RF View

RF concepts

The ideas behind RF work, such as why 50 ohm, what S-parameters mean and how matching works. Expert detail first, then a plain-language version.

14 articles
  • TRL Calibration Explained: Thru, Reflect and Line2026-10-26TRL needs only the line impedance and line length to be known. The reflect can be any high-reflection device, equal at both ports. The line must be longer than the thru by 20° to 160° of phase, so one line covers about an 8:1 frequency band.
  • VNA Dynamic Range and Noise Floor in Stop-Band Measurements2026-10-26Transmission dynamic range is the source power minus the receiver noise floor, in dB. Raising the source power by 10 dB or narrowing the IF bandwidth by a factor of 10 each gains 10 dB, until the receiver compresses.
  • How a Vector Network Analyzer Measures S-Parameters2026-10-25A VNA measures complex wave ratios: S11 = b1/a1 and S21 = b2/a1 when a2 = 0. Because it reports ratios, S-parameters do not depend on source power while the receivers stay linear, and a magnitude of 0.1 reads -20 dB.
  • The 12-Term Error Model: What VNA Calibration Corrects2026-10-25A full two-port VNA calibration solves 12 error terms, 6 forward and 6 reverse. A one-port calibration needs 3. Measuring known standards lets the VNA solve these repeatable errors and remove them, but not noise or connector repeatability.
  • Amplifier Stability: How to Read the Rollett K Factor2026-10-18An amplifier is unconditionally stable at a frequency when K is greater than 1 and |delta| is less than 1. With S12 = 0.1 and S21 = 2, K is 1.256 and the amplifier passes; with S12 = 0.15 and S21 = 4, K falls to 0.519 and it does not.
  • dB and dBm in a Signal Chain: Adding Gains and Losses2026-10-18A gain or loss in dB is added to a level in dBm to give the new level in dBm. A -10 dBm signal through a 20 dB amplifier and a cable with 3 dB loss ends at +7 dBm, about 5 mW.
  • SMA Connectors: Frequency Limits, Torque and Reflections2026-10-18SMA connectors are 50 ohm and commonly specified up to 18 GHz, and 5 in-lb (about 0.56 N*m) is a common torque for repeatable results. Two connectors with 30 dB return loss each can combine into a reflection of up to 0.063 (about -24 dB), seen as ripple in S11.
  • VSWR Explained: Reflection Coefficient and Mismatch Loss2026-10-17VSWR = (1 + |G|) / (1 - |G|), so a VSWR of 2 means |G| = 0.333, a return loss of 9.54 dB, 11.1 % of the power reflected and 0.512 dB of mismatch loss. SWR and VSWR are the same number.
  • How to Measure RF Power: Power Meters, Analyzers and Losses2026-10-16A reading of -12.3 dBm after 10 dB of attenuator and 1.5 dB of cable means the source is -0.8 dBm, because losses in dB are added back. Mismatch with a VSWR of 1.2 at both ends adds up to about ±0.07 dB of uncertainty.
  • RF Matching Networks: L, Pi and T Compared2026-10-15An L-network has its loaded Q fixed by the resistance ratio: Q = sqrt(R_high / R_low - 1), which is 1.73 for 50 ohm to 200 ohm. Pi and T networks add a third part so you can choose a higher Q and a narrower, more selective match.
  • Smith Chart Basics: How to Read Impedance and Reflection2026-10-15The Smith chart plots the complex reflection coefficient inside a unit circle, with the center as a perfect match. A load of 100 + j50 ohm in 50 ohm gives z = 2 + j1, a reflection coefficient of 0.447 at 26.6 degrees and a VSWR of 2.62.
  • S11, S21, S12 and S22 Explained for a Two-Port2026-10-14In Sxy the first number is the port where the wave comes out and the second is the port where it was applied, so S21 is transmission from port 1 to port 2. A magnitude of 0.5 is about -6.02 dB, and a passive cable or plain filter has S21 equal to S12.
  • Return Loss vs Insertion Loss: What Each Number Tells You2026-10-13Return loss, from S11, tells you how much power bounces back. Insertion loss, from S21, tells you how much fails to arrive. A 10 dB return loss reflects 10 % of the power, yet a matched 10 dB attenuator still has 10 dB of insertion loss.
  • What S21 and S11 of a Low-Pass Filter Look Like2026-10-13At the -3 dB cutoff a Butterworth low-pass filter transmits half the power and reflects half, so S21 and S11 are both -3.01 dB. Above the cutoff S21 falls and S11 climbs towards 0 dB, because a lossless filter reflects what it cannot pass.