Currents & Concepts
Every time you tune an antenna, use an SWR meter, build a filter, or adjust a tuner, you’re interacting with these three concepts — often without realizing it.
Reactance describes how inductors and capacitors resist AC.
Impedance is the grand total of resistance and reactance.
Resonance is when these opposites cancel and energy flows freely.
This section teaches you how electricity behaves dynamically — not just how much flows, but how it flows, at what frequency, and how efficiently.
Reactance – The Frequency-Dependent Resistance
What It Is:
Reactance is the opposition to alternating current (AC) due to energy storage in electric or magnetic fields.
Capacitive Reactance (XC) — from capacitors
- Inductive Reactance (XL) — from inductors
- Measured in ohms (Ω) like resistance, but doesn’t consume power (it's reactive, not resistive).
Formulas:.
Where:
ƒ = frequency (Hz)
C = capacitance (F)
L = inductance (H)
Behavior Summary:
|
Component |
Low Frequency |
High Frequency
|
|---|---|---|
| Capacitor | High reactance (blocks) |
Low reactance (passes)
|
| Inductor | Low reactance (passes) |
High reactance (blocks)
|
Impedance – The Complete Opposition to Current
What It Is:
Impedance (Z) is the total opposition a circuit offers to AC. It combines:
Resistance (R) – dissipates energy as heat
Reactance (X) – stores energy in fields temporarily
Where j is the imaginary unit (don’t worry—visualize it as a direction, not a number).
Practical Form:
Think of impedance as a vector — it has both magnitude and direction (resistive vs reactive).
Why Impedance Matters in Ham Radio:
Transmitters expect a specific impedance (typically 50 Ω). If the antenna system isn’t matched to this, reflected power occurs → inefficiency and possibly damage.
Coax cable is designed for a particular impedance; mismatches cause standing waves and loss.
Impedance matching ensures maximum power transfer — vital for DXing and QRP.
Resonance – Where Reactance Cancels and Power Flows
What It Is:
Resonance occurs when capacitive and inductive reactances are equal and opposite:
Resistance (R) – dissipates energy as heat
Reactance (X) – stores energy in fields temporarily
Where j is the imaginary unit (don’t worry—visualize it as a direction, not a number).
Practical Form:
Then:
In other words, the circuit behaves as if there’s no reactance at all — pure resistance, and energy flows most efficiently.
Resonant Frequency Formula (LC circuit):
Where:
L = inductance (H)
C = capacitance (F)
What Happens at Resonance?
Series circuit: Impedance is minimum → current is maximum
Parallel circuit: Impedance is maximum → voltage is maximum
At resonance:
No power is reflected
Energy flows cleanly
Signals pass without attenuation
Tuning circuits hit their sweet spot
Real-World Ham Radio Applications
|
Component/System |
Concept Involved |
Description
|
|---|---|---|
| Antenna Resonance | Resonance & impedance |
An antenna must be resonant at your transmission frequency to efficiently radiate.
|
| Antenna Tuners (ATUs) | Reactance & impedance |
Match the reactive load of the antenna to the resistive load your radio expects (50 Ω).
|
| SWR meters | Impedance mismatch detection |
High SWR means impedance isn’t matched; power is bouncing back.
|
| Band-pass filters | Resonance |
Allow only desired frequencies through (e.g., 7 MHz for 40m).
|
| Traps in antennas | Resonant reactance cancellation |
Act like blocks at certain frequencies, enabling multiband performance.
|
Visualizing the Concepts
Imagine impedance like terrain:
Pure resistance = flat ground
Capacitive reactance = uphill
Inductive reactance = downhill
Resonance = the point where the road levels out again — perfect energy flow
Summary Table
|
Concept |
Definition |
Key Behavior
|
Importance in Ham Radio
|
|---|---|---|---|
| Reactance | Frequency-dependent opposition from C or L |
Opposes AC without wasting power
|
Shapes filters, affects tuning
|
| Impedance | Total opposition (R + X) |
Determines current flow in AC
|
Needs to match for max power transfer
|
| Resonance | X_L = X_C (reactance cancels out) |
Pure resistance, max efficiency
|
Foundation of antenna and circuit design
|
Optional Add-ons
Would you like:
A Smith chart explainer (for visualizing impedance changes)?
An animated circuit showing resonance forming between L and C?
A resonance curve showing amplitude response across frequencies?