G3: Radio Wave Propagation – Ham Radio General License Study Guide

G3 covers how radio waves travel from one point to another — the physics of the ionosphere, the influence of the Sun, and the operating limits that determine which bands work at any given time and place. Three exam questions come from this subelement, one from each group.

G3A addresses solar activity and its effects on HF propagation: sunspot numbers, the solar flux index, sudden ionospheric disturbances, solar flares and coronal mass ejections, the 26–28 day solar rotation cycle, geomagnetic storms, the K-index and A-index, and how aurora and coronal holes affect radio communication. G3B covers the Maximum Usable Frequency (MUF), the Lowest Usable Frequency (LUF), short-path and long-path propagation, how to determine current propagation conditions, ionospheric refraction, hop distances for F2 and E region propagation, and what happens when the LUF exceeds the MUF. G3C examines the ionospheric regions themselves — D, E, F1, and F2 — the critical frequency and critical angle, why D-region absorption limits lower-band propagation during daytime hours, the characteristics of HF scatter propagation, and near vertical incidence skywave (NVIS) techniques used for regional communication.

G3A: Solar Activity and Effects

The Sun is the primary driver of HF propagation conditions. Sunspot numbers indicate solar activity levels: higher sunspot numbers mean more ionizing radiation reaching Earth's ionosphere, which raises the critical frequency and generally supports better propagation at higher HF frequencies — 10, 12, and 15 meters open more reliably during solar maximum. During periods of low solar activity, these high bands become unreliable for long-distance communication, while lower bands like 80 and 40 meters remain more dependable.

The solar flux index measures solar radiation at a wavelength of 10.7 centimeters and is a widely used indicator of ionospheric conditions. It correlates with sunspot numbers but is measured continuously. A geomagnetic storm is a temporary disturbance in Earth's geomagnetic field, usually caused by charged particles from the Sun. Geomagnetic storms degrade HF propagation — especially at high latitudes — by disturbing the ionosphere. However, high geomagnetic activity can create auroras that are capable of reflecting VHF signals, enabling a different type of communication.

The K-index measures the short-term stability of Earth's geomagnetic field (updated every 3 hours), while the A-index measures its long-term stability over a 24-hour period. Both are used to assess current HF propagation quality — low values indicate quiet conditions favorable for HF communication; high values indicate disturbed conditions.

Solar events cause propagation disturbances on different timescales. The ultraviolet and X-ray radiation from a solar flare travels at the speed of light and reaches Earth in approximately 8 minutes, immediately affecting ionospheric propagation — particularly through sudden ionospheric disturbances (SIDs) that disrupt signals on lower frequencies more than higher frequencies. A coronal mass ejection (CME), by contrast, consists of charged particles that travel much more slowly and take 15 hours to several days to reach Earth after a solar event. The 26–28 day periodic variation in HF conditions is caused by the rotation of the Sun's surface layers around its axis — active regions rotate into and out of Earth-facing positions on that timescale. Charged particles from solar coronal holes disturb HF communication when they arrive at Earth.

G3B: MUF, LUF, and Propagation

The Maximum Usable Frequency (MUF) is the highest frequency that will be refracted back to Earth between two specific points. The Lowest Usable Frequency (LUF) is the lowest frequency at which a usable signal can be maintained between two specific points — below the LUF, signals are attenuated before reaching the destination. When the frequency is between the LUF and the MUF, signals are refracted back to Earth and communication is possible. The frequency just below the MUF provides the least attenuation for long-distance skip propagation.

When the LUF exceeds the MUF — which can happen during disturbed conditions — propagation via ordinary skywave communication is not possible over that path. The MUF is affected by all of the following factors: path distance and location, time of day and season, and solar radiation and ionospheric disturbances.

Determining current propagation on a desired band can be accomplished by using a network of automated receiving stations on the internet (such as the Reverse Beacon Network or PSKreporter) to see where your transmissions are actually being received. This gives real-time, operational data about actual propagation conditions.

When skywave signals arrive at a location via both short-path and long-path simultaneously, a slightly delayed echo may be heard, because the long-path signal travels approximately 40,000 km further and arrives a fraction of a second later. The maximum distance covered in one F2 region hop is approximately 2,500 miles; the E region maximum hop is approximately 1,200 miles. During summer, lower HF frequencies (80 and 160 meters) experience high levels of atmospheric noise or static.

G3C: Ionosphere and NVIS

The ionosphere consists of several regions at different altitudes. The D region is the lowest, closest to Earth's surface, and is present only during daylight hours — it is also the most absorbent region for signals below 10 MHz. This is why long-distance communication on 40, 60, 80, and 160 meters is more difficult during the day: the D region absorbs these lower-frequency signals before they can reach the higher refracting layers. The E region is above the D region and can support propagation up to about 1,200 miles per hop. The F2 region is the highest ionospheric layer and supports the longest skip distances — up to approximately 2,500 miles per hop — precisely because of its altitude.

The critical frequency is the highest frequency that will be refracted back to Earth at a given incidence angle. The critical angle is the highest takeoff angle from which a radio wave will still be returned to Earth under specific ionospheric conditions — above this angle, the wave passes through the ionosphere and is lost to space.

HF scatter propagation allows signals to reach the skip zone — the area between the end of the ground wave and the nearest point where a sky wave returns to Earth. Scatter signals are generally weak because only a small fraction of the transmitted signal energy is scattered into the skip zone. They also have a characteristic fluttering sound and may be distorted because energy arrives via several different paths simultaneously, each with a slightly different delay.

Near vertical incidence skywave (NVIS) propagation is a technique that uses high elevation angles — nearly straight up — to achieve short-distance MF or HF coverage. By transmitting at a high angle, the signal goes straight up into the ionosphere and comes back down over a wide area within a few hundred miles. NVIS is particularly useful for emergency communications in mountainous or forested terrain where line-of-sight VHF is impractical, providing coverage in areas that are in the skip zone of normal HF and beyond the range of ground wave.

Study These Topics