How radio signals travel

A radio signal can reach another station in several ways: along the ground, in a straight line, or by bending back from the ionosphere (ITU-R ionosphere handbook, chapter 3). Which path works depends on the distance, the time of day, the season and the Sun (General pool, G3B02).

Ground wave

Below about 30 MHz, part of the signal can travel directly along the Earth’s surface. The ITU notes that broadcasting uses this ground wave, but that it “has limited range at the higher frequencies and over land” (ITU-R ionosphere handbook, chapter 3). The same handbook describes ground-wave distances as typical of the MF (AM) broadcast band, 535 to 1705 kHz, while MF sky waves reach much farther at night (section 4.1). Ground wave supports vertical polarization (Extra pool, E3B13), and its maximum range decreases as frequency rises (Extra pool, E3B08). An ARRL article puts typical amateur ground-wave range on HF at up to 50 miles (QST, December 1983).

Line of sight and the radio horizon

VHF and UHF signals normally travel in straight lines and pass through the ionosphere into space (ITU-R ionosphere handbook, chapter 3), so simplex UHF signals are rarely heard beyond the radio horizon (Technician pool, T3C01). The atmosphere bends these signals slightly (Technician pool, T3C11). Radio engineers model this by pretending the Earth is larger than it is: the “k-factor”, the ratio of that effective radius to the real one, has a median value of about 4/3 (ITU-R point-to-point handbook, section 3.3).

For a given antenna height, the distance to the horizon grows with the square root of the Earth’s radius. The square root of 4/3 is about 1.15, so the radio horizon works out about 15 percent farther than the visual horizon, the same figure the exam pool gives (Extra pool, E3C06). Signals can also bend around a sharp ridge or rooftop edge, which is called knife-edge diffraction (Technician pool, T3C05). If buildings block the direct path to a repeater, a directional antenna may reach it by finding a path that reflects signals to it (Technician pool, T3A05).

The ionosphere

The ionosphere is the part of the upper atmosphere, between about 80 and 600 km up, where solar X-rays and extreme ultraviolet light ionize atoms and molecules and create a layer of free electrons (NOAA SWPC). The ionosphere refracts (bends) HF radio waves; if a ray is bent enough, it returns to Earth far away (ITU-R ionosphere handbook, section 3.2). The ionosphere has regions that behave differently (ITU-R ionosphere handbook, sections 2.2 and 3.2):

D-region absorption depends strongly on frequency: the ITU gives an example where 1 dB of loss at 30 MHz becomes 10 dB or more at lower HF frequencies, and low-angle signals lose more because they spend longer in the layer (ITU-R ionosphere handbook, section 6.5.2). This is why long-distance contacts on 160, 80, 60 and 40 meters are harder during the day (General pool, G3C05).

Skip, the skip zone and the MUF

The critical frequency is the highest frequency a layer returns when you transmit straight up; higher frequencies pass through (SWPC glossary). Above it, a signal is returned only if it meets the layer at a shallow enough angle. The highest takeoff angle that still comes back is the critical angle (General pool, G3C04), and lower takeoff angles give longer hops (Extra pool, E3B07).

Close to the transmitter, steep signals pass through the layer, so there is an area around the station where the sky wave cannot be received, and the skip distance grows as frequency rises (ITU-R ionosphere handbook, section 3.2). This skip zone is too distant for ground waves and too close for sky waves (ARRL glossary). Stations in the skip zone sometimes hear a weak, fluttery copy of the signal, because a small part of the energy is scattered there from the ground or from irregularities in the ionosphere (ITU-R ionosphere handbook, section 3.2; General pool, G3C06 and G3C08).

Two numbers describe a given path (SWPC glossary):

• MUF, maximum usable frequency: the highest frequency that gives reliable communication over that distance by ionospheric refraction. Higher frequencies go through to space.
• LUF, lowest usable frequency: the lowest frequency that gives reliable long-range communication by ionospheric refraction. Below it, signals are attenuated before they reach the other station (General pool, G3B06).

Between the two, signals are refracted back to Earth (General pool, G3B05), and frequencies just below the MUF have the least attenuation for long-distance skip (General pool, G3B03). When the LUF climbs above the MUF, ordinary skywave on that path is not possible (General pool, G3B11). A signal can also arrive both by the short path and the long way around the globe, and then a slightly delayed echo might be heard (General pool, G3B01).

Day and night by band

These are typical patterns, not guarantees:

• 160, 80, 60 and 40 meters: the D region absorbs these signals during daylight, so long-distance contacts are harder by day (General pool, G3C05). The D layer usually disappears within 30 minutes after sundown, and at night 160, 80 and 40 meters can often support contacts of several thousand miles (QST, December 1983). On 160 meters, a path entirely in darkness is the most likely to support long-distance propagation (Extra pool, E3B05).
• 20 meters: usually supports worldwide propagation during daylight at any point in the solar cycle (General pool, G3A07).
• 15, 12 and 10 meters: the least reliable bands for long-distance contacts when solar activity is low (General pool, G3A04). During high sunspot activity, 10 meters supports long-distance F-region contacts from dawn to shortly after sunset (Technician pool, T3C09).
• 6 meters: long-distance F-region openings are possible during the peak of the sunspot cycle (Technician pool, T3C10). Sporadic E and meteor scatter, described below, also carry 6 meter signals beyond the horizon.

Two daily effects are worth knowing. The first is the change at dusk and dawn. The D layer usually disappears within about 30 minutes after sundown (QST, December 1983), while the F2 layer is the only layer that persists to any significant degree at night (ITU-R ionosphere handbook, section 2.2.5). As darkness falls, low-band absorption drops while the F2 layer can still return signals.

The second is NVIS (near vertical incidence skywave): short-distance MF or HF propagation at high elevation angles (General pool, G3C10). A wave sent straight up is returned only if its frequency is below the layer’s critical frequency (ITU-R ionosphere handbook, section 3.2). The exam pool’s example of an NVIS antenna is a horizontal dipole between 1/10 and 1/4 wavelength above the ground for daytime contacts on 40 meters (General pool, G9D01), and an ARRL article describes a low 80 meter antenna that is “ideal for 80-meter contacts ranging from next door out to 150 or 200 miles” (QST, April 2001). Because the signal comes down steeply, NVIS covers the short distances that lower-angle skip passes over.

The Sun drives HF

The number of sunspots rises and falls over a cycle of about 11 years (NOAA SWPC). The extreme ultraviolet output that builds the ionosphere varies by nearly a factor of ten over the cycle, and the density of the ionosphere changes with it (NOAA SWPC). Higher sunspot numbers generally mean a greater probability of good propagation at higher frequencies (General pool, G3A01).

Solar Cycle 25 began in December 2019 (NOAA SWPC). In SWPC’s observed solar cycle data, the smoothed sunspot number reached its highest value of the cycle so far, 160.9, in October 2024. As of October 2026 the latest smoothed value in that data, for March 2026, was 94.5, so the cycle is declining. SWPC expects Cycle 26 to begin some time between January 2029 and December 2032 (solar cycle progression). Conditions also tend to repeat about every 27 days, the Sun’s average rotation period as seen from Earth (SWPC glossary).

The numbers you will see in propagation reports:

• Solar flux index (SFI): the Sun’s radio output at 10.7 cm (2800 MHz), in solar flux units. It correlates with the sunspot number and with the extreme ultraviolet output that affects the ionosphere, and it ranges from below 50 to above 300 over a cycle (NOAA SWPC).
• K index: a 3-hourly measure of disturbance in the Earth’s magnetic field, from 0 to 9. Kp is the planetary version, and 5 or more means a geomagnetic storm (NOAA SWPC). It describes the short-term stability of the field (General pool, G3A12).
• A index: a daily index, the average of the eight 3-hourly readings, and SWPC calls 0 to 7 quiet, 8 to 15 unsettled, 16 to 29 active, and 30 or more a storm (SWPC glossary). It describes the longer-term stability of the field (General pool, G3A13).

A rising K or A index means increasing disturbance of the geomagnetic field, and paths through the auroral oval are the most likely to suffer high absorption when it is elevated (Extra pool, E3C02 and E3C03).

When the Sun misbehaves

• Solar flares are bursts of electromagnetic radiation that travel at the speed of light, so their effects on the sunlit side of the Earth begin at the moment the flare is seen (NOAA SWPC), about 8 minutes after it leaves the Sun (General pool, G3A03). The extra X-rays and extreme ultraviolet ionize the D region, which then absorbs HF signals, and NOAA rates these radio blackouts from R1 (an M1 flare) to R5 (an X20 flare) (NOAA SWPC). Lower frequencies are disrupted more than higher ones (General pool, G3A02).
• Coronal mass ejections usually take several days to reach Earth, though some have arrived in as little as 18 hours. They, and high-speed solar wind streams, cause geomagnetic storms, especially when the solar wind’s magnetic field points southward (NOAA SWPC). Coronal holes are a source of high-speed solar wind (SWPC glossary). NOAA rates storms from G1 (Kp 5) to G5 (Kp 9); at G2 HF can fade at higher latitudes, and at G5 HF may be impossible in many areas for one to two days (NOAA scales). Energetic protons from the Sun can also block HF at high latitudes (NOAA SWPC).
• Aurora is most likely during severe geomagnetic storms (Extra pool, E3A12), and it can reflect VHF signals (General pool, G3A09). Signals received this way are distorted, with a characteristic raspy sound (Technician pool, T3C03), and the exam pool names CW, rather than SSB, FM or RTTY, as the mode for auroral contacts (Extra pool, E3A13).

VHF and UHF beyond the horizon

• Sporadic E is a thin, dense patch of ionization in the E region, most often near 110 km. At mid-latitudes it is most common in the local summer months, with daily peaks in mid-morning and near sunset (ITU-R ionosphere handbook, section 2.2.4.1). It is the propagation most commonly associated with occasional strong signals on the 10, 6 and 2 meter bands from beyond the radio horizon (Technician pool, T3C04).
• Tropospheric ducting is caused by temperature inversions (Technician pool, T3C08), layers of air that get warmer with height (NWS glossary). Unusual temperature or moisture gradients can bend a radio beam back down, so signals return from farther away than normal (NWS glossary). Ducting regularly allows over-the-horizon VHF and UHF contacts to about 300 miles (Technician pool, T3C06).
• Meteor scatter uses the brief ionized trails left by meteors. The WSJT-X guide says contacts can be made “any time on the VHF bands at distances up to about 2100 km (1300 miles)”, that completing one takes longer in the evening than in the morning and longer at higher frequencies, and that its MSK144 mode is designed to use these brief meteor “pings” (WSJT-X User Guide). The exam pool names 6 meters as the band suited to meteor scatter (Technician pool, T3C07).

Checking conditions right now

• NOAA SWPC radio dashboard: solar wind, the 10.7 cm solar flux, D-region absorption predictions and an aurora forecast.
• PSK Reporter: a map of reception reports for FT8 and other digital modes, which you can filter to show who has received your call sign.
• Reverse Beacon Network: a network of stations that listen to the bands and report which stations they hear, when and how well. You can call a quick CQ and see which of them hear you.
• WSPRnet map: reports from the Weak Signal Propagation Reporter network, selectable by band from LF to 23 cm.
• NCDXF/IARU beacons: 18 beacons around the world take turns on 14.100, 18.110, 21.150, 24.930 and 28.200 MHz (NCDXF). Each one transmits on each band once every three minutes, sending its call followed by dashes at 100 W, 10 W, 1 W and 100 mW. The project notes that three minutes of listening can show where a band is open (NCDXF).
• VOACAP Online: point-to-point predictions from VOACAP, a model of HF propagation (Extra pool, E3C11), for planning.