Explain skin effect, parasitic reactance, electrical length, and the difference between real and reactive power.
Syllabus: RF effects in components and circuits: skin effect; real and reactive power; electrical length of conductors
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
As frequency rises, RF current is pushed into a thin layer at the conductor's surface. Less metal carries it, so resistance rises with frequency. It is also the main RF loss in film capacitors.
RememberHigher frequency: current nearer the surface, resistance higher.
Leads and windings add small parasitic reactances: wire has inductance, and neighbouring turns act as a capacitor. The part's own (nominal) reactance and its parasitic reactance combine into a self-resonance. Beyond it the part behaves like its opposite.
| Part | Parasitic | Consequence |
|---|---|---|
| Inductor | inter-turn capacitance | creates its self-resonance |
| Electrolytic capacitor | inductance | unsuitable at RF |
| Any part at VHF and up | lead inductance | keep leads short to minimise inductive reactance |
RememberSelf-resonance = nominal reactance meeting parasitic reactance. Short leads keep parasitics small.
Electrical length is length measured in wavelengths. A signal shifts 360° across each wavelength, so the phase shift along a connection grows with its length and the frequency.
RememberPhase shift = 360° × length ÷ wavelength. Higher frequency, shorter wavelength, bigger shift.
An ideal inductor or capacitor stores energy in a field, then hands it back. Voltage and current are 90° out of phase, so over a cycle the power averages zero. This wattless, nonproductive reactive power is not dissipated as heat. Only resistance consumes real power.
P = I² × R100 Ω resistor in series with 100 Ω XL at 1 A: P = 1² × 100 = 100 W. Total is 141.4 VA, but only the resistor's 100 W is real.
RememberReal power uses R only. Reactive power is wattless and nonproductive.