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2016 Extra Class study guide: E5D - AC and RF energy, behavior of
conductors at high frequencies

Posted: 17 Jan 2016 04:11 PM PST
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Â*Questions E5D02 E5D05 were changed from questions about skin effect and
capacitors and inductors to questions about how conductors behave at high
frequenciesDan

E5D AC and RF energy in real circuits: skin effect; electrostatic and
electromagnetic fields; reactive power; power factor; electrical length of
conductors at UHF and microwave frequencies

In AC circuits–and RF circuits are just a type of AC circuit–capacitors and
inductors store and release energy as the voltages and currents change.
Because of this, calculating power and energy in an AC circuit is not as
straightforward as it is for DC circuits.

Capacitors store electrical energy in an electrostatic field. During the
positive portion of an AC cycle, the capacitor stores energy in its
electrostatic field, but during the negative portion of the cycle, it
returns that energy to the circuit.

Inductors store electrical energy in a magnetic field. The current through
the inductor creates the magnetic field. The amount of current determines
the strength of a magnetic field around a conductor. (E5D07) The direction
of the magnetic field oriented about a conductor in relation to the
direction of electron flow runs in a direction determined by the left-hand
rule. (E5D06)

A similar thing happens to the magnetic field created by the current flow
through an inductor that happens to the electrostatic field in a capacitor.
When the current flows in one direction, a magnetic field is created. When
the current changes direction, the energy stored in that magnetic field
gets returned to the circuit.

The type of energy that is stored in an electromagnetic or electrostatic
field is potential energy (E5D04)

Reactive power

When talking about the power consumed by AC circuits, an important concept
is reactive power. Reactive power is wattless, nonproductive power. (E5D14)

As noted above, during some portions of an AC cycle, inductors and
capacitors will draw current and store energy, but during other portions of
the cycle, it will return that energy to the circuit. So, what happens to
reactive power in an AC circuit that has both ideal inductors and ideal
capacitors is that it is repeatedly exchanged between the associated
magnetic and electric fields, but is not dissipated. (E5D09) In other
words, the net power dissipation is zero.

Of course, very few circuits contain only capacitors and inductors. In AC
circuits where there is a resistance, that resistance will dissipate real
power. For example, in a circuit consisting of a 100 ohm resistor in series
with a 100 ohm inductive reactance drawing 1 ampere, the power consumed is
100 Watts. (E5D13) (P = I2 × R = 1A2 × 100 ohms = 100 W.)

In an AC circuit with inductors and capacitors, the voltage is out of phase
with the current. You determine the true power an AC circuit where the
voltage and current are out of phase by multiplying the apparent power
times the power factor. (E5D10) For example, if a circuit has a power
factor of 0.71 and the apparent power is 500 VA, the watts consumed is 355
W. (E5D18)

The power factor, or PF, is the cosine of phase angle between the voltage
and current. For example, if an R-L circuit has a 60 degree phase angle
between the voltage and the current, the power factor is the cosine of 60
degrees, or 0.5 (E5D11) The power factor of an RL circuit having a 45
degree phase angle between the voltage and the current is the cosine of 45
degrees, or 0.707. (E5D15) The power factor of an RL circuit having a 30
degree phase angle between the voltage and the current is the cosine of 30
degrees, or 0.866. (E5D16)

Let’s look at a few examples:

If a circuit has a power factor of 0.2, and the input is 100-VAC at 4
amperes, the watts consumed is V × I × PF = 100 V × 4 A × 0.2 = 80 watts.
(E5D12)

If a circuit has a power factor of 0.6 and the input is 200V AC at 5
amperes, the watts consumed is V × I × PF = 200 V × 5 A × 0.6 = 600 watts.
(E5D17)

The behavior of conductors at high frequencies

At RF frequencies, the current in a conductor tends to flow near the
surface of that conductor. This is the reason this phenomenon is called
skin effect. The result of skin effect is that as frequency increases, RF
current flows in a thinner layer of the conductor, closer to the surface.
(E5D01)

At VHF, UHF, and microwave frequencies, the inductance of conductors must
be taken into account. The reason for this is that inductive reactance
increases with frequency, and at high frequencies, this reactance is no
longer negligible. Inductance is a parasitic characteristic that increases
with conductor length. (E5D05) It is, therefore, important to keep lead
lengths short for components used in circuits for VHF and above to avoid
unwanted inductive reactance. (E5D02)

Another phenomenon that occurs at high frequencies is that printed circuit
board traces begin to act like transmission lines instead of just simple
conductors. To properly connect components and circuits, printed circuit
board designers carefully lay out the traces so that they have a constant
impedance. Microstrips are precision printed circuit leads above a ground
plane that provide constant impedance interconnects at microwave
frequencies. (E5D03)

At microwave frequencies, it is also import to keep connections as short as
possible. Short connections are necessary at microwave frequencies to
reduce phase shift along the connection. (E5D04)

The post 2016 Extra Class study guide: E5D AC and RF energy, behavior of
conductors at high frequencies appeared first on KB6NUs Ham Radio Blog.