G3: Radio Wave Propagation

The Sun is the engine behind everything in this section. The more energy the Sun pours onto the upper atmosphere, the more charged that ionosphere becomes, and the better it bends high-frequency signals back to Earth. So this group is about reading the Sun's mood. Good news: the test mostly wants you to know which way each number pushes propagation, not to calculate anything.

Sunspots: the Sun's freckles

If you safely look at the Sun through a proper filter, you sometimes see dark blotches on its surface. Those are sunspots, cooler, magnetically active patches. They matter because more sunspots come along with more of the energy that charges up the ionosphere. We track them with a count called the sunspot number.

The key fact: a higher sunspot number generally means a greater probability of good propagation at higher frequencies. In plain words, lots of sunspots is great news for the upper shortwave bands (think 15, 12, and 10 meters), because a strongly charged ionosphere can bend even those high frequencies back down to Earth. When sunspots are scarce, those same upper bands often go quiet.

The solar flux index

Sunspots can be hard to count consistently, so scientists also measure the Sun's energy with a radio. The solar flux index is a measure of solar radiation at a wavelength of 10.7 centimeters. (A "wavelength" is the physical length of a radio wave; 10.7 cm corresponds to a frequency near 2.8 GHz.) You do not need to know the physics, just the definition: the solar flux index measures the Sun's radio energy at 10.7 centimeters, and a higher number, like a higher sunspot count, points to better high-band conditions.

The least reliable bands when the Sun is quiet

During stretches of low solar activity (few sunspots, low solar flux), the ionosphere is weakly charged and cannot reliably bend the highest frequencies back down. So the least dependable bands for long-distance work during a solar lull are 15 meters, 12 meters, and 10 meters. Remember it this way: when the Sun is sleepy, the highest bands suffer first.

The 20-meter band: the dependable workhorse

One band stands out as remarkably steady. At what point in the solar cycle does 20 meters usually support worldwide propagation during daylight hours? At any point. Whether the Sun is busy or quiet, 20 meters tends to be open somewhere in the daytime. That is why hams reach for 20 meters as a reliable long-distance band year-round.

Solar flares: a fast punch of radiation

Sometimes the Sun erupts with a sudden burst of energy called a solar flare. A flare blasts out ultraviolet light and X-rays, which are forms of light, so they travel at the speed of light. How long does that increased ultraviolet and X-ray radiation take to reach Earth and affect propagation? About 8 minutes (the same time sunlight itself takes to reach us). When that radiation arrives, it can cause a sudden ionospheric disturbance, a temporary upset to the ionosphere. Its effect is lopsided: it disrupts signals on lower frequencies more than those on higher frequencies. So during such a disturbance the low bands suffer most while higher bands hold up better.

Coronal mass ejections and coronal holes: slow clouds of particles

Besides light, the Sun also hurls out actual matter, streams of charged particles. A big eruption of such particles is a coronal mass ejection (CME). Because particles are far slower than light, a CME takes 15 hours to several days to reach Earth and affect radio propagation. (Contrast that with the roughly 8 minutes for a flare's light.)

The Sun also has coronal holes, openings that let a steady wind of charged particles escape toward Earth. When those particles arrive, HF (shortwave) communication is disturbed. So both CMEs and coronal holes send us particles that tend to rough up the shortwave bands.

The 27-day rhythm

You may notice propagation conditions repeat on a roughly monthly cycle. What causes HF conditions to vary periodically in a 26- to 28-day pattern? The rotation of the Sun's surface layers around its axis. The Sun spins about once a month, so an active region (or a coronal hole) facing us today swings back around to face us again about 27 days later, bringing similar conditions with it.

Earth's magnetic field gets stormy too

Earth has its own magnetic field, the same one a compass needle follows. The streams of particles from the Sun can shake it up. A geomagnetic storm is a temporary disturbance in Earth's geomagnetic field. ("Geomagnetic" just means "Earth's magnetic.") How does such a storm affect shortwave? It tends to degrade high-latitude HF propagation, meaning paths that cross near the poles (toward the far north or south) get hit hardest.

But there is a silver lining. High geomagnetic activity can also create auroras (the northern and southern lights) that can reflect VHF signals. So an event that wrecks shortwave near the poles can briefly hand VHF operators a fun bonus: bouncing signals off the glowing aurora.

Reading the dials: the A-index and the K-index

Two numbers tell you how calm or stormy Earth's magnetic field is right now. They sound alike, so pin them down:

• The K-index measures the short-term stability of Earth's geomagnetic field. Think of K as the "right now" reading, updated every few hours.
• The A-index measures the long-term stability of Earth's geomagnetic field. Think of A as the "overall day" summary.

Memory trick: K is "quick" (short-term), A is the "average" (long-term). For both, lower numbers mean a calmer field and usually better, more stable propagation.

Now we get practical. Given the Sun's current mood, which frequency should you actually use to reach a particular place? The answer lives between two limits called the MUF and the LUF. Master those two and you understand most of day-to-day band picking.

How the ionosphere bends a signal

First, the core mechanism. When a shortwave signal travels up to the ionosphere at the right frequency, the charged layer gradually bends its path until it heads back down to the ground far away. The proper word for that gentle bending is refraction (the same word for how a straw looks bent in a glass of water). We often loosely say the signal is "reflected" like off a mirror, but technically it is refracted back to Earth. Keep that word in mind, the test uses it.

The MUF: the high ceiling

MUF stands for the Maximum Usable Frequency for communications between two points. It is the highest frequency that will still get bent back down to Earth on a given path right now. Go above the MUF and your signal punches straight through the ionosphere and out into space instead of coming back, so the far station never hears it.

What sets the MUF? Lots of things, which is why "all of these choices are correct" is the answer when the test asks what factors affect the MUF: the path between the stations, the time of day and season, and the state of the ionosphere (driven by the Sun) all play a part.

Here is a pro tip the test rewards. Which frequency has the least attenuation (the least weakening or loss) for long-distance skip? The answer is just below the MUF. Operating near the top of the usable range means your signal takes the most efficient path through the ionosphere, so it arrives strongest. Hams chasing distance aim just under the MUF.

The LUF: the low floor

LUF stands for the Lowest Usable Frequency for communications between two specific points. It is the lowest frequency that will still make it through. Why is there a floor at all? Because the lower the frequency, the more the ionosphere (especially its bottom layer) soaks up, or absorbs, the signal. Drop below the LUF and your signal gets attenuated (weakened) before reaching the destination, fading away before it arrives.

The usable window: between LUF and MUF

Put the two together and you get a window. A radio wave with a frequency below the MUF and above the LUF is refracted back to Earth, that is the sweet spot where sky-wave communication works. Pick a frequency inside that window for the path you want, and you are in business.

Now the unhappy case. What happens when the LUF actually exceeds the MUF, when the floor rises above the ceiling? Then the window slams shut: propagation via ordinary sky-wave communications is not possible over that path. There is simply no usable frequency left. (This often happens at night on long paths or during disturbed conditions.)

Short path and long path

Between any two points on the globe there are two directions you could send a signal: the short way around the Earth, called the short path, and the long way around, called the long path. Sometimes your signal arrives both ways at once. What is the telltale sign? A slightly delayed echo might be heard. The long-path signal travels much farther, so it shows up a fraction of a second late, producing a faint echo of the same transmission.

How far is one hop?

A signal that bounces off the ionosphere once and comes back down has made one hop. How far a single hop reaches depends on which layer did the bending. The two numbers to memorize:

• Using the high F2 region: about 2,500 miles in one hop.
• Using the lower E region: about 1,200 miles in one hop.

Memory trick: the higher the mirror, the longer the throw, so the high F2 layer gives the longer 2,500-mile hop. (We will meet these layers properly in group G3C.)

Checking conditions before you call

How can you tell, right now, whether a band is actually open from your station? A great modern tool: use a network of automated receiving stations on the internet to see where your transmissions are being received. These networks (often called "reverse beacon" or "spotting" networks) listen for your signal worldwide and post a map of who heard you. It is like dropping a pebble and watching the internet tell you exactly where the ripples reached.

A seasonal nuisance: summer static

One last practical note. What is typical of the lower HF bands during summer? High levels of atmospheric noise (static). Summer thunderstorms, even distant ones, fill the low bands with crackling and hiss, which can bury weak signals. So on a summer evening the lower bands may be open yet still hard to use because of all that natural noise.

The ionosphere is not one smooth blanket, it has layers, stacked like floors in a building. Knowing what each layer does, and the angles and frequencies that make skip work, ties this whole section together. We will also cover two special tricks: scatter and NVIS.

The layers, from bottom to top

Scientists label the ionosphere's layers (called regions) with letters. From lowest to highest:

• The D region is closest to the surface of Earth, the lowest layer. It does not bend signals back; instead it mostly absorbs them.
• The E region sits above the D region.
• The F region is the highest. During the day it splits into two parts, F1 (lower) and F2 (higher); at night they merge back into a single F layer.

The troublesome D region

The low D region only exists in daylight, and it is a signal-eater. It is the most absorbent of signals below 10 MHz during daylight hours. This single fact explains a classic frustration: why is long-distance work on the 40-, 60-, 80-, and 160-meter bands harder during the day? Because the D region absorbs signals at those frequencies during daylight hours. Those low bands soak up in the daytime D region and only come alive after sunset, when the D region fades away and signals can reach the higher reflecting layers.

Why F2 gives the longest skip

Of all the layers, the F2 region produces the longest single-hop distances. Why is skip via the F2 region longer than via the other regions? Because it is the highest. A higher reflecting layer lets the signal angle out farther before it returns to Earth, the same way a higher bank shot on a pool table travels a longer distance. That is why F2 gives that roughly 2,500-mile hop from group G3B.

Critical frequency and critical angle

Two "critical" terms sound similar but mean different things. Take them one at a time.

• The critical frequency (at a given incidence angle) is the highest frequency that is refracted back to Earth. Send a signal straight up: below the critical frequency it comes back down, above it the signal escapes to space. It is the ceiling for that straight-up test, and it helps predict the MUF for slanted paths.
• The critical angle is the highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. ("Takeoff angle" is how steeply your antenna launches the signal toward the sky.) Aim steeper than the critical angle and the signal goes straight through the ionosphere instead of coming back. So for normal long-distance skip you want a low takeoff angle, below the critical angle.

Memory tip: critical FREQUENCY is about how high in frequency you can go; critical ANGLE is about how steep an angle you can launch. Both are "highest that still comes back."

HF scatter: filling in the dead zone

When you make a sky-wave contact, there is a ring around your station that is too far for your ground wave yet too close for your skip to land, signals seem to leap right over it. That ring is the skip zone (sometimes called the "dead zone"). Surprisingly, you can sometimes still hear stations inside that zone, thanks to scatter.

Scatter is the type of propagation that allows signals to be heard in the transmitting station's skip zone. Instead of one clean bounce, a small bit of the signal's energy gets scattered in many directions by irregularities in the ionosphere, and some of it trickles back down into that otherwise-dead ring. Its fingerprints:

• A characteristic of HF scatter is that signals have a fluttering sound, a wavery, watery quality.
• Scatter signals often sound distorted because the energy is scattered into the skip zone through several different paths, arriving slightly out of step with itself.
• Scatter signals in the skip zone are usually weak because only a small part of the signal energy is scattered into the skip zone. Most of the signal carries on as normal skip; only crumbs fall into the dead ring.

NVIS: bouncing straight up for medium-range coverage

Sometimes you do not want extreme distance, you want to cover everyone within a few hundred miles, including the folks in that skip zone, like during a regional emergency. The trick is near vertical incidence skywave, or NVIS.

NVIS is short-distance MF or HF propagation at high elevation angles. ("MF" means medium frequency, the band just below HF; "high elevation angle" means you aim the signal nearly straight up.) You fire the signal almost vertically, it bends off the ionosphere directly overhead, and it rains back down in a wide circle around you with no skip zone. The result is reliable coverage out to a few hundred miles, perfect for blanketing a whole region. NVIS works best on the lower HF bands (like 40 and 80 meters), where signals are steep enough to come straight back down.