Tell gamma, hairpin and stub matches apart, size a quarter-wave section, and see what Wilkinson dividers and phasing lines do.
Syllabus: Impedance matching: matching antennas to feed lines; phasing lines; power dividers
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
| System | Signature |
|---|---|
| Gamma | shield to element center, center conductor tapped to one side, series capacitor cancels inductive reactance. Also shunt-feeds a grounded tower at its base |
| Beta / hairpin | element insulated from the boom, an inductance across the feed point. Needs a capacitive feed point (element shorter than ½ λ) |
| Stub | short piece of line in parallel with the feed line at the feed point |
RememberSeries C = gamma. Hairpin inductance needs a capacitive element.
A quarter-wave line turns a load into Zq² ÷ Z load. To match 100 Ω to 50 Ω, you need Zq = √(100 × 50) = 70.7 Ω: pick the closest available, 75 Ω. What is left over is described by the reflection coefficient Γ = (Z load − Z₀) ÷ (Z load + Z₀), which sets the SWR.
RememberZq = √(Z1 × Z2). Γ describes how a load meets a line.
A Wilkinson divider splits power equally between two 50 Ω loads while the input stays 50 Ω. Each output sits behind a quarter-wave 70.7 Ω section that transforms its 50 Ω load up to 100 Ω, and two 100 Ω in parallel make 50 Ω. A resistor between the outputs keeps them isolated from each other.
RememberWilkinson: equal split, 50 Ω in, 50 Ω out.
Drive two or more elements through phasing lines (extra line length = delay) and the signals add in some directions and cancel in others. That controls the radiation pattern: it doesn't change band coverage, harmonics or take-off angle by itself.
RememberPhasing lines set element phase. Phase sets the pattern.