USB vs LSB: Which Sideband to Use on Each Ham Band and Why
Tune across the 40 metre phone band and every voice sounds natural on lower sideband. Move to 20 metres and the same radio needs upper sideband. Switch the wrong way and speech turns into a squeaking, unintelligible warble. The rule that decides which to use is simple and universal, but newcomers rarely get an explanation of why it exists or of the places where it does not apply. This guide covers USB vs LSB: what the two sidebands are, the convention for each band, the exceptions, and the practical consequences for band edges, digital modes and satellite work.
What a Sideband Is
When an audio signal modulates a radio carrier, the result has two mirror-image copies of the audio, one above the carrier frequency and one below. These are the upper and lower sidebands. Both carry the same information, and a conventional amplitude-modulated signal sends both, plus the carrier itself, which wastes power and bandwidth. As the general article on single-sideband modulation explains, an SSB transmitter filters out one sideband and, in the usual form, suppresses the carrier. What remains uses roughly half the bandwidth and puts the power into the part of the signal that carries the voice.
The wanted sideband is either the one above the carrier frequency, the upper sideband (USB), or the one below it, the lower sideband (LSB). Take a simple example. A 2 kHz audio tone applied to a 5 MHz carrier produces a signal at 5.002 MHz in USB and 4.998 MHz in LSB. In USB, rising audio pitch moves the radio frequency up, and in LSB, rising audio pitch moves it down, which is why the signal is described as inverted.
To demodulate it, the receiver must put the carrier back at exactly the right place in the spectrum. A radio set to the wrong sideband places it on the wrong side, and the audio comes out inverted: the lowest voice frequencies become the highest and the speech is no longer recognisable. That is the squeaking you hear when you choose the wrong mode.
The Convention: LSB Below 10 MHz, USB Above
For voice operation on the amateur bands, the common practice is:
- Lower sideband below 10 MHz. That covers 160, 80 and 40 metres, including the 75 metre phone area.
- Upper sideband at 10 MHz and above. That covers 20, 17, 15, 12 and 10 metres, and 6 metres and the bands above it where SSB is used.
The same description appears in the general reference on SSB, which also gives examples: voice contacts around 7.100 MHz on 40 metres use LSB, and contacts around 14.200 MHz on 20 metres use USB. It is a convention, not a law, and there is no technical reason why one sideband must be used above 10 MHz and the other below. It grew up in the early days of SSB, and amateurs standardised on it so that every station could talk to every other without arranging a sideband in advance.
The value of a convention is that everyone follows it. A station that insists on the other sideband on a busy band simply cannot be understood by the majority. If you hear distorted speech in the middle of a normal QSO, the fix is almost always to change your own mode.
Where the Rule Does Not Apply
| Situation | Sideband | Why |
|---|---|---|
| 60 metre channels | USB | An exception to the 10 MHz rule; in the United States the rules specify USB for the five channels, and USB is the accepted practice elsewhere on this band |
| Digital modes such as FT8, FT4 and RTTY by sound card | USB, regardless of band | Programs assume an upper-sideband audio-to-frequency relationship; the radio’s data mode usually selects USB |
| CW | Either; many radios have a reverse CW setting | The audio pitch is chosen by the receiver, not by the signal; the choice affects which side interference falls on |
| Linear satellite transponders | Depends on the transponder; often an inverting one | The satellite flips the spectrum, so uplink and downlink sidebands are opposite |
| VHF and UHF SSB | USB | The weak-signal community at 50 MHz and above uses USB by convention |
The 60 metre exception is the one most often forgotten. On the five discrete channels in the United States the FCC rules specifically require USB, as the SSB reference notes, and elsewhere the same sideband is commonly used on that band. See a dedicated band guide for the rules of your own country, because the allocations and channel arrangements differ.
Digital Modes Use USB
When you operate FT8 or most other sound-card digital modes, the radio goes in USB, usually through a data mode that bypasses microphone processing. Many radios call this USB-D, DATA-U or something similar. The software generates tones between roughly 200 and 3000 Hz, and in USB those audio tones map directly to frequency offsets above the dial frequency. When the software says the frequency of an FT8 signal is 14.074 MHz, it means the dial frequency, and the tones sit above it.
This matters for two reasons. First, there is no reason to switch to LSB on 40 or 80 metres just because voice would use it there. A station on 40 metre FT8 uses USB like everyone else. Second, if you use a radio with a separate data mode, check that the mode is the upper-sideband variety and not the lower one. If your decodes suddenly vanish, a mode that quietly changed to LSB is a common cause. Our guide to a WSJT-X and FT8 setup that works first time lists the radio settings that must match, and the article on digital modes beyond FT8 covers the others.
Where Your Signal Sits: Band Edges
The sideband you use decides which side of the dial frequency your signal occupies, and that matters at the edges of a band or sub-band.
- In USB, your signal extends upward from the dial frequency, so a typical voice signal occupies the dial frequency plus about 300 Hz to 2.7 kHz or so.
- In LSB, your signal extends downward, so it occupies the dial frequency minus about 300 Hz to 2.7 kHz or so.
The consequence is that at the lower edge of a phone segment you must not use LSB at the very edge, or your signal will fall outside it. Keep the dial frequency at least the width of your signal above the lower limit. At the upper edge of a segment the reverse applies for USB, and the dial frequency has to stay a few kilohertz below the limit. Remember that the dial reading shows the suppressed carrier frequency, not the edge of your signal. A good rule is to stay at least 3 kHz inside any limit. Our article on the IARU band plan explains the segments in each region, and your national regulator publishes the legal limits.
Satellites: When the Sideband Flips
Linear satellite transponders retransmit a whole slice of spectrum, and many of them invert it. If the satellite inverts, a signal you send on upper sideband comes back on lower sideband, and the other way round. That is why satellite operators often send on one sideband and receive on the other. Some transponders do not invert, in which case both are the same. Look up the details for each satellite before you use it, and see our guide to getting started with amateur radio satellites for the practical routine, including Doppler shift.
Sideband and Interference
When two stations are close together, the choice of sideband can make a difference to what you hear. An unwanted signal on one side of your frequency lands in the audio passband, while a signal on the other side is rejected by the filter. In SSB, switching the sideband on the receiver will not help, because you must use the correct one to understand the speech. In CW, however, you can reverse the sideband on many radios, and moving the beat note to the other side of the carrier can move an interfering signal out of the way or into a different place in the filter. That is the idea behind reverse CW and the narrow filter shapes available on modern receivers. A display that shows the spectrum helps you decide, and our article on the panadapter and spectrum scope shows what to look for.
Tuning an SSB Signal Correctly
Because an SSB signal has no carrier to tune to, finding the right frequency takes a little practice. A voice that sounds too low and rumbling is tuned too far one way, a voice that sounds thin and high is tuned too far the other. Tune slowly with the receiver in the correct sideband until the voice sounds natural, and then leave it. Small errors of a few tens of hertz are tolerable for speech, but larger ones make a voice sound like a cartoon character, and the same error is more troublesome when the other station is on a different mode. In a contest or a pileup, do not keep retuning in the middle of an over. Set the frequency once, and use the receiver’s RIT or clarifier control for fine adjustment while leaving the transmit frequency alone.
A Quick Reference by Band
For voice on the common bands, the habit is as follows. On 160, 80 and 40 metres, use LSB. On 30 metres, voice is not normally used and the band is for CW and digital modes. On 20, 17, 15, 12 and 10 metres, use USB. On 6 metres and above, use USB for SSB. On 60 metres, use USB. For digital modes on any band, use USB through the radio’s data mode. Always check the band plan of your own region, because some countries limit phone to certain segments or certain bands, and operators in some regions may find that a particular band has its own local custom.
If you are ever in doubt, listen for a minute. Other stations on the frequency are the best guide to the convention in use, and on a busy band you will quickly hear whether the voices are natural. If they are, you are on the right sideband.
Practical Tips
- Check the mode when you change band. A radio that remembers a mode per band will often follow the convention automatically, but a manual change or a memory recall can leave it wrong.
- If speech sounds inverted, change the mode first. The cause is almost always the sideband, and it takes one button press to confirm.
- Watch your sideband in a contest. It is easy to carry a data mode across to a voice contest and call CQ on the wrong sideband.
- Use the correct sideband when transmitting through a transverter or SDR software. Some programs show the sideband labels differently, so test with a local contact or a dummy load and a monitor receiver.
- Keep your signal inside the band. Remember that the dial shows the carrier position, not the edge of the signal.
The Bottom Line
The rule for USB vs LSB is to use lower sideband below 10 MHz and upper sideband at and above 10 MHz, with 60 metres, digital modes, VHF and UHF all on upper sideband, and satellites depending on whether the transponder inverts. It is a convention, but a universal one, and following it means you will be understood by everyone. Check the mode after each band change, watch the band edges and remember that a sudden loss of decodes on a digital mode may be a sideband that quietly changed.
Frequently Asked Questions
Which sideband should I use on 40 metres?
Lower sideband for voice. The convention is LSB below 10 MHz and USB at 10 MHz and above, and 40 metres is below that line. Digital modes on 40 metres still use upper sideband.
Which sideband should I use on 20 metres?
Upper sideband for voice. 20 metres is above 10 MHz, and the convention is USB there. The same applies to 17, 15, 12 and 10 metres.
Why does FT8 use USB on every band?
Digital programs generate audio tones that the radio places above the dial frequency in USB, and everyone uses the same arrangement so that stations on a given dial frequency can decode one another. The 10 MHz voice rule does not apply to digital modes.
What happens if I use the wrong sideband?
The audio is inverted, because the receiver places the carrier on the wrong side. Speech sounds like a squeaky, unintelligible warble. Changing to the correct sideband on your radio restores normal speech.
Which sideband is used on 60 metres?
Upper sideband. It is an exception to the 10 MHz rule. In the United States the rules require USB on the five channels, and the same sideband is the accepted practice on the band elsewhere.
Does the sideband matter for CW?
Not for the signal itself, because CW is an on-off carrier, but your receiver’s sideband setting changes where the beat note falls. Many radios offer a reverse CW setting that can help move interference out of the passband.