Understanding Single Sideband (SSB)
A new Technician Class operator is likely to get started in ham radio with VHF and UHF phone operations using FM simplex channels and repeaters. The channelized world of VHF/UHF FM offers relative simplicity of operations and is a great way to get on the air immediately upon earning the Technician Class license. However, after mastering repeaters and gaining comfort with on-air FM phone QSOs, the next step for many hams is the more challenging domain of single sideband (SSB) phone operations.
Single sideband phone ops offer a broader range of radio contact opportunities, including long distance and international communications. Generally, SSB signals tend to propagate greater distances and exhibit more graceful degradation over distance than FM signals. Single sideband phone may be used on the VHF and UHF bands available to the Technician Class licensee, on the 10-meter band phone segment available to Technicians (28.3 to 28.5 MHz), and on all HF phone sub-bands available to higher license classes. Single sideband is the predominant phone mode used for over-the-horizon skip propagation via the ionosphere. This article takes a closer look at the basics of SSB phone mode for better understanding of its complexities and operating nuances relative to FM channelized ops.
Take a look at our Technician Learning - Section 1.1 media list and listen to the "Mode Characteristics" audio to hear the differences between FM and SSB signals, including signal degradation, noise, and audio quality. In the same section, watch the video "Operations with SSB Mode" to see the operating differences in tuning and signal characteristics with SSB.
What is SSB? Single sideband is a special form of amplitude modulation (AM). What’s so ‘special’ about it? Besides just encoding voice information with variations in signal amplitude, or power, SSB consumes a little less than half the bandwidth of a full “double band” AM signal. Let’s unfold that last statement for the uninitiated new ham.
First, some bandwidth basics: A radio signal is comprised of a range of transmitted frequencies. When an operator tunes up a specific frequency on a transceiver, that displayed frequency value is the carrier frequency. The carrier may be thought of as a reference position for a small, contiguous band of spectrum (frequency range) that will all be transmitted simultaneously when the push-to-talk button is depressed and some voice audio is provided to the microphone. So, a transmitter does not emit only that singular tuned carrier frequency, but rather it emits an entire little band of frequencies near the carrier value that is used to encode the information of all the various audio frequencies of a voice. The extent of this little transmitted band of signals will vary with different types of modulation, or modes, and we refer to the extent or total range of frequencies emitted as the signal’s bandwidth, in units of hertz.
Consider this bar chart comparison of the bandwidth consumed by the signals of common operating modes, including SSB. Notice that FM consumes the widest band of frequencies and is variable from roughly 10 kHz to 15 kHz. (The FM bandwidth varies with the power or ‘loudness’ of the voice audio provided.) Although not a phone mode, CW has the narrowest bandwidth since it must produce only a simple tone and not a wide range of audio frequencies to represent a human voice.
The AM signal is about 6 kHz wide, and if we examine it in more detail we will find that it is actually comprised of two bands, one on each side of the carrier frequency, and they are ‘mirror imaged’ redundant bands or “sidebands.” That is, a complete voice signal is carried by each of the two sidebands comprising the AM signal. Additionally, the AM signal includes transmission of the carrier frequency itself. While this redundant double band AM signal provides robust and high quality audio, it consumes a relatively wide band of spectrum.
As the name implies, single sideband mode utilizes only one of the two AM sidebands and also suppresses the carrier frequency in transmission. So, the SSB signal is just under one-half the bandwidth of the double sideband conventional AM signal. The narrower bandwidth of SSB has a couple of important implications: 1) The SSB signal consumes less of the available spectrum within an amateur band, thereby allowing more signals simultaneously on the band without interference; and 2) The power of a transmission is more densely applied in the narrower band, providing a higher average effective power across the transmitted band, and thereby giving the SSB signal more ‘punch’ than a comparably powered FM or AM signal in which the power is spread across a much broader range of frequencies.
Take a look at our Technician Learning - Section 1.1 media list and see the video "SSB Bandwidth Advantage" for a visual illustration of the two advantages listed above.
You may now be asking, “Which sideband is used with SSB mode?” The convention used by hams is that bands above the 30-meter band (frequencies higher than 10 MHz), including all VHF and UHF bands, use the upper sideband (USB) – the band of frequencies adjacent to, and higher than, the carrier frequency. For bands below 30-meters (frequencies lower than 10 MHz), the lower sideband is used. [The 30-meter band is a digital modes-only band where SSB is not used, and another exception occurs in the 60-meter band (5.3 MHz) where only five USB channels are allowed.]
The trade-off with SSB as compared to conventional double-sideband AM and especially to FM phone mode is the quality of the audio. Narrower bandwidth dictates a reduction in audio information carried by the SSB signal. As a result, SSB audio will sound a bit thinner and less rich, but it is still quite intelligible and more than sufficient for weak signal phone communications.
Weak Signals: Signals that are transmitted great distances, such as those refracted by the ionosphere over the horizon and back to earth hundreds or thousands of miles distant, become very weak in comparison to the initial output power at the transmitting antenna. As radio waves expand their power is distributed over a greater volume of space, reducing the effective power at a distant receiving station. Further, signal polarization essentially becomes randomized during transit through the ionosphere and earth's magnetic field, further reducing the signal’s ability to induce currents on a distant receiving antenna at which the polarization is unlikely to be matched. These so-called ‘weak signal’ operations benefit from the relatively high power density of the SSB signal noted above.
Even signals that do not utilize ionospheric skip can benefit from the extended reach offered by SSB, such as VHF or UHF signals transmitted across a more local region. It is common for local VHF/UHF SSB signals to be viable well over 100 miles, depending upon specific terrain features and polarization. For non-skip SSB, operators use horizontal polarization with antenna elements parallel to the surface of the earth.
Take a look at our Technician Learning - Chapter 3 and Chapter 5 lists to learn more about using SSB on VHF and UHF bands in the two articles: "SSB on 2-meters" and "VHF Multi-mode Transceiver."
No SSB Channelization: Unlike FM phone operations, SSB tuning does not use predefined channels. Rather, tuning across a phone sub-band is contiguous. That means that an operator may select any carrier frequency desired across the extent of the sub-band and transmit and receive signals on the 3 kHz bandwidth adjacent to the carrier frequency. The typical receiver will normally employ a SSB receive band of the standard 3 kHz SSB bandwidth, demodulating whatever signals are received within that 3 kHz band into audio. In order to receive a signal properly, the receiver’s receive band must be aligned across the frequencies with the precise band position of the signal being received. Otherwise, if there is misalignment between the receive band position and the signal position, the signal will sound distorted and may be unintelligible. Audio distortion of a received signal is clearly heard as it is gradually ‘tuned in’ to correct registration with the receive band, as you will see and hear in the Technician Learning video "Operations with SSB Mode" recommended near the beginning of this article.
Further, since no predefined channels exist across the band to keep different stations’ signals properly separated from one another as with FM ops, it is easy for signals to ‘overlap’ in the same spectrum space and cause interference. The receiving station will demodulate any and all signals within its receive band of 3 kHz, and sometimes that will be multiple different signals. Some portion of each signal will be heard in the receive audio, with various degrees of distortion and interference to the desired signal. A narrower receive bandwidth may be used by implementing receive filters, helping to reduce any such interference from nearby signals on the band. Receive filters can be implemented to attenuate undesired signals or noise in nearby portions of the band so that the desired signal is more easily demodulated and heard by the operator.
Take a look at our Technician Learning - Section 6.2 lists to learn more about using receive filters with SSB mode in our article, "SSB Receive Filters."
These are some of the challenges of SSB operations, carefully tuning in a contiguous manner rather than readily snapping through predefined channels, and cleverly implementing various filtering techniques to isolate and select only the single desired signal out of possibly several near the band location in which are operating. It requires a bit more attention to detail to make sure that your signal remains within proper portions of the bands for SSB modes, and that you keep within sub-bands compliant with your license privileges. Good amateur practice also requires that you be considerate on the air, taking care to ensure adequate unused band near your selected carrier frequency before you transmit in order to minimize interference with others. And when interference does occur, be polite and willing to move along to another location on the band for your operations.
-- Stu WØSTU