Ham Radio: Impedance-3: Reactance

[last updated: 2020-11-25]
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• Reactance opposes current flow for AC signals. ∿
  Reactance is measured in ohms, and the actual value of the reactance depends on the frequency of the AC signal.
        (see calculation formula below)

• Reactance is created by inductors and capacitors.
  Inductance (L) is measured in Henrys, and creates "Inductive Reactance" (X_l) measured in ohms.
  Capacitance (C) is measured in Farads, and creates "Capacitive Reactance" (X_c), also measured in ohms.

• Calculate Reactance:
  Inductive Reactance: X_l = 2 * pi * F * L
  Capacitive Reactance: X_c = -1 / (2 * pi * F * C)
       (Note Capacitive Reactance is a negative value)
  where:
  F is frequency in Hz
  L is inductance in Henrys
  C is capacitance in Farads

• Circuits with both inductive and capacitive reactance:
  Since reactances are "like" resistances, if you want to combine series or parallel reactances,
  use the same rules that you'd use to combine resistances.
  • Total reactance in a series circuit is the sum of the reactances of the individual components.
    For example if you have a 50ohm inductive reactance in series with a 100ohm capacitive reactance,
    the net reactance will be:      50 + (-100) = -50ohms capacitive
  • Total reactance in a parallel circuit with two components uses the "product-over-the-sum" formula.

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• So what's going on when an inductor or a capacitor creates Reactance?

• The Answer: Because of the way capacitors and inductors are built,
  and the way that electric and magnetic fields work,
  it happens that both inductors and capacitors,
       when they are in a circuit with an AC waveform,

  
       Shift the current waveform to be out of phase with the applied voltage signal.
       They create a phase-shift between voltage and current.
  
       This is the essence of Reactance: Voltage and Current are out-of-phase with each other.

• The shift can happen in either direction: Capacitors shift the current to lead the voltage waveform. Inductors shift the current to lag the voltage.
• By convention, phase angle is measured from voltage to current. So if voltage leads (inductors), the phase angle is positive. If voltage lags (capacitors), the phase angle is negative.
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  More explanation:
  go to: Impedance-3a - Reactance Physics

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