Modulation Manual
What Is Bandwidth?
In radio terms, bandwidth refers to the range of frequencies a signal occupies. It's measured in hertz (Hz), kilohertz (kHz), or megahertz (MHz).
Think of the radio spectrum as a highway, and each transmission as a vehicle:
A narrow signal is like a motorcycle — small footprint, easy to fit
A wide signal is like a truck with a trailer — takes up more space
If two vehicles (signals) get too close, they can collide — this is interference
Bandwidth by Mode (Quick Reference)
|
Modulation Type |
Name |
What It Modifies |
|---|---|---|
| CW (Morse Code) | ~150 Hz | Extremely narrow; fits nearly anywhere |
| SSB (voice) | ~2.4 kHz | Narrow and efficient voice mode |
| AM (voice) | ~6 kHz | Double sidebands + carrier = wider |
| FM (voice) | ~10-15 kHz | Very clear audio, but wide |
| RTTY | ~170-300 Hz | Narrow; uses frequency shift |
| PSK31 | ~31 Hz | Ultranarrow, ideal for low-power text |
| FT8 | ~50 Hz | Incredibly narrow, time-synced bursts |
| Digital voice (DMR, Fusion) | ~6.25-12.5 kHz | Efficient but varies with system |
Why Narrow Is Powerful
Narrow modes (like CW, PSK31, FT8) are more efficient and resistant to noise. Here's why:
Less noise: Narrow bandwidth = less random interference picked up
Lower power needed: Energy is concentrated in a small slice of spectrum
More signals per band: Narrow signals let more hams share the space
Example: In a 3 kHz slice of spectrum, you could fit:
1 SSB conversation, or
100 FT8 transmissions!
Why Is SSB Considered Efficient?
Single Sideband (SSB) strips away:
The carrier (constant tone)
One of the sidebands (mirrored audio information)
This leaves just the useful voice data, reducing the bandwidth to ~2.4 kHz.
Less power and less spectrum used, while still intelligible voice.
Interference & Adjacent Channel Impact
If two signals are too close in frequency, they can bleed into each other, causing:
QRM (man-made interference)
Splatter (distorted signals spilling out)
"Adjacent-channel" interference (like hearing a neighbor’s radio through the wall)
Wide signals, like FM or improperly tuned AM, can ruin space for others nearby.
That’s why we care about:
Good filters (in both radios and antennas)
Clean signal practices
Choosing narrow modes in crowded bands
Real-World Examples
During a contest, hams switch to CW or FT8 because hundreds can operate without overlapping
In emergencies, narrow digital modes get crucial info through even when voice fails
On VHF/UHF, repeaters may allocate strict 15 kHz channels — so FM fits, but not AM
Analogies to Visualize It
Guitar strings: Thin strings (narrow bandwidth) produce higher, focused notes; thick strings (wider bandwidth) rumble deeper, broader tones.
Painting: A fine brush (narrow mode) draws clean lines; a wide brush (broad mode) covers more area but can smudge neighboring colors.
Wi-Fi routers: Ever had two on the same channel? That’s what happens when signals overlap — dropped packets, lag, noise.
Summary
|
Narrow Bandwidth |
Wide Bandwidth |
|---|---|
| Low power, efficient | Clearer audio, more data |
| Many users can coexist | Needs more spectrum |
| Great in noisy conditions | May bleed into neighbors |
| Modes: FT8, CW, PSK31 | Modes: FM, AM, SSTV |