Below the Noise Floor — Episode 4: "The HF Bands in Practice"

Not all HF bands behave the same way. Some come alive at night, some peak at noon, some only open during high solar activity. This episode is a practical band-by-band tour of the amateur HF spectrum - what each band is used for, when to listen, and where to point your attention when you are just getting started.

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Last episode we talked about reading the waterfall, what different signal types look like, and how to use the display as a propagation tool. This episode takes that one step further and gives you a practical map of the HF spectrum itself. Not all bands are created equal. Each one has its own personality, its own typical hours of activity, its own propagation characteristics, its own culture of operators and modes, learning that personality is one of the more rewarding parts of getting on HF for the first time. The bands are not just numbered slices of spectrum, they are distinct places, each with something different happening depending on the time of day and the state of the sun. I'm going to go through the main amateur HF bands, roughly from low to high frequency. I will skip 160 meters, 630 meters, and 2200 meters for now. We touched on those in episode 1, and they require specialized antennas and equipment that most newcomers are not starting with. The bands we are going to cover are the ones where the vast majority of HF amateur activity happens: 80 meters, 40 meters, 30 meters, 20 meters, 17 meters, 15 meters, 12 meters, and 10 meters. Let's start at the low end. 80 meters covers 3.5 to 4.0 MHz. It is a nighttime band. During the day, the D layer of the ionosphere absorbs signals at these frequencies before they can reach the F layer that would reflect them skyward. After sunset, the D layer dissipates and 80 meters opens up. Skip distance on 80 meters at night is relatively short, typically a few hundred to around a thousand miles, which makes it ideal for regional contacts. If you are on the west coast, 80 meters at night will reach the mountain states, the Pacific Northwest, into Canada, down into Mexico. It will not reliably reach Europe or Asia under most conditions. 80 meters is also one of the noisier bands. At those frequencies, atmospheric noise, lightning from thunderstorms anywhere in the hemisphere, contributes a constant background crackle that varies with season and weather. Summer nights tend to be noisier than winter nights for this reason. Despite the noise, 80 meters carries a lot of traffic. Voice operations are common, as is FT8 and other digital modes. There is also a tradition of informal nets, scheduled group conversations, that have been running on 80 meters for decades. 40 meters covers 7.0 to 7.3 MHz and is arguably the most versatile band on HF. It transitions. During the day, it behaves similarly to 80 meters, mostly regional, with skip distances of a few hundred to around a thousand miles. After dark, it extends dramatically. On a good night, 40 meters from the west coast will reach Europe, South America, the Pacific, and sometimes beyond. The skip distance lengthens as the ionosphere settles after sunset, and the band can stay open to long haul paths well into the night. 40 meters is heavily used, it carries voice, CW, FT8, and various other digital modes. If you're going to pick one band to start listening on in the evenings, 40 meters is a strong choice. There is almost always something happening, the propagation is relatively predictable, and the noise levels are lower than 80 meters. It is also where you will encounter a lot of broadcast interference. Shortwave broadcasters still use portions of the 7 MHz range internationally, and you will hear them. 30 meters is a narrow band, 10.1 to 10.15 MHz, and amateur use is limited to CW and digital modes only. No voice. This is a band that many operators overlook, but it rewards attention. 30 meters has a useful middle ground propagation character, it behaves better than 40 meters during the day and better than 20 meters at night. FT8 and WSPR are very active here. If you are interested in propagation monitoring specifically, 30 meters is worth watching because it often shows paths that are marginal or closed on the adjacent bands. 20 meters covers 14.0 to 14.35 MHz and is the workhorse of HF Amateur Radio. It is open to somewhere in the world at almost any hour. During the day, it supports long-distance contacts. Europe from the west coast is routine on 20 meters in the morning and early afternoon. As the afternoon progresses, the long haul paths toward Europe close and paths toward Asia and the Pacific open. In the evening it starts to fade for those very long paths, but remains open regionally. During the current part of the solar cycle, we are near solar maximum, which means the solar flux index is elevated and the ionosphere is well charged, 20 meters is particularly productive. The FT8 segment, around 14.074 MHz, is extremely active during daylight hours. If you pull up either SDR and tune to 20 meters on a weekday morning and the band is open, you will see the waterfall fill up with the characteristic FT8 comb pattern across that segment. It can look almost overwhelming at first. That density of activity is actually a useful calibration point. A busy 20-meter waterfall means propagation is good and you should take note of what conditions produced it. 20 meters is also where you will hear the most voice activity, the most DX, long-distance contacts, and the most contest activity on weekends when organized competition events fill the band with rapid fire exchanges. 17 meters covers 18.068 to 18.1168 MHz. It is a wark band, one of three bands allocated to amateur radio at the 1979 World Administrative Radio Conference that by convention are not used for organized contests. This gives 17 meters a notably different character than 20 meters. It tends to be quieter and more conversational. Propagation on 17 meters tracks closely with 20 meters, but generally requires slightly better conditions to be productive. When 20 meters is wide open, 17 meters is often open as well. When conditions are marginal, 17 meters may be thin, while 20 meters is still usable. 15 meters covers 21.0 to 21.45 MHz. This band is strongly dependent on the solar cycle. During solar minimum, when the sun is less active and the ionosphere is less energized, 15 meters can be essentially dead for days or weeks at a time. During solar maximum, which is close to where we are right now, 15 meters can be spectacular. Long distance paths open that are not possible on lower bands. The band supports worldwide contacts with lower power and modest antennas when conditions cooperate. If you check 15 meters and find the waterfall mostly empty, it does not necessarily mean a problem with your setup. It may simply mean conditions are not there today. 12 meters covers 24.89 to 24.99 MHz, another work band. It sits between 15 and 10 meters in propagation behavior. Like 17 meters, it tends to be less crowded and more relaxed than its neighbors. During good solar conditions, 12 meters can produce excellent long-distance contacts. It is a band that rewards checking during the middle of the day when the solar elevation is highest and ionospheric ionization is strongest. 10 meters covers 28.0 to 29.7 MHz and is the highest of the traditional HF bands. At solar maximum, 10 meters can be astonishing. The skip distance can be extremely long, contacts spanning 15,000 miles or more are possible. The band can open suddenly and dramatically, filling the waterfall with signals from every direction and then close just as quickly. At solar minimum, 10 meters is frequently completely dead for extended periods. Right now, with solar activity elevated, 10 meters is worth checking regularly. The FM subband at the top of 10 meters, around 29.6 MHz, carries local FM repeater traffic and is a different flavor of activity from the HFDX lower in the band, a few practical observations that cut across all of these bands. Gray line propagation is worth knowing about. The gray line is the terminator, the boundary between day and night on the Earth's surface. Twice a day, for a brief period around sunrise and sunset, the gray line passes over any given location. During this time, propagation conditions can be briefly exceptional because the D layer is either just forming or just dissipating, and signals that would normally be absorbed can instead reach the reflective F layer. If you want to try for a difficult path, say, a contact with Japan from the East Coast, or with Europe from the West Coast in the evening, checking the bands right around your local sunrise or sunset is often productive. Band openings can be sudden, you can check 10 meters at noon and find nothing, come back two hours later and find it full of signals. The ionosphere responds to changes in solar activity with a lag of hours, not days, and the difference between a closed band and an open one can be dramatic and fast. One of the habits that experienced HF operators develop is checking the bands briefly and regularly rather than assuming conditions are constant. Finally, and this is something the waterfall makes obvious in a way that a traditional radio does not, you will often see activity on bands you did not expect to be open. The visual overview lets you scan multiple bands quickly. A brief sweep across 10, 12, 15, and 17 meters in sequence takes about 30 seconds in Ether SDR and immediately tells you which ones have anything happening. That quick scan becomes a natural part of any operating session. Next episode, we go into receiving voice specifically SSB and AM. How to tune a signal until it sounds right, and what Ether SDR's mode and bandwidth controls are doing when you use them. This is below the noise floor.