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Cascade noise figure calculator

Why the first stage decides everything

The noise of each subsequent stage is divided by the gain of everything before it. So a 0.5 dB preamplifier at the masthead and the same preamplifier in the shack behind three decibels of cable are two different receivers. The calculator shows each link's contribution separately, and the difference becomes obvious.

CableNF 3 · G -3Masthead LNANF 1 · G 20ReceiverNF 10 · G 0the stage's share of the excess noise
The first stage sets almost all the noise: what the later ones add is divided by the gain ahead of them
Stages in signal order
NF, dB / gain, dB
Result
Noise figure of the chain
Noise temperature
Total gain

📐 The formulas​

Friis's formula:

F = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁·G₂) + …
NF = 10·log10(F)

Conversions:

F = 10^(NF/10)        G = 10^(G_dB/10)
Noise temperature:  Te = 290 · (F − 1)

A passive element (cable, attenuator, filter) with a loss of L dB:

NF = L        G = −L

That is, a cable with 3 dB of loss has a noise figure of exactly 3 dB.

Check points:​

  • LNA (NF 1 dB, G 20 dB) → receiver (NF 10 dB): NF = 1.30 dB
  • 3 dB of cable → LNA (NF 1 dB): NF = 4.00 dB
  • NF 1 dB → Te = 75.1 K; NF 0.5 dB → Te = 35.4 K
Where this calculator stops:
  • The formula holds for matched stages: with a mismatch the real noise figure differs
  • Noise figure depends on frequency — use the datasheet value for yours
  • On HF the calculation is almost pointless: external band noise is tens of decibels above the receiver's own
  • The noise the antenna itself picks up (its noise temperature) is not accounted for

🎯 In practice​

  • Masthead preamp or one in the shack? Press “swap the first two” and the difference is immediate.
  • Assessing an existing chain: what a low-noise preamplifier really buys you after a long cable.
  • Choosing cable for VHF: every decibel of loss before the preamp is a decibel of noise.
Practical advice:
  1. Put the preamplifier as close to the antenna as you can — that is the formula's main conclusion.
  2. A bandpass filter ahead of the preamp adds its loss to the noise: another trade-off.
  3. From 50 MHz upwards receiver noise starts to matter; below that, hardly at all.
  4. Noise temperature is more convenient than noise figure at small values: 0.5 dB and 0.6 dB sound the same, whereas 35 K and 43 K do not.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Cascade noise figure calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/noise_figure (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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