E3: Radio Wave Propagation

This first group is a grab-bag of specialized propagation modes, plus a little physics about what an electromagnetic wave really is. Let's start with the most jaw-dropping mode of all.

EME: bouncing your signal off the Moon

EME stands for Earth-Moon-Earth, and it is exactly what it sounds like: you aim a big antenna at the Moon, transmit, your signal travels roughly a quarter-million miles up, reflects off the Moon's surface, and comes back down to be heard by someone else on Earth. Hams also nickname this moonbounce. Because the Moon is so far away and reflects only a tiny fraction of what hits it, EME demands strong stations, but it works.

How far apart can two stations be and still hear each other this way? The answer is about 12,000 miles, as long as the Moon is "visible" (above the horizon) to both stations at the same time. Think of the Moon as a giant mirror in the sky: any two stations that can both see that mirror at once can use it to talk, and that geometry tops out around 12,000 miles of separation along the Earth's surface.

EME signals have a strange, distinctive wobble called libration fading. ("Libration" is the slow rocking and turning of the Moon as we see it; the Moon does not face us perfectly still.) Because the Moon's rough surface is slowly shifting, the many little reflections add up differently moment to moment, so an EME signal sounds like a fluttery, irregular fading. Picture sunlight glinting off rippling water, the sparkle dances unpredictably; libration fading is the audio version of that.

To make EME as easy as possible, you want the least path loss (the weakening of a signal as it travels). The Moon's distance from Earth changes over its orbit, and path loss is lowest when the Moon is closest to us. The point in the orbit where the Moon is nearest Earth is called perigee. So scheduling an EME contact when the Moon is at perigee gives you the least path loss. ("Peri-" means "near," a handy memory hook: perigee = nearest point.)

What a radio wave actually is

Let's pause for the physics the test wants. A radio wave is an electromagnetic wave, meaning it is made of two things traveling together: an electric field and a magnetic field. (A "field" is just a region where a force can be felt, like the invisible pull around a magnet.) Two facts to memorize:

• The electric field and the magnetic field are at right angles to each other (90 degrees apart, like the corner of a square).
• The wave itself travels at a right angle to both of those fields. So if the two fields make a flat "plus sign," the wave shoots straight out of the page, perpendicular to both.

How fast does it go? The speed of an electromagnetic wave through any material is set by the material's index of refraction. ("Refraction" is the bending of a wave as it passes into a different material; the "index of refraction" is just a number describing how much a material slows and bends a wave.) In empty space a wave travels at the speed of light, but in glass, water, or air it goes a bit slower, and the index of refraction is what determines that speed.

Polarization, including circular polarization

Polarization describes the direction the electric field points. If the electric field stays vertical, the wave is "vertically polarized"; horizontal, and it is "horizontally polarized." But there is a third, fancier kind. A circularly polarized wave is one whose electric and magnetic fields rotate as the wave moves forward, corkscrewing through space instead of staying flat. So circularly polarized electromagnetic waves are simply waves with rotating electric and magnetic fields. This corkscrew property is wonderful for satellite and EME work, because it does not matter how the far antenna is tilted, a rotating wave couples into it either way.

Daily change in ionospheric propagation

The Sun charges up the ionosphere during the day and lets it relax at night, so the highest frequency that will bounce back, the MUF (Maximum Usable Frequency), rises and falls on a daily cycle. As night falls on a path, the MUF drops. If you are in the middle of a long-distance HF contact and the MUF for that path decreases because of darkness, the fix is to switch to a lower HF band. Why? Because a falling MUF means the higher bands stop bouncing first; a lower frequency stays below the MUF and keeps reflecting. Think of the MUF as a ceiling that is slowly dropping, you simply duck to a lower band to stay underneath it.

Meteor scatter: talking on shooting stars

Every day, countless tiny meteors burn up in our atmosphere. When a meteor streaks in, it leaves behind a brief, glowing trail of ionized (electrically charged) air. That trail can reflect radio signals for a second or two before it fades, just long enough to squeeze in a quick contact. This mode is called meteor scatter.

Where in the atmosphere does that ionized trail form? At the E region of the ionosphere. (The ionosphere has layers labeled D, E, and F from lowest to highest; the E region sits in the middle, around 60 to 70 miles up, right where most meteors vaporize.) So when a meteor strikes the atmosphere, the linear ionized region it forms is in the E region.

Which frequencies work best for meteor scatter? The range 28 MHz to 148 MHz, which covers the 10-meter, 6-meter, and 2-meter bands. These VHF-ish frequencies are high enough to normally pass through the ionosphere, so a meteor trail gives them a rare, brief chance to be reflected back.

Microwave ducting and tropospheric scatter

Down low in the atmosphere, weather can create a special path for VHF, UHF, and microwave signals. Sometimes a layer of warm air sits over a layer of cool, moist air, forming an invisible tube, called a duct, that traps and guides radio waves for long distances, like water flowing down a pipe. Where do these ducts most often form? Over large bodies of water, where the air just above the surface stays cool and humid under warmer air aloft. A typical range for tropospheric duct propagation of microwave signals is 100 miles to 300 miles. ("Tropospheric" refers to the troposphere, the lowest layer of the atmosphere where weather happens.)

Auroral propagation

When the Sun hurls charged particles at Earth, they can light up the sky near the poles with the shimmering glow of an aurora (the Northern or Southern Lights). That same charged region high in the atmosphere can also reflect radio signals, a mode called auroral propagation. What triggers it? A severe geomagnetic storm, meaning a big disturbance of Earth's magnetic field caused by the Sun. (We will dig into geomagnetic storms in group E3C.)

One quirk: the aurora is a churning, unstable reflector, so it badly distorts a signal, smearing voice into mush. Because of that, the best mode for auroral propagation is CW (Morse code). A simple on-off beep survives the distortion far better than voice does, so even when the aurora garbles everything, you can still copy the dots and dashes.

This group continues the tour with more long-distance HF modes: signals that vault across the equator, signals that travel the "long way" around the planet, the surprise summer band openings called sporadic-E, the efficient chordal hop, and the down-to-earth ground wave.

Transequatorial propagation (TEP)

Transequatorial propagation, abbreviated TEP, is a mode where signals cross the equator unusually well. ("Trans-" means "across," so trans-equatorial literally means "across the equator.") It happens between two stations that are roughly the same distance on opposite sides of the geomagnetic equator, along a path that runs roughly north-south. Specifically, TEP is most likely between points separated by 2,000 to 3,000 miles over a path perpendicular to (crossing straight over) the geomagnetic equator. The geomagnetic equator is the magnetic version of the equator, a line tied to Earth's magnetic field rather than its geography.

The approximate maximum range for TEP is about 5,000 miles. And the best time of day for it is the afternoon or early evening, after the Sun has had all day to charge up the ionosphere over the tropics. So picture two stations a couple thousand miles apart on either side of the equator, getting on the air in late afternoon, and reaching each other when the bands otherwise seem dead.

Ordinary and extraordinary waves

Here is a subtle bit of physics. When a single radio wave enters the ionosphere, Earth's magnetic field actually splits it into two separate waves that travel a little differently. These are nicknamed the "ordinary" wave and the "extraordinary" wave. The exam definition: they are independently propagating, elliptically polarized waves created in the ionosphere. Let's translate. "Independently propagating" means the two halves go their own separate ways once split. "Elliptically polarized" means their fields rotate in a stretched, oval pattern (a cousin of the circular polarization from group E3A). The one-sentence takeaway: the ionosphere can split one incoming wave into two differently behaving waves.

Long-path propagation

Between any two points on Earth there are two great-circle routes: the short way and the long way around the globe. Sometimes a signal arrives best by going the long way, all the way around the planet the opposite direction. This is long-path propagation. It is most frequent on the 40-meter and 20-meter bands, the classic long-haul HF bands. (You can sometimes hear a station with a faint echo, where you receive both the short-path and long-path signals a fraction of a second apart.)

160 meters and the value of darkness

The 160-meter band (often called "top band") is a nighttime, long-distance band. Its signals are heavily absorbed by the Sun-charged lower ionosphere during the day, so the path that best supports long-distance work on 160 meters is one that is entirely in darkness. The absorbing daytime layer (the D region) relaxes after dark, opening the band for distant contacts. Rule of thumb: on the lowest bands, follow the night.

Elevation angle and hop distance

When you launch a signal at the ionosphere, the angle above the horizon matters a lot. Lowering the elevation angle (aiming the signal flatter, closer to the horizon) makes each ionospheric bounce, called a hop, cover more ground. So lowering a signal's transmitted elevation angle means the distance covered by each hop increases. Picture skipping a stone: throw it flat and low across the water and each skip carries it much farther than a steep, plunking throw. A low takeoff angle is exactly why DX (long-distance) antennas are designed to radiate close to the horizon.

Chordal-hop propagation

Normally a multi-hop HF signal bounces ionosphere-to-ground-to-ionosphere-to-ground, losing a chunk of strength each time it reflects off the ground. Chordal-hop propagation is a more efficient trick: the signal makes successive refractions in the ionosphere without an intermediate reflection from the ground in between. ("Refraction" here is the ionosphere bending the wave back down.) Picture the signal skimming along under the ionosphere like a ball rolling along the inside of a curved bowl, instead of bouncing down and back up. The named effect is that the signal experiences less loss compared to ordinary multi-hop propagation, which uses the Earth as a reflector. Skipping those lossy ground bounces means the signal arrives stronger.

Sporadic-E propagation

Sporadic-E (often written "Es") is one of the most exciting surprise modes in ham radio. Patches of intensely ionized air form in the E region of the ionosphere, seemingly at random, and briefly reflect VHF signals that would normally sail right through. When it happens, the 6-meter and even 2-meter bands suddenly come alive with distant stations. The name fits: "sporadic" means unpredictable and occasional.

• Time of year: sporadic-E is most likely around the solstices, and especially the summer solstice. (A "solstice" is the longest or shortest day of the year; summer is the prime season for Es.)
• Time of day: it is most likely between sunrise and sunset, that is, during the daylight hours.

So if you want to catch a sporadic-E opening on 6 meters, the odds are best on a long summer day.

Ground-wave propagation

Not every signal goes up to the sky. A ground wave hugs the surface of the Earth and follows its curvature for a modest distance, no ionosphere involved. Two facts the test wants:

• How range changes with frequency: as the frequency goes up, the maximum ground-wave range goes down. Higher frequencies are absorbed by the ground more quickly, so they do not crawl as far. (This is why AM broadcast stations on the low frequencies can cover a whole region by ground wave.)
• Polarization: ground-wave propagation works with vertical polarization. A vertical electric field stays in step with the conducting ground and survives the trip; a horizontal one would be shorted out by the Earth almost immediately.

The last group of E3 is the practical, modern side of propagation: the numbers and tools that tell you what the bands are doing right now and what they will do soon. Most of this comes down to one thing, the Sun, and how its moods ripple out to affect our radios.

Solar flares and short-term blackouts

A solar flare is a sudden, intense burst of energy from the Sun's surface. When a big flare erupts, it floods the daylight side of Earth with radiation that drastically increases absorption in the lower ionosphere, and HF signals can simply vanish. So the cause of short-term radio blackouts is solar flares. The blackout typically lasts minutes to a few hours, however long the flare's effects persist.

Solar flares are graded by strength, and the test wants the top of the scale. The classes run (from weaker to stronger) B, C, M, and X, so Class X indicates the greatest solar flare intensity. ("X" for extreme, an easy way to remember the most powerful class.)

The A-index and K-index

Two numbers describe how disturbed Earth's magnetic field is, which directly affects propagation. The K-index is a short-term (every few hours) measure, and the A-index is a daily summary. When either one rises, it signals an increasing disturbance of the geomagnetic field ("geomagnetic" means Earth's magnetism). A rising A or K is bad news for HF: the more disturbed the field, the worse the absorption, especially on paths near the poles.

Speaking of the poles: which signal path suffers the most absorption when the A-index or K-index is high? A path that goes through the auroral oval, the ring-shaped region around each magnetic pole where auroras form. Signals that must cross that turbulent, energized zone get badly absorbed during disturbed conditions, so polar paths are the first to fade out in a geomagnetic storm.

The Bz of the interplanetary magnetic field

The Sun's magnetic field stretches out through the solar system, carried along by the solar wind; out near Earth it is called the interplanetary magnetic field. The piece we care about is its north-south component, written Bz (said "B sub z"). So the value of Bz represents the north-south strength of the interplanetary magnetic field.

Why does this matter? Because of how it lines up with Earth's own field. When Bz points southward, it is opposite to Earth's field and "unlocks the gate," making it far easier for the Sun's charged particles to pour in and stir up disturbed conditions. So a southward Bz increases the likelihood of disturbed (stormy) geomagnetic conditions. Northward Bz, by contrast, tends to keep those particles out.

Naming the storms: the G-scale

Geomagnetic storms get their own intensity scale, labeled G1 through G5 from minor to extreme. The strongest, an extreme geomagnetic storm, is called G5. (Notice the pattern: flares use X for their extreme, storms use G5 for theirs.)

The 304A solar parameter

One handy way to gauge the Sun's output is to watch its ultraviolet light. The 304A parameter measures UV emissions at a wavelength of 304 angstroms, which closely tracks the solar flux index (a common measure of how active the Sun is). (An "angstrom" is a tiny unit of length used for light wavelengths.) In short, 304A is a UV-light stand-in for solar activity: higher numbers generally mean the higher HF bands are more likely to be open.

Sudden HF noise: a CME or flare has arrived

Sometimes the background hiss across a huge stretch of the HF spectrum suddenly jumps up all at once. What does that mean? It indicates that a coronal mass ejection (CME) impact or a solar flare has occurred. A coronal mass ejection is an enormous cloud of charged particles blasted off the Sun; when it (or a flare's radiation) hits Earth, it can wash radio noise across the bands. So a sudden, broad rise in HF noise is a real-time alarm that the Sun just threw something at us.

The VHF/UHF radio horizon

At VHF and UHF, signals travel almost in a straight line, but not quite, because the atmosphere bends them slightly downward, letting them reach a little past the visible horizon. The radio horizon (how far a VHF/UHF signal effectively reaches) is therefore about 15 percent farther than the geographic (visual) horizon. So whatever distance you can see to the horizon, your radio can reach roughly 15 percent beyond it. This small bonus is why VHF/UHF coverage maps stretch a bit past line-of-sight.

Prediction software and reporting networks

Finally, the modern tools. VOACAP is a well-known piece of software that models HF propagation, predicting which bands will be open between two points at a given time. (The name comes from "Voice of America Coverage Analysis Program," but you just need to know it models HF propagation.)

And to see what is happening live, hams use automated propagation reporting networks (such as the Reverse Beacon Network and PSK Reporter) that listen for signals and post who is hearing whom. The type of data these networks report is digital-mode and CW signals, because those modes are easy for a computer to decode and log automatically. So between VOACAP for forecasts and the reporting networks for live spotting, you can know the band conditions before you even key up.