Skin effect
What is the result of conductor skin effect?
Resistance increases as frequency increases because RF current flows closer to the surface
At DC, current uses the whole cross-section. At RF, induced eddy currents push it outward until it flows only in a thin surface layer — so the effective cross-section shrinks and resistance rises with frequency.
Three consequences worth carrying:
- Flat strap beats round wire for RF bonding, because what matters is surface perimeter rather than cross-sectional area. That is the Technician station-setup answer, now with its reason.
- Silver plating helps at RF and not at DC — only the surface carries current, so only the surface’s conductivity matters.
- Hollow tubing is as good as solid rod at RF and much lighter, which is why beam elements and amplifier tank coils are tubing.
What is the primary cause of loss in film capacitors at RF?
Skin effect.
Everything has parasitics
What combines to create the self-resonance of a component?
The component’s nominal and parasitic reactance.
Every real component is the thing it is supposed to be plus a small amount of the thing it is not. Those parasitics resonate against the nominal value, and above that self-resonant frequency the component behaves as its parasitic instead.
What parasitic characteristic creates an inductor’s self-resonance?
Inter-turn capacitance.
Capacitance between adjacent turns. Above self-resonance the inductor is capacitive — the General components lesson stated that result; this is the mechanism.
What parasitic characteristic causes electrolytic capacitors to be unsuitable for use at RF?
Inductance.
An electrolytic is a long strip of foil rolled up — which is a coil. Its series inductance means it stops being a capacitor at a few hundred kilohertz. This is why a power supply needs both a large electrolytic for bulk smoothing and a small ceramic in parallel for RF bypass: neither works over the whole range.
Why is it important to keep lead lengths short for components used in circuits for VHF and above?
To minimize inductive reactance.
A wire’s inductance is roughly 20 nH per inch. At 145 MHz that is about 18 ohms per inch of lead — comparable with the impedances in the circuit, so lead length is a circuit element whether you intended it or not.
Electrical length
Why are short connections used at microwave frequencies?
To reduce phase shift along the connection.
At microwave a connection is a transmission line, and its length is a meaningful fraction of a wavelength. A signal arrives phase-shifted, which changes what the circuit does.
As a conductor’s diameter increases, what is the effect on its electrical length?
It increases.
A fatter conductor is electrically longer than a thin one of the same physical length, because increased capacitance to its surroundings slows the wave. This is why a fat dipole is cut shorter than a thin-wire one for the same frequency — the same end effect that gives 468 rather than 492 in the dipole formula.
Real and reactive power
What is reactive power?
Wattless, nonproductive power.
What happens to reactive power in ideal inductors and capacitors?
Energy is stored in magnetic or electric fields, but power is not dissipated.
What is the phase relationship between current and voltage for reactive power?
They are 90 degrees out of phase.
Reactive power flows into a reactance and back out again each cycle. Nothing is consumed — the current is real and does real work in the wire’s resistance, but the reactance itself dissipates nothing.
The quantitative consequence:
How much real power is consumed in a circuit consisting of a 100-ohm resistor in series with a 100-ohm inductive reactance drawing 1 ampere?
100 watts.
P = I²R = 1² × 100 = 100 watts. The reactance contributes nothing to real power, even though it contributes to impedance and therefore to how much current flows for a given voltage.
That is why power in a reactive circuit is P = E × I × cos(θ), and why the foundations power lesson flagged the qualifier. Here only the resistance dissipates, and I²R with the resistive part alone is the whole answer.
Check yourself
- Why is silver plating useful on an RF coil and pointless on a DC busbar?
- Your bypass capacitor is a 10 µF electrolytic and the circuit still has RF on the supply rail. What is wrong?
- A circuit draws 2 A through 50 Ω of resistance in series with 200 Ω of reactance. Real power?
Answers
- Skin effect — at RF the current flows only in the surface layer, so surface conductivity is what matters. At DC the whole cross-section conducts.
- An electrolytic’s series inductance makes it useless above a few hundred kilohertz. Add a small ceramic in parallel.
- P = I²R using the resistance only: 4 × 50 = 200 watts. The reactance dissipates nothing.