Radio Propagation Conditions: The Complete Guide for Radio Amateurs

📡 1. Introduction to radio propagation

Radio propagation is the physical phenomenon by which electromagnetic waves travel from a transmitter to a receiver. For radio amateurs, understanding propagation conditions is the key to long-distance communication, whether on HF (shortwave), VHF or UHF.

Unlike satellite or Internet communications, HF links depend entirely on atmospheric and solar conditions. The ionospheric layer of the Earth’s atmosphere acts as a natural mirror that sends radio waves back to Earth, allowing contacts over thousands of kilometres without any intermediate infrastructure.

Factors that influence propagation

🌐 2. The ionosphere: the engine of propagation

The ionosphere is the region of the Earth’s atmosphere between 60 and 1000 km altitude. Ultraviolet radiation and X-rays from the Sun ionise the atmospheric gases, creating layers of plasma capable of reflecting (refracting) HF radio waves.

The ionospheric layers

The ionosphere is made up of four main layers, each playing a specific role in radio propagation:

D layer (60-90 km)

The D layer is the lowest layer of the ionosphere. It is mainly responsible for the absorption of HF radio waves, particularly on the low frequencies (160m and 80m). This layer exists only during the day and disappears a few minutes after sunset.

E layer (90-150 km)

The E layer allows propagation over medium distances (500 to 2000 km), normally reflecting frequencies up to about 5 MHz. It is present mainly during the day and weakens considerably at night.

The most interesting phenomenon linked to the E layer is sporadic E (Es): clouds of intense, localised ionisation that form unpredictably, mainly in summer (May-August in the northern hemisphere). Sporadic E can reflect frequencies up to 150 MHz and beyond, allowing exceptional VHF contacts on 6m and sometimes 2m.

F1 layer (150-250 km)

The F1 layer exists only during the day in summer. It merges with the F2 layer at night and in winter. Its role in HF propagation is secondary, but it contributes to absorption on the highest frequencies during the day.

F2 layer (250-500 km)

The F2 layer is the most important layer for long-distance HF propagation. It is what makes intercontinental contacts possible on the bands from 40m to 10m. Its key characteristics:

• The highest ionisation density of all the layers
• Present 24 hours a day, although weaker at night
• Its altitude varies with the time of day, the season and solar activity
• The critical frequency (foF2) determines the maximum usable frequency at vertical incidence
• The MUF (Maximum Usable Frequency) is about 3 to 3.5× the foF2 for an oblique path

The reflection mechanism

Contrary to what you often hear, HF waves are not “reflected” as if by a mirror. They are gradually refracted by the ionised layer. The refractive index decreases as the electron density increases, bending the signal until it is sent back towards the ground. If the frequency is too high for the ionisation density (above the MUF), the signal passes through the ionosphere and is lost in space.

🔄 3. Types of propagation

Radio waves can travel between transmitter and receiver in several ways. Each type of propagation has its own characteristics and uses.

Ground wave

The ground wave follows the curvature of the Earth, travelling along the surface. It dominates on the low frequencies (LF and MF) and typically covers 100 to 500 km on the 160m and 80m bands. Attenuation increases with frequency, making the ground wave negligible above 10 MHz. It is reliable and little affected by ionospheric conditions.

Sky wave

This is the main propagation mode for HF DX. The signal is sent skywards at a certain elevation angle, refracted by the ionosphere and then sent back to Earth. There are several kinds:

Skip zone

The skip zone is the region between the limit of the ground wave’s range and the point where the first ionospheric reflection returns to Earth. In this zone, the signal cannot be heard. The size of the zone depends on the frequency (higher = longer skip), the radiation angle of the antenna and ionospheric conditions.

📐 Approximate skip distance
Skip distance ≈ 2 × h × tan(θ)
where h = height of the layer (250-350 km for F2)
and θ = complement of the angle of incidence

Example on 20m (foF2 = 8 MHz):
- Elevation angle 5° → skip ≈ 2800 km
- Elevation angle 20° → skip ≈ 1200 km
- Elevation angle 45° → skip ≈ 500 km

Grey line (day/night terminator)

The grey line is the transition zone between day and night that circles the Earth. This band, about 200 km wide, creates exceptional propagation conditions because:

• The (absorbing) D layer disappears on the night side while the F2 layer remains ionised
• A “duct” forms along the terminator, guiding signals over very long distances
• The low bands (80m, 40m) are particularly favoured
• Openings are short (15-45 minutes) but spectacular
• Occurs at local sunrise and sunset

Sporadic E (Es)

Sporadic E is a fascinating propagation phenomenon caused by dense clouds of ionisation in the E layer (100-120 km). These clouds form mainly in summer (peaking in June-July in the northern hemisphere) and allow contacts that are normally impossible:

Tropospheric propagation (tropo)

Tropospheric propagation occurs in the lowest layer of the atmosphere (0-12 km) and mainly affects VHF and UHF frequencies. It is caused by temperature inversions that create atmospheric “ducts”. Tropo can allow VHF contacts over 500 to 2000+ km, especially in summer and autumn during stable high-pressure periods.

Other propagation modes

📊 4. Solar and geomagnetic indices

To assess propagation conditions, radio amateurs keep an eye on several indices that reflect the state of the Sun and the Earth’s magnetic environment. Understanding these indices lets you anticipate the quality of HF propagation.

SFI: Solar Flux Index

The Solar Flux Index (SFI) measures the intensity of the Sun’s radio emission at 2800 MHz (10.7 cm). It is the most widely used indicator of the overall level of ionisation.

SSN: Sunspot Number

The sunspot number (SSN) is the oldest historical indicator of solar activity. Sunspots are magnetically active regions on the surface of the Sun, correlated with the UV emission that ionises the atmosphere.

K-index

The K-index measures the disturbance of the Earth’s geomagnetic field over a 3-hour period. It uses a quasi-logarithmic scale from 0 to 9. It is the key indicator for short-term disturbances.

A-index

The A-index is the daily average of geomagnetic activity, derived from the 8 K-index values over 24 hours. It gives a broader picture than the instantaneous K.

Index summary: ideal conditions for DX

📊 Quick assessment formula
Conditions = high SFI + low K + low A

✅ Excellent: SFI > 150, K ≤ 1, A < 7
✅ Good: SFI > 100, K ≤ 2, A < 15
⚠️ Average: SFI 80-100, K = 3, A = 15-25
❌ Poor: SFI < 80, K ≥ 4, A > 30

☀️ 5. Solar cycle 25

Solar activity follows a cycle of about 11 years, alternating between periods of minimum (few sunspots) and maximum (many sunspots). We are currently in solar cycle 25, which began in December 2019.

Cycle 25 timeline

Comparison with previous cycles

📻 6. Band-by-band propagation (HF)

Each HF band has its own propagation characteristics. Here is a detailed guide to each band, from the lowest to the highest.

160 metre band (1.8-2.0 MHz): “Top Band”

80 metre band (3.5-3.8 MHz)

40 metre band (7.0-7.2 MHz)

30 metre band (10.1-10.15 MHz): WARC

20 metre band (14.0-14.35 MHz)

17 metre band (18.068-18.168 MHz): WARC

15 metre band (21.0-21.45 MHz)

12 metre band (24.89-24.99 MHz): WARC

10 metre band (28.0-29.7 MHz)

6 metre band (50-52 MHz in Switzerland): “The Magic Band”

📡 7. VHF/UHF propagation

Beyond HF, the VHF (30-300 MHz) and UHF (300-3000 MHz) bands have their own propagation modes. Conventional ionospheric propagation is not normally possible (except for sporadic E and exceptional conditions), but other phenomena allow long-distance links.

Line-of-sight propagation

The normal propagation mode on VHF/UHF is line of sight (LOS). The range is limited by the radio horizon, which depends on the height of the antenna:

📐 Radio horizon range (open terrain)
Distance (km) ≈ 4.12 × √(h in metres)

Examples:
Antenna at 10m → range ~13 km
Antenna at 25m → range ~20 km
Antenna at 100m → range ~41 km
Summit at 1000m → range ~130 km

Between 2 stations: D ≈ 4.12 × (√h1 + √h2)

Tropospheric ducting

Temperature inversions create ducts in the troposphere that trap VHF/UHF waves. This phenomenon is favoured by:

• Stable anticyclones (high pressure)
• Late summer and autumn in Europe
• Maritime conditions (surface ducting over the sea)
• Calm nights with a ground-level temperature inversion

Ducting can allow contacts on 2m and 70cm over 500 to 2000+ km, with signals that are sometimes surprisingly strong.

Meteor scatter

Meteor scatter uses the brief ionisation created by meteors burning up in the atmosphere (80-120 km). It is a reliable mode on 6m and 2m thanks to fast digital modes:

Auroral propagation

During geomagnetic storms (K ≥ 5), the northern lights create an ionised zone above the polar regions that can reflect VHF signals. Signals reflected by the aurora have a characteristic “raspy” sound (Doppler distortion). This mode works mainly on 6m and 2m, with contacts possible over 800-2000 km towards the north.

🛠️ 8. Real-time forecasting tools

Many free tools allow radio amateurs to monitor propagation conditions in real time and anticipate openings. Here are the most important ones.

PSK Reporter

VOACAP Online

Other essential tools

Mobile apps

📈 9. How to read a propagation forecast

Knowing how to read a propagation bulletin or widget correctly is essential. Here is how to interpret the information.

The solar widget (HamQSL / N0NBH)

The famous solar banner found on many amateur radio websites shows:

Interpreting an ionogram

An ionogram is the result of a vertical sounding of the ionosphere by an ionosonde. The key information is:

• foF2: critical frequency of the F2 layer, the vertical MUF
• foE: critical frequency of the E layer (typically 2-4 MHz during the day)
• hmF2: height of the F2 density peak (250-400 km)
• foEs: critical frequency of sporadic E (if present), can reach 10+ MHz
• MUF(3000): estimated MUF for a distance of 3000 km ≈ foF2 × 3.0-3.5

NOAA alert scales

🎯 10. A practical guide to DX

Here is some practical advice for making the most of propagation conditions and maximising your DX contacts.

The DXer’s daily routine

1. Check the indices: look up the SFI, K-index and A-index first thing in the morning on hamqsl.com or SolarHam.
2. Check PSK Reporter: look at the FT8 spots on the band(s) you are interested in to see real propagation.
3. Consult the DX Cluster: see which rare stations are active and which bands are open.
4. Plan with VOACAP: for a specific DX target, use VOACAP to find the best band and the optimum time.
5. Listen before transmitting: tune across the band to assess the noise and activity before calling CQ.
6. Make the most of the windows: openings can be short. Be ready to transmit quickly when the band opens.

Which band at what time?

Optimising your antennas for DX

Digital modes and weak-signal propagation

Modern digital modes such as FT8 and FT4 (WSJT-X) have revolutionised weak-signal operating. With a sensitivity of -24 dB below the noise (FT8), these modes can detect openings that are invisible in SSB or CW.

🇨🇭 11. Propagation from Switzerland (HB9)

Switzerland enjoys a privileged geographical position in the centre of Europe for radio propagation. Here are the specifics for HB9 stations.

Advantages of the HB9 location

• Centre of Europe: easy access to the whole of Europe without a troublesome skip zone
• Mid-latitude (~47°N): a good compromise between ionisation and polar paths
• Altitude: many high-altitude QTHs (Alps) offer an exceptional radio horizon
• Grey line: from HB9, the terminator covers southern Africa at dusk and the Americas at dawn
• Long path: excellent for VK (Australia) via the South Pole on 20m late in the day

Main DX directions from HB9

Repeaters and amateur radio activity in Switzerland

SOTA activations from HB9

Switzerland is a paradise for SOTA (Summits On The Air) activations. With more than 1300 listed summits, propagation from Alpine summits is exceptional thanks to the altitude and the clear horizon. The 40m and 20m bands are the most used in SOTA, combining local coverage (40m NVIS) and DX (20m).

📖 12. Glossary of terms

📋 13. Summary

Summary table: conditions by band

🤝 Our partners

Phonicom.ch: amateur radio specialist in Switzerland

Phonicom offers HF transceivers to take advantage of the best propagation conditions, directional and omnidirectional antennas, and all the accessories you need for DX. Personal advice and delivery within Switzerland.

Visit phonicom.ch →