Currents & Concepts
If voltage, current, and resistance are the backbone of DC circuits, capacitance and inductance are the beating heart of AC and RF circuits. They store energy, shape signals, and define how systems respond to changing voltages and currents.
Without capacitors and inductors:
Radios couldn’t tune
Antennas couldn’t resonate
Filters wouldn’t exist
Oscillators and amplifiers wouldn’t work
Let’s break it down.
Capacitance – Storing Energy in an Electric Field
Definition:
A capacitor stores electrical energy in the form of an electric field between two conductive plates separated by an insulator (dielectric).
Units:
Farad (F) — typically microfarads (μF), nanofarads (nF), or picofarads (pF) in practice.
What It Does:
Resists changes in voltage by temporarily storing and releasing energy.
In DC: Acts like an open circuit once charged.
In AC/RF: Allows higher-frequency signals to pass more easily (blocks DC, passes AC).
Key Formula:
Q=C×V
Where:
Q= charge stored (coulombs)
C = capacitance (farads)
V = voltage across plates
Behavior:
Charging: Voltage builds up slowly (RC time constant).
Discharging: Voltage drops off as stored energy is released.
Ham Radio Applications:
Filter circuits: Block or pass certain frequencies.
Tuning circuits: In antenna tuners, capacitors adjust resonance.
DC blocking: Prevent DC from entering sensitive amplifier stages.
Bypass capacitors: Route noise to ground in power supply lines.
Inductance – Storing Energy in a Magnetic Field
Definition:
An inductor stores energy in a magnetic field created by current flowing through a coil of wire.
Units:
Henry (H) — usually measured in mH or μH for radio applications.
What It Does:
Resists changes in current by generating a counter-voltage (self-induced EMF).
In DC: Acts like a short circuit after current stabilizes.
In AC/RF: Opposes higher-frequency currents (passes DC, blocks AC depending on frequency).
Key Formula:
V=L(dI/dt)
Where:
V = voltage across the inductor
L = inductance
dI/dt = rate of current change
Behavior:
Increasing current causes voltage opposition.
Decreasing current causes voltage to push forward (tries to keep current flowing).
Ham Radio Applications:
Chokes: Block RF in power lines (used in baluns).
Filters: Combine with capacitors to allow or reject frequency ranges.
Tuned circuits: Set the resonant frequency in transmitters and receivers.
Matching networks: Help antennas match transmitter output impedance.
Capacitors vs. Inductors — A Tale of Opposites
|
Property |
Capacitor |
Inductor |
|
|---|---|---|---|
| Stores energy in | Electric field |
Magnetic field
|
|
| Reacts to | Changes in voltage |
Changes in current
|
|
| In DC circuits | Acts as open circuit after charging |
Acts as short circuit after settling
|
|
| In AC circuits | Allows high freq, blocks low (HPF) |
Blocks high freq, passes low (LPF)
|
|
| Key component in | Tuning, filtering, bypassing |
Chokes, tuning, impedance matching
|
Capacitance & Inductance in Tuning and Resonance
When a capacitor and inductor are placed together in a circuit — usually in parallel or series — they can resonate at a specific frequency:
f = 1 / 2π√LC
This forms the basis of:
LC filters
Antenna tuning units (ATUs)
Oscillators in transmitters and signal generators
Crystal filters and bandpass selectors in receivers
This is literally how your radio "tunes in" to a station.
Real-World Ham Radio Examples
|
Use Case |
Component Type |
Function |
|---|---|---|
| Antenna tuner (manual) | Capacitor & Inductor |
Matches transmitter impedance to antenna for efficient power
|
| Bandpass filter | Both |
Removes unwanted signals outside a target frequency range
|
| RF choke | Inductor |
Prevents RF from riding along shielded cables (coax outer)
|
| Voltage regulation | Capacitor |
Smooths power supply output; absorbs spikes
|
Summary
Capacitors store and release energy based on voltage changes.
Inductors store and release energy based on current changes.
Together, they filter, shape, and control the flow of AC and RF signals.
These elements form the heart of radio signal control, tuning, and filtering.
Optional Visual Add-ons
Would you like:
A resonance diagram showing energy oscillating between L and C?
A comparison chart of AC behavior across frequencies?
A circuit animation of a low-pass and high-pass filter at work?