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G3 Radio Wave Propagation

All 37 pool questions in this section, each with its answer, from the General Class study guide. Read the notes, cover the answers, work the questions, and see how fast they stick.

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3 of your 35 exam questions · 3 groups (G3A–G3C) · 37 questions in the pool

The most genuinely interesting subelement, and the one where understanding beats memorizing by the widest margin. Build the mental model of the ionosphere once and most answers fall out of it.

The core model. The Sun ionizes the upper atmosphere. Ionized gas refracts radio waves back toward Earth — that is skip. Four regions, stacked bottom to top: D (absorbs below ~10 MHz, daytime only), E (~1,200 miles per hop), F1, and F2 (highest, ~2,500 miles per hop, the workhorse of HF (high frequency) DX (long-distance communication)). More solar activity means more ionization, which means the higher bands open.

Table of Contents

G3A — Sunspots and solar radiation; geomagnetic field and stability indices

One exam question comes from this group. 14 questions in the pool.

The indices. Two pairs that get confused:

Index Measures
Sunspot number Higher means better propagation at higher frequencies
Solar flux index Solar radiation at a wavelength of 10.7 cm
K-index Short-term stability of Earth's geomagnetic field
A-index Long-term stability of Earth's geomagnetic field

Both K and A measure Earth's field, not the Sun's. The only difference is the time scale — K is Kwick.

Three different delays, and this is the most-missed set in G3:

Event Time to reach us
Solar flare (UV and X-rays, at light speed) 8 minutes
Coronal mass ejection (particles, much slower) 15 hours to several days
Solar rotation cycle 26–28 days

The 26–28 day cycle happens because the Sun's surface layers rotate, bringing the same active regions back around.

Disturbances. A sudden ionospheric disturbance hurts lower frequencies more. A geomagnetic storm is a temporary disturbance in Earth's geomagnetic field and it degrades high-latitude HF paths — but its upside is auroras that reflect VHF signals (VHF = very high frequency). Coronal hole particles disturb HF (high frequency). At solar minimum, 15, 12, and 10 meters are least reliable, while 20 meters works worldwide in daylight at any point in the cycle.

All 14 pool questions for G3A

G3A01

How does a higher sunspot number affect HF propagation?

G3A02

What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?

G3A03

Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth?

G3A04

Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?

G3A05

What is the solar flux index?

G3A06

What is a geomagnetic storm?

G3A07

At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours?

G3A08

How can a geomagnetic storm affect HF propagation?

G3A09

How can high geomagnetic activity benefit radio communications?

G3A10

What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?

G3A11

How long does it take a coronal mass ejection to affect radio propagation on Earth?

G3A12

What does the K-index measure?

G3A13

What does the A-index measure?

G3A14

How is long distance radio communication usually affected by the charged particles that reach Earth from solar coronal holes?

G3B — Maximum and Lowest Usable Frequency; short and long path; ionospheric refraction

One exam question comes from this group. 12 questions in the pool.

The window. Two frequencies bracket what works on a path right now. The MUF is the maximum usable frequency between two specific points; the LUF (lowest usable frequency) is the lowest. Both definitions are point-to-point — "during a 24-hour period" is the distractor on both questions.

Frequency Result
Between LUF and MUF Refracted back to Earth — it works
Above the MUF Passes through into space
Below the LUF Attenuated before arriving
LUF above MUF No skywave propagation possible on that path

The best frequency to use is just below the MUF, because that path takes the shallowest trip through the absorbing lower layers.

Determining conditions practically: use a network of automated receiving stations on the internet to see where your own transmissions land. Reverse beacon networks and PSK (phase-shift keying) Reporter, not the A-index and not listening for your own echoes.

Hop distances: E region 1,200 miles, F2 region 2,500 miles. Signals arriving by both short and long path produce a slightly delayed echo. And the low bands in summer suffer from high atmospheric noise and static.

All 12 pool questions for G3B

G3B01

What is a characteristic of skywave signals arriving at your location by both short-path and long-path propagation?

G3B02

What factors affect the MUF?

G3B03

Which frequency will have the least attenuation for long-distance skip propagation?

G3B04

Which of the following is a way to determine current propagation on a desired band from your station?

G3B05

How does the ionosphere affect radio waves with frequencies below the MUF and above the LUF?

G3B06

What usually happens to radio waves with frequencies below the LUF?

G3B07

What does LUF stand for?

G3B08

What does MUF stand for?

G3B09

What is the approximate maximum distance along the Earth’s surface normally covered in one hop using the F2 region?

G3B10

What is the approximate maximum distance along the Earth’s surface normally covered in one hop using the E region?

G3B11

What happens to HF propagation when the LUF exceeds the MUF?

G3B12

Which of the following is typical of the lower HF frequencies during the summer?

G3C — Ionospheric regions; critical angle and frequency; HF scatter; NVIS

One exam question comes from this group. 11 questions in the pool.

Two "critical" terms, both meaning highest. The critical frequency is the highest frequency refracted back to Earth at a given angle. The critical angle is the highest takeoff angle that returns a wave to Earth. If you remember that both are "highest," you get both questions even if you mix up which is which.

The D region explains the low bands. It is the closest region to Earth, it is the most absorbent below 10 MHz during daylight, and it is why 40, 60, 80, and 160 meters are hard in the daytime. It vanishes after dark, which is when those bands come alive.

F2 gives the longest skip because it is the highest — shallower geometry, longer hop. Not density, not Doppler.

Scatter fills the skip zone, and every scatter question follows from it being multipath: signals flutter, they sound distorted because energy arrives by several different paths, and they are weak because only a small part of the energy is scattered in there.

NVIS (near vertical incidence skywave) is short-distance MF or HF propagation at high elevation angles (MF = medium frequency; HF = high frequency) — fire nearly straight up, drop signals back down in a circle around you, fill in the skip zone. The matching antenna is in G9D01.

All 11 pool questions for G3C

G3C01

Which ionospheric region is closest to the surface of Earth?

G3C02

What is meant by the term “critical frequency” at a given incidence angle?

G3C03

Why is skip propagation via the F2 region longer than that via the other ionospheric regions?

G3C04

What does the term “critical angle” mean, as applied to radio wave propagation?

G3C05

Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?

G3C06

What is a characteristic of HF scatter?

G3C07

What makes HF scatter signals often sound distorted?

G3C08

Why are HF scatter signals in the skip zone usually weak?

G3C09

What type of propagation allows signals to be heard in the transmitting station’s skip zone?

G3C10

What is near vertical incidence skywave (NVIS) propagation?

G3C11

Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?

Bottom line for G3

D absorbs, E hops 1,200 miles, F2 hops 2,500 miles and is highest. K is Kwick (short-term), A is long-term, both measure Earth's field. Solar flux is measured at 10.7 cm. Flare 8 minutes, CME (coronal mass ejection) 15 hours to days, solar rotation 26-28 days. Work just below the MUF (MUF = maximum usable frequency). LUF above MUF means no path. (LUF = lowest usable frequency) Both "critical" terms mean highest. Scatter fills the skip zone, weak and fluttery. NVIS (near vertical incidence skywave) is high-angle short-range HF (high frequency).

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