How HF signals travel

HF radio signals (3-30 MHz) bounce off the ionosphere — layers of ionized gas in the upper atmosphere. The ionosphere acts as a mirror for radio waves, reflecting them back to Earth hundreds or thousands of miles from the transmitter. This is called skywave propagation, and it is why amateur operators can communicate worldwide with modest equipment.

The ionospheric layers

D layer (60-90 km altitude)

Absorbs lower HF frequencies during the day. The D layer is why 80m and 160m are noisy during the day and quiet at night — the D layer disappears after sunset, removing the absorption. The D layer has no useful reflection for amateur purposes.

E layer (90-120 km altitude)

Reflects signals on 6m and sometimes 10m. Sporadic-E propagation occurs when patches of the E layer become highly ionized, creating mirror-like reflection for VHF signals. Sporadic-E is how operators work 6m and occasionally 2m over distances of 500-1500 miles.

F layer (150-500 km altitude)

The primary layer for HF propagation. The F layer splits into F1 and F2 during the day. The F2 layer is the highest and provides the longest-distance reflections. At night, the F layers merge into a single layer. The F layer's ionization depends on solar radiation — more sunspots means more ionization means higher frequencies are reflected.

Which bands work when

Daytime bands (best during daylight)

20m (14 MHz): The workhorse DX band. Open during daylight hours, often worldwide. The most reliable HF band for DX.
17m (18 MHz): Similar to 20m but less crowded. Opens slightly later, closes slightly earlier.
15m (21 MHz): Needs moderate solar activity. Excellent when open. Can be dead during solar minimum.
12m (24.9 MHz): Needs good solar conditions. Sporadic but exciting when open.
10m (28 MHz): The solar-cycle band. During solar maximum, 10m provides worldwide propagation with modest antennas. During solar minimum, it is often dead. Sporadic-E provides occasional short-distance openings regardless of solar cycle.

Nighttime bands (best after sunset)

80m (3.5 MHz): Regional at night (500-1500 miles). The ragchew band. Noisy.
160m (1.8 MHz): The hardest band. Requires large antennas. Best in winter nights. Very noisy.
40m (7 MHz): The transition band. Regional during the day, intercontinental at night. Excellent for medium-distance contacts.
60m (5.3 MHz): Channelized in the US (5 fixed frequencies). Shared with government users. Good for regional emergency communications.
30m (10.1 MHz): CW and digital only. Open day and night. The band that is always open.

The solar cycle

The sun's activity follows an approximately 11-year cycle. During solar maximum, sunspot numbers are high, ionospheric ionization is strong, and the higher HF bands (10m, 12m, 15m) are open worldwide. During solar minimum, only 20m and below are reliable for DX.

Current cycle: Solar Cycle 25 began in December 2019. Peak activity is expected around 2025-2026. As of 2024-2025, conditions on 10m and 15m are excellent.

Practical propagation tools

PSKReporter (pskreporter.info) — real-time map of FT8/CW signal reception worldwide
DX Maps (dxmaps.com) — real-time HF propagation based on reported contacts
VOACAP (voacap.com) — HF propagation prediction by time, frequency, and path
Solar indices — Solar Flux Index (SFI), A-index, K-index available from spaceweather.com

HF propagation guide