Work Out Your RF Front-End Noise Figure, Gain, OIP3 & OP1dB
Building an RF front end usually means stacking several components in a row: from the antenna there's likely a coaxial cable, a changeover relay, perhaps an LNA, a filter, and then either a long cable run down to the shack or, for a masthead-mounted transverter, just a short lead. Each stage changes the signal a little before passing it to the next. The Cascade Calculator below works out what the whole chain does together — overall gain, overall noise figure, overall linearity (OIP3 and OP1dB), and, if you know your antenna or sky temperature, your full system noise temperature.
Enter each stage in signal order, hit calculate, and the tool does the rest using the industry-standard Friis noise formula.
Why Cascade Math Matters
Noise figure and gain don't add up the simple way. A low-loss switch right at the front of the chain costs you noise figure almost one-for-one, because nothing downstream has amplified the signal yet to swamp that loss. The same loss placed after a good LNA barely matters at all, because by then the signal is already well above the noise floor. Where you put a component in the chain matters as much as how good that component is on its own — which is exactly why a proper cascade calculation, rather than just adding up datasheet numbers, is worth doing before you buy or build anything. For a real worked example of this — including why a big noise-figure improvement doesn't always translate into an equally big real-world signal-to-noise gain — see 24GHz Noise Figure: How Much Does an LNA Help on Tropo?
How to Use This Calculator
- Name your cascade (optional) — useful if you’re comparing a few different front-end options and want to keep the PDF exports straight.
- Add a row for every stage, in signal order, from the antenna/feed end through to your receiver. For each stage enter:
- Gain (dB) — positive for an amplifier, negative for anything lossy (a switch, a length of cable, a filter).
- Noise Figure (dB) — for a passive lossy stage, this is the same number as its loss.
- P1dB and IP3 (dBm) — optional. Leave these blank for a stage that isn’t the linearity bottleneck (most passives are); fill them in for amplifiers where compression or intermod actually matters.
- Use + Add stage to add as many stages as your chain needs, or the × button to remove one.
- If you know your antenna or sky noise temperature, enter it under Antenna / sky temperature (K) to see your full system noise temperature, not just the receiver’s own contribution.
- Not sure what realistic numbers look like? Click Load Example 1 or Load Example 2 to see two fully worked example front ends.
- Once you’ve got results you want to keep, click Save as PDF for a clean, downloadable summary you can file away or share.
Cascade Calculator
Add each stage of your RF front end in signal order, with its Gain, Noise Figure, and (optionally) P1dB and IP3, all in dB / dBm. For a passive lossy stage (a switch, relay, or cable), enter its loss as the Noise Figure and as a negative Gain, and leave P1dB / IP3 blank — they're optional, and a blank field is treated as "not limiting."
| # | Stage name | Gain (dB) | NF (dB) | P1dB (dBm) | IP3 (dBm) |
|---|
Stages are cascaded in the order listed, from the antenna/feed end first through to the final stage. Leave the antenna temperature blank to skip the system-temperature figure.
| Stage | Cum. Gain (dB) | Cum. NF (dB) | Cum. OIP3 (dBm) | Cum. OP1dB (dBm) |
|---|
Friis cascade: F_total = F1 + (F2−1)/G1 + (F3−1)/(G1·G2) + … OIP3/OP1dB use the standard reciprocal-sum combining rule referred to the system output. "Cumulative" columns show the running system figure considering only the stages up to and including that row.
Reading Your Results
- System Gain — the net gain (or loss) of the whole chain, end to end.
- System Noise Figure — the cascaded noise figure referred to the input of stage 1.
- System OIP3 / System OP1dB — the chain’s overall third-order intercept and 1dB compression point, referred to the system output. These only appear if you’ve entered at least one stage’s P1dB or IP3.
- Receiver Noise Temperature (T_rx) — your noise figure converted to temperature (T_rx = 290 × (F − 1)), useful for comparing against antenna/sky noise.
- System Noise Temperature (T_sys) — T_rx plus your antenna/sky temperature, if you entered one. This is the number that actually determines your signal-to-noise ratio on a real path.
The Stage Breakdown table underneath the calculator shows these same figures calculated cumulatively after each stage, so you can see exactly where in the chain your noise figure is being set (usually the first one or two stages) and where it's essentially already locked in.