Single-Ended (Unbalanced) vs Differential (Balanced)
| Characteristic | Unbalanced (SE) | Balanced (Differential) |
|---|---|---|
| Signal reference | Single conductor vs ground | Two conductors: V+ and V− |
| Common-mode rejection | None | High (CMRR typically 20–40 dB) |
| Radiation | Ground current varies → radiates | Differential currents cancel → less radiation |
| Interface | Coaxial SMA, PCB microstrip | Twin-lead, stripline pair, differential pair |
| Conversion | Balun converts SE↔differential | — |
Differential S-Parameters (Mixed-Mode)
For a differential pair (4-port), define: Differential mode: V_d = (V+ − V−)/√2 Common mode: V_c = (V+ + V−)/√2 Mixed-mode S-matrix (Sdd, Scc, Sdc, Scd): Sdd11: differential input return loss Sdd21: differential insertion loss (wanted signal path) Scc21: common-mode insertion loss (noise leakage) CMRR = Sdd21 − Scc21 [dB, higher = better noise rejection] Standard .s4p file contains single-ended parameters; post-process to mixed-mode using Touchstone 2.0 or manual calculation.
Why Baluns Are Needed at Antenna Feeds
A dipole antenna is a balanced structure (equal and opposite currents on each arm). Feeding it with coaxial cable (unbalanced) causes common-mode current to flow on the outside of the cable shield — this distorts the antenna pattern and causes RF interference. A balun at the feed point forces equal and opposite currents, recovering the balanced antenna behavior.
Balun S-Parameter Analysis
Load balun .s3p file (Port 1=SE, Port 2=balanced+, Port 3=balanced−): S21 = S31: both outputs equal amplitude (0 dB amplitude balance) Phase(S31) − Phase(S21) = 180° (differential output) S11: input port match <−15 dB S23: output-to-output isolation >20 dB (common-mode rejection)
RF View Balun Analysis: Load balun .s3p file, overlay S21 and S31 magnitude (should be equal) and check phase difference between outputs (target 180°). Phase delta marker quantifies amplitude/phase balance. Free on Android.