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Subelement E5
ELECTRICAL PRINCIPLES
Section E5D
RF effects in components and circuits: skin effect; real and reactive power; electrical length of conductors
What is the result of conductor skin effect?
  • Correct Answer
    Resistance increases as frequency increases because RF current flows closer to the surface
  • Resistance decreases as frequency increases because electron mobility increases
  • Resistance increases as temperature increases because of the change in thermal coefficient
  • Resistance decreases as temperature increases because of the change in thermal coefficient

The AC current density is strongest at the surface of a conductor, and the magnitude decreases exponentially as you get farther away from the surface. Several variables affect this distribution, with frequency being one of them. You just have to remember that the current density at the surface increases with increasing frequency, leading to a 'thinner' RF current.

The skin effect governs how far RF signals penetrate a given material.

Memory Aid: The question refers to conductor skin effect. The portion of the correct answer - “…current flows close to the surface,” refers to “skin” (on the surface).

Silly Hint: Where can you find a lot of skin on people? In their creases ("Increases" is in the right answer).

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Why is it important to keep lead lengths short for components used in circuits for VHF and above?
  • To increase the thermal time constant
  • Correct Answer
    To minimize inductive reactance
  • To maintain component lifetime
  • All these choices are correct

Any wire has self inductance, which increases with the length of the wire (among other things). Since the impedance of an inductor is proportional to frequency, it is usually safe to ignore the self inductance of short wires at low frequencies. But for VHF and above a wire's self inductance may have significant inductive reactance. This reactance is often unwanted and can be minimized by keeping connections short.

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What is the phase relationship between current and voltage for reactive power?
  • They are out of phase
  • They are in phase
  • Correct Answer
    They are 90 degrees out of phase
  • They are 45 degrees out of phase
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Why are short connections used at microwave frequencies?
  • To increase neutralizing resistance
  • Correct Answer
    To reduce phase shift along the connection
  • To increase compensating capacitance
  • To reduce noise figure

The answer is somewhat bogus as with microstrip and other high frequency designs, you use controlled lengths of connections (transmission lines) to purposely introduce phase shift which is part of tuning and matching.

In other words, short connections are not necessary other than to cut down on loss and parasitic radiation.

But if you wanted to minimize phase shift (which is rarely a design goal), then you would want short connections.

Just remember it is the only answer with "phase shift" in it.

Hint: It's also the only answer with the word "connection" in it, which is also in the question.

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What parasitic characteristic causes electrolytic capacitors to be unsuitable for use at RF?
  • Skin effect
  • Shunt capacitance
  • Correct Answer
    Inductance
  • Dielectric leakage

All capacitors have parasitic inductance, also known as equivalent series inductance (ESL). ESL is the result of the size of the capacitor's package. Smaller capacitors tend to have lower ESL than larger capacitors. For example, large electrolytic or tantalum capacitors have higher ESL than small ceramic capacitors.

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What parasitic characteristic creates an inductor’s self-resonance?
  • Skin effect
  • Dielectric loss
  • Coupling
  • Correct Answer
    Inter-turn capacitance

Self-resonance happens when you have both inductance and capacitance in series.

Real world components have parasitics-- every component contains resistance, inductance, and capacitance in addition to the values it is designed to have.

In an inductor, adjacent wire acts similarly to the plates of a capacitor to create a small amount of parasitic capacitance between each turn. This combination of inductance and capacitance causes.

HINT: Both "self-resonance" in the question "inter-turn" in the answer are hyphenated words.

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What combines to create the self-resonance of a component?
  • The component’s resistance and reactance
  • Correct Answer
    The component’s nominal and parasitic reactance
  • The component’s inductance and capacitance
  • The component’s electrical length and impedance
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What is the primary cause of loss in film capacitors at RF?
  • Inductance
  • Dielectric loss
  • Self-discharge
  • Correct Answer
    Skin effect
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What happens to reactive power in ideal inductors and capacitors?
  • It is dissipated as heat in the circuit
  • Correct Answer
    Energy is stored in magnetic or electric fields, but power is not dissipated
  • It is canceled by Coulomb forces in the capacitor and inductor
  • It is dissipated in the formation of inductive and capacitive fields

The question states both ideal inductors and capacitors, so think perfect. The current just passes from the inductors (magnetic field) to the capacitors (electric field), back and forth so none of the power is lost or dissipated.

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As a conductor’s diameter increases, what is the effect on its electrical length?
  • Thickness has no effect on electrical length
  • It varies randomly
  • It decreases
  • Correct Answer
    It increases

From the ARRL Antenna book:

the electrical length of a linear circuit such as an antenna wire is not necessarily the same as its physical length in wavelengths or fractions of a wavelength. Rather, the electrical length is measured by the time taken for the completion of a specified phenomenon.

As the diameter increases the resistance decreases, which in effect lengthens the effective "electrical length" of the wire. Thus you could have two wires of differing physical length which are both electrically e.g. "\(1\over2\) wavelength" at the same frequency because the shorter one has a larger diameter.

So, to restate again: the electrical length increases as the diameter increases.

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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?
  • 70.7 watts
  • Correct Answer
    100 watts
  • 141.4 watts
  • 200 watts

Only resistance (real component of impedance) consumes power. The values for the resistor, 100 ohms, and current, 1 A, are given.

\begin{align} P_{\text{real}} &= I^2 R\\ &= (1 \text{ A})^2(100 \:\Omega)\\ &= 100 {\text{ W}} \end{align}

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What is reactive power?
  • Power consumed in circuit Q
  • Power consumed by an inductor’s wire resistance
  • The power consumed in inductors and capacitors
  • Correct Answer
    Wattless, nonproductive power

Capacitors resist change in voltage and inductors resist change in current each by storing energy and releasing it as voltage and current fluctuate. This is called reactance. Unlike resistance, no actual power is dissipated by reactance. In purely reactive circuits there will still be measurable voltage and current. The product of this voltage and current is called "wattless" power, measured in volt-ampere reactive (VAR).

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