RF design frequently asked questions
Short answers to common RF design questions, each with a link-ready summary. Follow the article links in the glossary for detail.
Why is 50 ohms the standard RF impedance?
It is a compromise in air coax between minimum loss (about 77 Ω) and maximum power handling (about 30 Ω).
What is −174 dBm/Hz?
Thermal noise power kT at 290 K in a 1 Hz bandwidth.
What is a good noise figure for an LNA?
Discrete LNAs below a few GHz commonly achieve well under 1 dB; integrated LNAs often 1.5–3 dB.
What VSWR is acceptable for an antenna?
A VSWR of 2:1 (return loss about 9.5 dB, mismatch loss 0.5 dB) is a common threshold.
What does the Smith chart show?
The reflection coefficient plane overlaid with circles of constant resistance and reactance.
How are IP3 and P1dB related?
For a simple cubic nonlinearity, input P1dB is about 9.6 dB below IIP3; real devices vary.
What is phase noise?
Random phase fluctuation of an oscillator, expressed as single-sideband noise in dBc/Hz at an offset.
What is the free-space path loss at 2.4 GHz over 100 m?
About 80 dB.
How long is a half-wave dipole?
About 150/f(MHz) meters in free space, trimmed a few percent shorter in practice.
Can I use FR-4 for RF?
Often below a few GHz; low-loss laminates are preferred for higher frequency or loss-critical circuits.
What is the difference between a diplexer and a duplexer?
A diplexer separates two bands; a duplexer lets a transmitter and receiver share an antenna in FDD.
How do I check amplifier stability?
Check Rollett K > 1 with |Δ| < 1, or μ > 1, across all frequencies where the device has gain.
What is S21?
The forward transmission coefficient of a two-port; |S21|² is gain with matched terminations.
What does a VNA calibration remove?
Systematic errors of the instrument, cables and adapters up to the calibration plane.
Which rules govern unlicensed devices in the US?
47 CFR Part 15.