How Radio Waves Travel: HF Propagation Basics

How HF Radio Works

Unlike VHF/UHF which is mostly line-of-sight, HF radio signals can travel around the world by bouncing off layers of the atmosphere called the ionosphere.

The Ionosphere

The ionosphere is a region of the upper atmosphere (about 50-400 miles up) where solar radiation ionizes gas molecules. These ionized layers can refract (bend) radio waves back to Earth.

Ionospheric Layers

D Layer (50-90 km)

• Only present during daylight
• Absorbs lower HF frequencies
• This is why 80m and 160m work better at night

E Layer (90-150 km)

• Provides short-range skip (500-1500 miles)
• "Sporadic E" can enable VHF propagation in summer

F Layer (150-400 km)

• The main layer for long-distance HF communication
• Splits into F1 and F2 during daytime
• F2 is the most useful for DX

How Skip Works

When you transmit on HF, your signal goes up, hits the ionosphere, and bends back down to Earth. This is called "skip."

The distance depends on:

• Frequency (higher frequencies = longer skip)
• Angle of radiation (lower angles = longer skip)
• Ionospheric conditions

The Skip Zone

There's often a "skip zone" or "dead zone" where:

• Ground wave has faded out
• Sky wave hasn't come back down yet

This is why you might hear stations 2000 miles away but not 200 miles away.

How the Sun Affects Propagation

The sun controls propagation. More solar activity = more ionization = better HF conditions (usually).

Solar Flux Index (SFI)

Measures solar radio emissions. Higher is generally better for HF.

• Below 70: Poor conditions
• 70-100: Fair conditions
• 100-150: Good conditions
• Above 150: Excellent conditions

Sunspot Number

More sunspots generally mean better HF propagation, especially on higher bands (10m, 12m, 15m).

A and K Indices

Measure geomagnetic activity. Lower is better.

• K index: 0-2 is good, 5+ is disturbed
• A index: Under 10 is good, over 30 is poor

Solar Cycle

The sun goes through an 11-year cycle of activity. We're currently in Cycle 25, which is showing good activity.

Band-by-Band Propagation

160m (1.8 MHz)

• Primarily nighttime band
• Best in winter
• Subject to atmospheric noise

80m (3.5 MHz)

• Nighttime band
• Good for regional contacts
• Can go long distance at night

40m (7 MHz)

• Day and night band
• Reliable for medium distance
• Crowded but always active

20m (14 MHz)

• The "workhorse" band
• Often open to somewhere
• Good for DX during daylight

15m (21 MHz)

• Daytime band
• Excellent when open
• More dependent on solar conditions

10m (28 MHz)

• Opens during high solar activity
• Can be amazing or completely dead
• Sporadic E propagation in summer

Checking Conditions

Before getting on the air:

• Check SFI, A-index, K-index at hamqsl.com or 73qrz.com/solar.php
• Look at PSK Reporter maps
• Check DX clusters for activity reports
• Listen to propagation beacons

Propagation Modes

Short Path

The direct, shorter route to a station. Most common.

Long Path

Going the "wrong way" around the world. Sometimes stronger than short path!

Gray Line

Enhanced propagation along the twilight zone (where night meets day). Great for working DX.

Sporadic E

Patches of intense ionization in the E layer. Enables VHF propagation and unusual HF skip in summer.