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E5D

ELECTRICAL PRINCIPLES

RF effects in components and circuits: skin effect; real and reactive power; electrical length of conductors

E5D011 of 12

What is the result of conductor skin effect?

Why At RF, changing magnetic fields inside a conductor induce eddy currents that cancel current flow in the center and reinforce it near the surface, so the current crowds into a thin outer layer. The depth of that layer shrinks roughly as the inverse square root of frequency, which means less effective cross-sectional area is carrying the current. Less usable cross-section means higher AC resistance, and it keeps climbing as frequency climbs. This is why RF conductors are often hollow tubing, silver plated, or made of Litz wire.

Watch out The choices about temperature describe the temperature coefficient of resistance, a separate DC effect that has nothing to do with frequency, and electron mobility does not improve with frequency.

Skin effect: higher frequency, thinner skin, smaller effective wire, more resistance. Frequency, not temperature.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D022 of 12

Why is it important to keep lead lengths short for components used in circuits for VHF and above?

Why Any straight wire has inductance, roughly 20 nanohenries per inch, and inductive reactance grows with frequency (XL = 2πfL). At HF that stray inductance is negligible, but at VHF and above a half-inch lead can add tens of ohms of reactance, detuning tuned circuits, adding unwanted series impedance to bypass capacitors, and turning leads into small radiators. Keeping leads short keeps that stray inductance, and the reactance it produces, as small as possible.

Watch out Thermal time constant and component lifetime are heat-related concerns tied to the part's mass and dissipation, not to RF behavior, so the choice that lumps everything together is wrong too.

Short leads = less stray L. XL = 2πfL, and at VHF the f is big, so even 20 nH per inch matters.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D033 of 12

What is the phase relationship between current and voltage for reactive power?

Why Reactive power is the energy that sloshes back and forth between the source and a pure reactance (inductor or capacitor) without being consumed. In an ideal inductor or capacitor the current leads or lags the voltage by exactly 90 degrees, so the average of the instantaneous power product over a cycle is zero, meaning no net energy is dissipated. Real (dissipated) power, by contrast, occurs when voltage and current are in phase, as in a resistor; in general P = E * I * cos(theta), and at theta = 90 degrees cos(theta) = 0.

Watch out The vague choice saying simply "out of phase" is tempting, but any phase angle other than 0 gives some reactive component; the pool wants the specific 90 degree case that is purely reactive with zero real power.

Reactive = 90 degrees (cos 90 = 0, no real watts). Real power = 0 degrees, in phase.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D044 of 12

Why are short connections used at microwave frequencies?

Why At microwave frequencies a wavelength is only centimeters long, so even a half-inch lead is an appreciable fraction of a wavelength. Signals take measurable time to travel that distance, which shows up as phase shift (and series inductance) between parts of the circuit. Keeping connections physically short keeps their electrical length, and therefore the phase shift, small so feedback paths, matching networks and oscillators behave as designed.

Watch out Noise figure is dominated by the first-stage device and its input matching, not by lead length; the neutralizing-resistance and compensating-capacitance choices are invented terms that don't describe why microwave leads are kept short.

At 10 GHz a wavelength is 3 cm, so an inch of wire is most of a wavelength. Short leads = small phase shift.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D055 of 12

What parasitic characteristic causes electrolytic capacitors to be unsuitable for use at RF?

Why Real capacitors are not pure capacitance. An electrolytic is built from long strips of foil and paper rolled into a cylinder, and those turns plus the long leads act as a series inductance. At RF that parasitic inductance (along with high equivalent series resistance) dominates, and above the part's self-resonant frequency the capacitor actually behaves inductively, so it no longer bypasses or couples RF.

Watch out Dielectric leakage is a real electrolytic flaw, but it is a DC/low-frequency loss (self-discharge) and is not what kills their RF performance; skin effect describes current crowding in conductors, not a capacitor parasitic.

Rolled foil = a coil in disguise: electrolytics fail at RF because of series inductance and self-resonance.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D066 of 12

What parasitic characteristic creates an inductor's self-resonance?

Why Every inductor has small capacitances between adjacent turns, since the turns are conductors separated by insulation or air. This distributed capacitance appears effectively in parallel with the coil's inductance, forming a parallel resonant circuit at the self-resonant frequency. Below that frequency the part behaves like an inductor; above it the capacitance dominates and it acts capacitive, which is why real inductors are only usable well below their self-resonance.

Watch out Skin effect is tempting because it is another RF parasitic, but it only raises the conductor's AC resistance by pushing current to the surface; it adds loss, not resonance. Dielectric loss in the coil form likewise adds resistance rather than the capacitance needed to resonate.

Adjacent turns are tiny capacitor plates: L plus that stray C equals self-resonance.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D077 of 12

What combines to create the self-resonance of a component?

Why Every real component has the reactance it is supposed to have (nominal) plus an unintended stray reactance of the opposite kind (parasitic): an inductor has capacitance between its turns, and a capacitor has inductance in its leads and plates. At one particular frequency those two reactances are equal and cancel, which is the component's self-resonant frequency. Above that frequency the part behaves like the opposite type of component, so a choke can start acting capacitive and a bypass capacitor can start acting inductive.

Watch out Saying simply inductance and capacitance is close but incomplete, and it describes a deliberate tuned circuit; the point of self-resonance is that one of the two reactances is a parasitic that the designer never intended to put there.

Self-resonance = what you wanted plus what you got stuck with: nominal reactance versus parasitic reactance.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D088 of 12

What is the primary cause of loss in film capacitors at RF?

Why Film capacitors use low-loss plastic dielectrics (polypropylene, polyester, polystyrene) whose loss tangent stays very small well into the RF range, so the dielectric is not what limits them. What does degrade is the metal: the thin foil or metallized electrodes and the leads carry current only in a shallow outer layer as frequency rises, because skin depth falls off as 1/sqrt(f). That crowding raises the effective series resistance, and the I-squared-R heating in the electrodes and leads becomes the dominant loss mechanism.

Watch out Dielectric loss is the tempting pick because it dominates in many other capacitor types, but the plastic films used here have exceptionally low loss tangents, leaving conductor resistance as the main culprit.

Film dielectric = low loss; the metal is the problem. Skin effect raises ESR as sqrt(f).

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D099 of 12

What happens to reactive power in ideal inductors and capacitors?

Why Ideal inductors and capacitors have no resistance, so they cannot convert electrical energy into heat. Instead they take energy from the source during part of the AC cycle and store it in a magnetic field (inductor) or an electric field (capacitor), then return that same energy to the source during the next part of the cycle. That back-and-forth flow is called reactive power, measured in VAR, and its net average over a cycle is zero. Only real power, dissipated in resistance, produces heat.

Watch out The choices describing dissipation as heat or dissipation in forming the fields confuse reactive power with real power; only resistance dissipates energy, and building a field stores energy rather than consuming it.

Reactance borrows, resistance burns. Ideal L and C give the energy back.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D1010 of 12

As a conductor's diameter increases, what is the effect on its electrical length?

Why Electrical length is measured in wavelengths or degrees, not feet, so it depends on how fast the wave travels along the conductor. A fatter conductor has more distributed capacitance to its surroundings and a greater end effect, which slows propagation (lowers the velocity factor), so each foot of metal represents more electrical degrees. That is why a thick tubing dipole must be cut physically shorter than a thin wire dipole to resonate on the same frequency.

Watch out The choice that says it decreases confuses electrical length with the physical length needed for resonance: the physical cut gets shorter precisely because the electrical length per foot went up. Saying diameter has no effect ignores the well known shortening factor used when designing fat element beams.

Fat elements get cut short, because fat means electrically long.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D1111 of 12

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?

Why Only resistance dissipates real power; reactance stores and returns energy each cycle, so it consumes none. Real power is P = I squared times R, using just the resistive part: 1 A squared times 100 ohms = 100 watts. The 100-ohm inductive reactance contributes reactive power (VARs), not watts.

Watch out The 141.4 watt choice comes from using the impedance magnitude, sqrt(100^2 + 100^2) = 141.4 ohms, which gives apparent power in volt-amperes, not real power in watts. The 200 watt choice wrongly adds resistance and reactance as if both dissipated power.

Watts come from ohms of resistance only: P = I squared R. Reactance gets VARs, never watts.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E5D1212 of 12

What is reactive power?

Why In a purely reactive component, voltage and current are 90 degrees out of phase, so energy flows into the component during one part of the cycle and back out to the source during the next. Over a full cycle the net energy dissipated is zero, so this apparent power does no work; it is measured in VAR (volt-amperes reactive) rather than watts. That is why it is called wattless or nonproductive power.

Watch out The choices about power consumed in inductors and capacitors or in an inductor's wire resistance describe real power: ideal reactances consume nothing, and any heating in the winding resistance is genuine I squared R dissipation, not reactive power.

Reactive power just sloshes back and forth: measured in VAR, not watts. No work done.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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