RF View

RF library

Concepts, tutorials and component guides, written from handset RF experience.

15 articles

Concepts 10

What the numbers mean: short, exact explanations with the formulas.

  • 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.

Tutorials 5

Step-by-step guides for S-parameters, Smith charts, matching and measurement.

  • L-Network Matching: Low-Pass or High-Pass Version?2026-10-20Matching 50 ohm to 200 ohm needs Q = sqrt(3) = 1.732. The low-pass version uses 13.78 nH and 1.378 pF, the high-pass version 1.838 pF and 18.38 nH, and the choice depends on the DC path and the filtering you want.
  • Microstrip Width for 50 Ohm on a 1.6 mm FR-4 Board2026-10-19On FR-4 (er 4.4) a microstrip line reaches 50 ohm at a width-to-height ratio of about 1.9, which is about 3 mm on a 1.6 mm board. Wider strips lower the impedance and narrower strips raise it.
  • When to Put an Attenuator Pad in an RF Chain2026-10-19A fixed pad of A dB passes 10^(A/10) of the power, so a 10 dB pad passes 0.1 and turns the other 0.9 into heat. It can improve return loss by up to twice its attenuation, but it costs signal level and, in front of a receiver, noise figure.
  • How to Read One Line of an S2P File (MA, DB and RI Formats)2026-10-14A two-port S2P data line holds nine numbers: the frequency, then the pairs for S11, S21, S12 and S22 in that order. The option line tells you whether each pair is MA, DB or RI, and a magnitude of 0.9975 is about -0.022 dB.
  • How to Open an S2P (Touchstone) File: 6 Ways and a Free Viewer2026-10-13An S2P file is a plain-text Touchstone file holding the two-port S-parameters of a device at many frequencies. You can plot it right here in your browser (the file never leaves your computer), read it in a text editor, or open it in RF View, Python, MATLAB or a spreadsheet.