Before a radio wave ever leaves your antenna, it starts as a tiny electrical signal inside your radio's circuitry. To truly understand radio — whether building a transmitter, tuning a filter, or diagnosing distortion — you need to grasp how signals behave as they flow through resistors, capacitors, inductors, and active devices.

What Is a Signal?

A signal is any time-varying voltage or current that carries information — such as voice, Morse code, or digital data. These signals travel through circuits just like water flows through pipes — but with more complexity due to frequency, phase, and interference.

Analog signals are smooth and continuous (e.g., voice).
Digital signals are stepped or square (e.g., binary 0s and 1s).

Signals as Waves

Even within circuits, electrical signals are waves — often sinusoidal (sine waves), especially in RF.

A signal has:

Amplitude – its strength (voltage or current)
Frequency – how often it cycles per second
Phase – its position in time relative to other waves
Waveform – its shape (sine, square, triangle, etc.)

These properties can change as the signal passes through various components.

How Signals Interact with Basic Components

Component	Effect on Signal
Resistor	Reduces amplitude (voltage drop), but no shape change
Capacitor	Blocks DC, passes AC (more easily at high frequencies)
Inductor	Blocks AC at high frequencies, passes low-frequency signals
Diode	Clips or rectifies (cuts part of the wave)
Transistor	Amplifies or switches signals

Capacitors and inductors affect signal timing (phase and delay), not just strength.

Signal Attenuation and Amplification

Attenuation = signal gets weaker (resistors, filters, long cables)
Amplification = signal gets stronger (transistors, op-amps)

In RF:

A weak received signal needs to be amplified without adding noise.
A transmitted signal must be boosted to overcome antenna losses and distance.

Too much amplification → distortion
Too little → buried in noise

Signal Reflection and Transmission

When a signal encounters a mismatch in impedance, part of it reflects:

At an open or short circuit: total reflection
At a properly matched load: no reflection

This leads to:

Standing waves (seen in high SWR)
Loss of power or distortion
Feedback or oscillation in poorly isolated circuits

Signal Phase and Delay

Phase is a signal’s position in its cycle compared to another. Capacitors and inductors cause:

Phase shifts (capacitors advance, inductors delay)
Filters to shape a signal’s frequency response
Interference or reinforcement when signals combine

Phase is crucial in:

SSB modulation
Antenna design
Filter networks
Mixing and demodulating signals

Signal Mixing, Filtering, and Shaping

Circuits often change signals deliberately:

Mixing: Combine two signals to get new frequencies (e.g., LO + RF = IF)
Filtering: Remove unwanted frequencies (e.g., bandpass or low-pass)
Shaping: Modify the waveform (e.g., squaring a sine wave in digital modes)

Example:

In an SSB transmitter:

Microphone signal → audio amp (amplified)
Audio mixed with oscillator (modulated)
High-pass filters remove lower sideband (filtered)
Signal amplified again and sent to the antenna

Signal Noise and Distortion

Noise = random unwanted signals that interfere

Distortion = waveform altered (clipping, harmonics, compression)

Causes:

Improper gain settings
Power supply instability
Nonlinear components (intentional or accidental)

Good circuit design:

Minimizes noise
Keeps signals in their “linear range”
Filters out unwanted artifacts

Common Real-World Signal Issues

Symptom	Likely Cause
Hum on audio	AC ripple from power supply
Clipping or harsh audio	Overdriven amplifier
Signal drift	Oscillator instability
Distortion on SSB	Poor linearity or incorrect mic gain
High SWR	Signal reflected due to impedance mismatch

Summary Table

Concept	Description
Signal	Time-varying voltage or current that carries data
Attenuation	Signal getting weaker
Amplification	Signal getting stronger
Filtering	Selecting or removing frequency components
Phase shift	Delay or lead relative to another waveform
Reflection	Signal bouncing back due to mismatch
Distortion	Signal shape is altered (not desirable)

Optional Visuals or Add-ons

Would you like:

A dynamic animation of signal attenuation and amplification through a circuit?
A schematic walkthrough showing how a signal flows through a transmitter?
An interactive phase diagram?

Lesson 11: Signal Behavior in Circuits