper impedance match to prevent reflections and losses
Practical Example: HF SDR Preamp
| Scenario | Antenna Type | NF (dB) | Result |
|---|---|---|---|
| Quiet site, active whip | EchoTracer | 1.7 dB | Excellent SNR — atmospheric-limited |
| Urban, no CMR choke | Same whip | 0.5 dB | Poor SNR — common-mode noise dominates |
| VHF DX Yagi, no LNA | 4-el beam @ 144 MHz | – | Weak signals lost in receiver noise |
| VHF Yagi + 0.8 dB NF LNA | Same beam | 0.8 dB | Weak DX now decodable |
Conclusion: Focus on What Actually Affects SNR
NF is just one piece of the puzzle.
✅ NF is key:
- For VHF/UHF/SHF
- In low-noise, quiet environments
- With weak signal modes like EME, FT8 DX, satellites
❌ NF is overrated:
- For HF reception, especially below 10 MHz
- In urban QRM zones
- When CMR is poor
A 1 dB NF won't save you if your antenna hears more streetlights than sky.
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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.