HF Propagation Basics
Understanding how HF radio waves travel from your antenna to a station thousands of kilometres away requires understanding the ionosphere — the electrically charged upper atmosphere that acts as a natural mirror for radio frequencies between roughly 3 and 30 MHz. Without the ionosphere, HF signals would simply travel in straight lines into space and long-distance communication would require satellites. With it, a 100-watt station with a wire antenna can reach virtually anywhere on Earth under the right conditions.
Ionospheric layers
The ionosphere is divided into layers distinguished by altitude and ionisation density. The D-layer (60–90 km) exists only during daylight hours and absorbs lower HF frequencies — it is responsible for 40m and 80m being primarily nighttime DX bands, because when the D-layer disappears after dark, those frequencies can reach the higher F-layer and reflect to distant stations. The E-layer (90–150 km) supports shorter-range HF propagation and is the medium for sporadic-E (Es) events on VHF. The F-layer (150–500 km) is the most important layer for long-distance HF — it exists continuously, combines into a single F2-layer at night, and reflects HF signals over thousands of kilometres.
How reflection works
When an HF radio wave enters the ionosphere at an angle, it is gradually bent (refracted) by the increasing ionisation density. If the frequency is low enough relative to the ionisation density, the wave is bent enough to return to Earth — what we commonly call reflection. If the frequency is too high (above the MUF), the wave passes through the ionosphere into space. If the frequency is too low (below the LUF), D-layer absorption consumes the signal before it can reach the reflecting layer. The angle of the wave determines the skip distance — the ground distance from the transmitter to where the signal first returns to Earth.
Maximum Usable Frequency (MUF)
The MUF is the highest frequency that will be reflected back to Earth by the ionosphere on a given path at a given time. Above the MUF, signals pass through the ionosphere and are lost to space. The MUF varies continuously with solar activity, time of day, season, and the specific path between two stations. On a good day at solar maximum, the MUF on a transoceanic path may reach 28–30 MHz, making 10m and 12m excellent DX bands. At solar minimum, the same path's MUF may only reach 14–18 MHz, leaving 10m and 15m closed. Propagation prediction tools like VOACAP calculate the MUF for specific paths.
Skip distance and skip zone
Skip distance is the ground distance from the transmitter to where the reflected signal first reaches Earth. Between the transmitter and the skip distance is the skip zone — an area where the signal cannot be received on the ground because it has reflected too high to reach it. The skip zone moves outward as frequency increases — on 20m, the skip zone may be 500–800 km; on 10m, it may be 1,500–2,000 km. NVIS (Near Vertical Incidence Skywave) eliminates the skip zone entirely by directing signals nearly straight up, creating coverage within a few hundred kilometres of the transmitter.
| Band | Daytime | Night | Best For |
|---|---|---|---|
| 160m (1.8 MHz) | Absorbed — poor | Regional to DX | Winter nights, DX at night |
| 80m (3.5 MHz) | Regional NVIS | Regional to DX | Regional day, DX night |
| 40m (7 MHz) | Regional to continental | Continental to worldwide DX | Most versatile band |
| 20m (14 MHz) | Worldwide DX | Often closes near midnight local | Best daytime DX band |
| 17m (18 MHz) | Worldwide DX | Variable — often open at night | Quieter alternative to 20m |
| 15m (21 MHz) | Worldwide DX when open | Usually closed | Solar maximum daytime DX |
| 12m (24 MHz) | Good DX at solar max | Usually closed | Solar maximum periods |
| 10m (28 MHz) | Spectacular DX at solar max | Closed except Es | Solar maximum — best band when open |
Single vs multi-hop
A single-hop propagation path reflects once off the ionosphere and returns to Earth. Maximum single-hop distance is roughly 4,000 km — the practical range of one F2 reflection. For contacts beyond this distance — transoceanic contacts, contacts to the opposite side of the Earth — the signal reflects multiple times, bouncing between the ionosphere and Earth's surface. A 2-hop path reaches 8,000 km, a 3-hop path reaches 12,000+ km. Each hop introduces additional loss, so multi-hop contacts typically require better band conditions and more power than single-hop contacts at shorter distances.
Long path vs short path
Every point on Earth can be reached from your station by two paths — the short path (most direct great-circle route) and the long path (the complementary route going the other way around the planet, roughly 180 degrees in the opposite direction). For most contacts, the short path works best. But for some paths at certain times of day, the long path is actually better — particularly when the short path is all daylight (higher absorption) and the long path has a more favourable balance of daylit and dark segments. Long-path contacts are often identifiable by unusual arrival directions on receive that differ from the expected short-path bearing.
Why can I hear distant stations but they cannot hear me?
This asymmetry happens for several reasons. Propagation is not always reciprocal — path conditions can differ by direction. More commonly, the other station is running significantly more power or has a better antenna. Stations in the skip zone relative to your signal cannot hear you even if they are audible to you. On digital modes like FT8, the automatic nature of the exchange makes apparent asymmetry more noticeable — you may be able to decode a station at -20 dB that cannot decode your -15 dB signal because band conditions changed between their transmit and your transmit slot.
What is backscatter?
Backscatter occurs when a radio wave reflects off the ionosphere and some energy scatters back towards the transmitting station rather than continuing in the forward direction. The result is that stations within the skip zone can sometimes hear the transmitting station via backscatter — the signal going up to the ionosphere, forward to a scattering region, and back to a nearby receiver. Backscatter signals typically have a distinctive rough, noisy quality compared to direct propagation. It also allows stations in the same skip zone to communicate with each other via the common backscatter region.
How does propagation differ on digital modes vs phone?
Digital modes like FT8 can decode signals 15–20 dB below the threshold for phone copy — signals you literally cannot hear with your ears. This means digital modes effectively extend the range of propagation because they can use weaker reflected signals that phone cannot. A marginal band opening that produces unreadable phone signals will often allow many FT8 contacts. This is one reason FT8 appears to have better propagation than phone — it does not, but it can use propagation that phone operators would dismiss as closed.