6.18.5 Why Is Single-Sideband (SSB) Used for HF Radio?
- What Is Single-Sideband Modulation?
- Why Was SSB Developed?
- How Does SSB Differ from Conventional AM?
- Why Does One Sideband Contain All the Information?
- How Much Bandwidth Does SSB Save?
- Why Is Bandwidth So Important?
- How Much Power Does SSB Save?
- Why Is This Important for HF Communications?
- What Is Carrier Suppression?
- Why Do SSB Receivers Need Fine Tuning?
- What Is a Beat Frequency Oscillator?
- What Is the Difference Between USB and LSB?
- Why Are Both USB and LSB Used?
- Is SSB More Difficult to Generate?
- Does SSB Have Any Disadvantages?
- Is SSB Used for Broadcasting?
- Is SSB Still Important Today?
- Has SSB Influenced Modern Digital Communications?
- Why Is Understanding SSB Important?
Description
Understand why HF radio systems almost universally use single-sideband modulation. Learn how suppressing the carrier and one sideband dramatically improves both bandwidth and power efficiency while preserving the transmitted information.
Introduction
Conventional amplitude modulation (AM) was one of the earliest practical methods of transmitting speech by radio. It was simple, reliable, and easy to receive, making it the dominant modulation technique for broadcasting throughout much of the twentieth century. However, engineers soon recognised that AM possessed a significant weakness: much of the transmitted power and bandwidth was wasted.
As discussed in the previous FAQ, an AM signal consists of three components:
- the carrier;
- the upper sideband; and
- the lower sideband.
Although the two sidebands each contain the complete transmitted information, the carrier itself contains none. Nevertheless, the carrier typically consumes most of the transmitter's power. Furthermore, transmitting both sidebands duplicates the same information and doubles the required bandwidth.
Single-aideband (SSB) modulation was developed to eliminate this inefficiency. By suppressing the carrier and transmitting only one sideband, SSB greatly reduces both power consumption and bandwidth while preserving all of the transmitted information. These advantages have made SSB the preferred voice modulation technique for most high-frequency (HF) communication systems.
Today, SSB is used extensively by maritime operators, amateur radio operators, military organizations, long-distance aeronautical services, emergency communication networks, and many commercial HF communication systems.
What Is Single-Sideband Modulation?
Single-Sideband modulation is a form of amplitude modulation in which:
- the carrier is suppressed; and
- only one of the two sidebands is transmitted.
The transmitted sideband may be either:
- the upper sideband (USB); or
- the lower sideband (LSB).
Since either sideband contains the complete information, transmitting both is unnecessary.
SSB therefore achieves the same communication objective while using considerably less bandwidth and power than conventional AM.
Why Was SSB Developed?
The development of SSB was driven by efficiency.
Engineers recognised two important facts about conventional AM. First, the carrier consumes a large proportion of the transmitter power but carries no information. Second, the upper and lower sidebands are mirror images of one another. Each contains exactly the same information.
By eliminating the carrier and one sideband, transmitter efficiency increases dramatically without reducing the information transmitted.
How Does SSB Differ from Conventional AM?
A conventional AM signal contains:
- one carrier;
- one upper sideband; and
- one lower sideband.
An SSB signal contains one sideband only. The receiver recreates the missing carrier internally before demodulating the signal.
Although this makes the receiver slightly more complex, the improvements in efficiency are substantial.
Why Does One Sideband Contain All the Information?
When a carrier is modulated by a baseband signal, mathematical analysis shows that identical information appears in both sidebands.
The upper sideband contains every frequency component shifted above the carrier. The lower sideband contains exactly the same information shifted below the carrier. Removing either sideband therefore does not remove any information.
The remaining sideband alone is sufficient for complete communication.
How Much Bandwidth Does SSB Save?
Conventional AM requires a bandwidth equal to twice the highest modulating frequency.
For example, if speech occupies frequencies up to 3 kHz, an AM signal requires approximately 6 kHz. An SSB signal requires only 3 kHz. Thus, SSB reduces bandwidth requirements by approximately fifty percent.
This allows many more communication channels to occupy the same section of radio spectrum.
Why Is Bandwidth So Important?
Radio spectrum is a limited natural resource.
Only a finite range of frequencies is available for communication services. Reducing the bandwidth occupied by each transmission allows:
- more users;
- more channels; and
- greater spectrum efficiency.
These advantages become particularly important in crowded HF frequency bands where many users must share limited spectrum.
How Much Power Does SSB Save?
The power savings are even more significant than the bandwidth savings.
In conventional AM:
- most transmitter power is devoted to the carrier; and
- the remaining power is divided between the two sidebands.
Because the carrier contains no information, much of this transmitted power contributes nothing to the communication. In an SSB transmitter, nearly all the transmitted power contributes directly to carrying useful information.
Consequently, an SSB transmitter can often achieve similar communication performance using only a fraction of the power required by an equivalent AM transmitter.
Why Is This Important for HF Communications?
High-frequency communication often involves very long transmission paths.
Signals may travel thousands of kilometres by ionospheric reflection. Received signal strengths are frequently weak. Improving transmitter efficiency therefore provides several important advantages:
- increased communication range;
- reduced transmitter power;
- improved battery life for portable equipment; and
- better signal quality.
These benefits explain why SSB became the dominant form of HF voice communication.
What Is Carrier Suppression?
Unlike conventional AM, SSB transmitters normally do not radiate a carrier.
Instead, the receiver recreates a local carrier during demodulation. This process is called carrier reinsertion.
Although it increases receiver complexity, the improvement in transmitter efficiency more than compensates.
Why Do SSB Receivers Need Fine Tuning?
Because the transmitted carrier is absent, the receiver must generate its own carrier at precisely the correct frequency.
Even small tuning errors affect the recovered speech. If the receiver is tuned slightly too high or too low:
- voices sound unnatural;
- speech pitch changes; and
- intelligibility may suffer.
Experienced HF operators therefore tune carefully for natural-sounding speech.
Modern digital frequency synthesizers have made accurate tuning much easier than in earlier generations of radio equipment.
What Is a Beat Frequency Oscillator?
The locally generated carrier used by an SSB receiver is often called a Beat Frequency Oscillator (BFO).
The BFO supplies the missing carrier required for demodulation. When adjusted correctly, the recovered speech sounds perfectly natural.
If incorrectly adjusted, voices become noticeably higher or lower in pitch.
What Is the Difference Between USB and LSB?
Both sidebands contain identical information.
The difference lies only in which side of the suppressed carrier is transmitted. By convention:
- The upper sideband (USB) occupies frequencies above the suppressed carrier.
- The lower sideband (LSB) occupies frequencies below it.
The choice is primarily one of operating convention rather than technical necessity.
Why Are Both USB and LSB Used?
Historically, different communication services adopted different conventions.
For example:
- commercial and maritime HF services traditionally used USB;
- amateur radio operators commonly use LSB below 10 MHz and USB above 10 MHz; and
- many military systems use USB.
These conventions simplify interoperability between stations using the same frequency bands.
Is SSB More Difficult to Generate?
Yes.
Conventional AM can be produced using relatively simple circuits. Generating SSB requires additional processing to remove both the carrier and the unwanted sideband. Historically this was achieved using:
- highly selective filters;
- phasing networks.
Modern transmitters usually generate SSB digitally using digital signal processing techniques.
These methods provide excellent performance with much greater flexibility.
Does SSB Have Any Disadvantages?
Although highly efficient, SSB is not without limitations.
Compared with conventional AM:
- receivers are more complex;
- accurate tuning is required; and
- operation may be less intuitive for inexperienced users.
For these reasons, AM remains attractive for some broadcasting applications where receiver simplicity is particularly important.
Is SSB Used for Broadcasting?
Although most international short-wave broadcasters historically employed conventional AM, some specialised broadcasting services have used SSB to reduce operating costs.
However, because ordinary AM receivers cannot demodulate SSB correctly, conventional AM remained the preferred choice for mass-market broadcasting.
The widespread adoption of digital broadcasting has reduced the importance of this distinction in recent years.
Is SSB Still Important Today?
Very much so.
Despite the growth of digital communications, SSB remains the standard voice modulation technique for many HF services.
Applications include:
- amateur radio;
- maritime communications;
- aeronautical HF communications;
- military radio;
- emergency communications; and
- long-distance commercial radio networks.
Its combination of excellent power efficiency, narrow bandwidth, and long-range capability continues to make it well suited to HF propagation.
Has SSB Influenced Modern Digital Communications?
Although many modern systems employ digital modulation, the engineering philosophy behind SSB remains highly relevant.
One of the recurring themes in communications engineering is improving efficiency by eliminating unnecessary transmission. SSB demonstrated that careful analysis of a signal could significantly reduce both power and bandwidth without losing information.
The same principle continues to guide the design of modern digital modulation schemes and compression algorithms.
Why Is Understanding SSB Important?
Single-Sideband modulation illustrates how engineering innovation often results from recognising and eliminating unnecessary redundancy. By transmitting only the information-bearing portion of an AM signal, SSB achieves major improvements in efficiency while preserving communication quality.
Understanding SSB also provides valuable insight into bandwidth conservation, power efficiency, receiver design, and spectrum management—concepts that remain central to modern communications engineering.
Summary
Single-Sideband modulation improves upon conventional AM by suppressing the carrier and transmitting only one sideband. Because either sideband contains the complete information, this approach halves the required bandwidth while greatly improving transmitter power efficiency.
These advantages have made SSB the preferred voice modulation technique for most HF communication systems, including maritime, aeronautical, military, amateur, and emergency communications. Although more sophisticated digital modulation techniques are now common, SSB remains one of the most elegant and practical examples of efficient analog communication.
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