T3: Radio Wave Propagation

One signal can take many roads at once

Imagine you shout across a big empty gym. Your friend hears your voice come straight across the room, but a split second later they also hear it again as an echo bouncing off the back wall, and maybe a third time off a side wall. Your one shout reached them by several different roads, all at once.

A radio wave behaves the exact same way. Your signal can travel straight to the other station and, at the very same time, bounce off a hill, a tall building, a metal roof, or even off the sky. When all those copies of your signal arrive, the receiver simply adds them together. This "many roads at once" idea is the secret hiding behind almost every strange effect in this group, so hold onto that gym-echo picture.

Multipath: when copies help or hurt each other

The proper name for "the same signal arriving by more than one road" is multipath (it literally means "many paths"). Here is the part that makes it interesting. A radio wave is a wiggle, an up-and-down motion just like a ripple on the pond. When two copies of that wiggle arrive and both happen to be going "up" at the same instant, they stack on top of each other and make a bigger, stronger signal. But if one copy is going "up" while the other is going "down," they fight and cancel each other out, leaving you with a weaker signal, or sometimes almost nothing at all.

This explains a favorite exam question: why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet? The answer is that multipath propagation cancels or reinforces signals. Moving the antenna even a little changes how long each road is. A spot that happened to be a "cancel" can suddenly turn into a "stack," and the other way around. That is exactly why nudging an antenna a foot or two can take a signal from noisy and broken to crystal clear, almost like magic.

Multipath also makes a mess of computer data sent over the radio. The echoes smear the tiny signal pulses into one another, so the receiver gets confused about which is which. That means when data is sent over a multipath path, the error rates are likely to increase — more mistakes creep in, just like trying to read a note that has been printed twice, slightly out of line, on the same piece of paper.

"Picket fencing": the flutter you hear in a moving car

Now picture yourself riding in a car while using a radio. As the car rolls down the road, the antenna is constantly sliding into new spots in that "stack here, cancel there" pattern. So the signal jumps up and down very fast, making a rapid chopping or fluttering sound. Hams have a name for it: picket fencing, which means rapid flutter on mobile signals due to multipath propagation. The name comes from the "brrrrrt" noise you would make by dragging a stick along a picket fence as you walk past it — that fast, choppy rattle is just what the signal sounds like.

Slow fading from the sky

Signals that travel by bouncing off the sky (off the ionosphere, which we will meet soon) also rise and fall, but much more slowly. The sky layer is always gently shifting and rippling, so the number of roads and how long they are keeps changing. That makes the signal drift slowly louder, then softer, then louder again. The likely cause of this irregular fading of signals propagated by the ionosphere is the random combining of signals arriving via different paths. It is the very same multipath idea you just learned, only happening far overhead and changing in a slower, gentler way.

Polarization: which way the wave "leans"

A radio wave has a direction that it leans, much like a jump rope can be wiggled either side-to-side or up-and-down. This lean has a name: polarization, which just means "the direction the wave is wiggling." The simple rule to remember is that the wave leans the same way the antenna points.

• An antenna standing straight up and down (a vertical antenna) makes a wave that leans up-and-down. We call that vertical polarization.
• An antenna lying flat across (a horizontal antenna) makes a wave that leans side-to-side. We call that horizontal polarization.

For the best signal, both stations should use the same lean, just like two kids turning a jump rope should agree to swing it the same way. Two test facts come straight out of this idea:

• For long-distance CW and SSB contacts on the VHF and UHF bands, the polarization normally used is horizontal. (CW is Morse code and SSB is a voice mode; both are quiet, weak-signal modes used to reach far away. Everyday repeater chatter is usually vertical, but this question is about that long-distance weak-signal work, and the answer there is horizontal.)
• If one station is set up vertical and the other is horizontal on a straight, line-of-sight VHF or UHF path, that mismatch is called cross-polarization. The effect of antenna cross-polarization is that the received signal strength is reduced. The waves do not line up, so a big chunk of the signal is simply wasted. Try to match the other station's lean.

The sky scrambles the lean

When a wave bounces off the ionosphere, it gets tumbled and twisted on the way, so it comes back down spinning instead of leaning one clean direction. Scientists call this spinning wave elliptically polarized (think of the lean as smeared into an oval rather than a straight line). Because the lean has been scrambled by the time the wave returns to Earth, carefully matching it no longer matters much. So one handy result of signals being elliptically polarized is that either vertically or horizontally polarized antennas may be used for transmission or reception on those sky paths. You do not have to fuss over matching.

Bouncing around an obstacle on purpose

What happens if a hill or a tall building sits right between you and a repeater you want to reach, blocking the direct road? If you are using a directional antenna (a beam, one that points its signal in a chosen direction), there is a clever trick. The best move is to try to find a path that reflects signals to the repeater. Aim your beam at a big building face or a hillside off to one side, and let your signal bounce off it and around the obstacle — exactly like bouncing a ball off a wall to reach a friend you cannot throw to in a straight line. (Cranking up power, increasing SWR, or flipping your polarization will not solve this one; finding a bounce path is the answer.)

Things that soak up your signal (absorption)

Some materials eat radio waves, quietly turning the signal into a tiny bit of heat. The word for this is absorption, which means "soaking up." As a rule, the higher the frequency, the more easily a signal gets soaked up by stuff in its way.

• Vegetation — that is trees, bushes, and leaves — absorbs UHF and microwave signals, leading to poor reception of weak signals. At these very short wavelengths, a wall of green leaves works almost like a sponge.
• At microwave frequencies, precipitation — rain, snow, and the like — can decrease your range, because those tiny waves get soaked up and scattered by water droplets.
• But down on the lower bands, water barely matters at all. Fog or rain has little effect on 10-meter and 6-meter band signals. Their waves are several meters long, far too big to be bothered by little raindrops.

The ionosphere: a mirror in the sky

High above the clouds, sunlight slams into the thin air and knocks some electrons loose from their atoms. This electrified region is called the ionosphere (the name comes from "ion," which is just an atom that has gained or lost an electron). The ionosphere can act like a giant mirror for certain radio waves. In fact, the region of the atmosphere that can reflect HF radio waves back down to Earth is the ionosphere. This sky-mirror is the whole reason a ham in Indiana can chat with someone in Australia, and we will lean on it heavily in group T3C. (Watch out for trick answers like "troposphere," "stratosphere," or the made-up word "electrosphere." The correct answer is the ionosphere.)

A radio wave is a cousin of light

Here comes a small surprise: a radio wave and the visible light from a flashlight are actually the same family of thing! Both are electromagnetic waves. That big word splits neatly into two pieces — "electro" (meaning electric) and "magnetic" (meaning magnet) — and that tells you exactly what a radio wave is made of. The two components of a radio wave are an electric field and a magnetic field, two invisible partners that travel side by side and lean on each other to keep moving.

These two partners point in directions that are perfectly square to one another. The relationship between the electric and magnetic fields of a wave is that they are at right angles to each other. (A right angle is the sharp square corner you see at the corner of this page, the kind an "L" makes.) So while the electric field wiggles, say, up-and-down, the magnetic field wiggles left-and-right, and together they push the wave forward. And remember polarization from the last group? The thing that decides a wave's polarization is the orientation of the electric field — whichever way that electric partner is pointing is the wave's lean.

How fast does it go? At the speed of light

A radio wave travels at the very fastest speed there is in the universe: the speed of light. When a radio wave moves through free space (empty air or the vacuum of space), its velocity is the speed of light. (The word velocity is just a slightly fancy word for speed.) That speed is about 300,000,000 meters every single second — that is three hundred million meters per second. To get a feel for how fast that is, light could zip all the way around the entire Earth roughly seven and a half times in just one second. That is almost too fast to imagine.

Here is a point the test loves to ask: every radio frequency travels at this same speed in free space. A microwave does not race ahead of a VHF wave, and a high note does not outrun a low note. So if a question asks "which of these frequencies travels at the highest velocity?", the answer is always that all radio frequencies travel at the same velocity. They tie, every time.

What is wavelength?

Go back to those pond ripples one more time. A wavelength is the distance from the top of one ripple to the top of the very next ripple — in other words, the length of one complete wiggle. We measure it in meters. (A meter is roughly the distance from your nose to your fingertips when you stretch your arm straight out to the side.)

Frequency is a different idea: it is how many of those wiggles go zooming past every second. We measure frequency in a unit called hertz, and most of the time in megahertz (written MHz), which means millions of wiggles per second.

Now think it through. Every wave covers the same speed-of-light distance in one second. So if a wave is wiggling very fast (a high frequency), each wiggle has to be squished short to fit. If it wiggles slowly (a low frequency), each wiggle gets to stretch out nice and long. That gives us the key relationship between wavelength and frequency:

Wavelength gets shorter as frequency increases.

The two are opposites. When one goes up, the other goes down, like the two ends of a seesaw. High frequency means short waves; low frequency means long waves.

The one bit of math to memorize

Here is the only real formula in this entire lesson, and it is genuinely easy:

Wavelength in meters = 300 divided by frequency in MHz

That is the whole thing. You take the number 300, and you divide it by the frequency. The exam writes it out in exactly those words: wavelength in meters equals 300 divided by frequency in megahertz. (Curious where the 300 comes from? It is just the speed of light, 300,000,000 meters per second, dressed up in friendly units. You do not need to know why for the test — only that the magic number is 300.)

Let us walk through it together, baby steps, no algebra and no stress:

• Step 1: Find the frequency in MHz. Let us say it is 150 MHz.
• Step 2: Type 300 into a calculator.
• Step 3: Press the divide button.
• Step 4: Type your frequency, 150.
• Step 5: Press equals. You get 2. So the wavelength is about 2 meters. Done!

Here are more easy examples you can check yourself on a calculator:

• At 300 MHz: 300 divided by 300 = 1. The wavelength is 1 meter. (Nice and tidy.)
• At 146 MHz (this is the 2-meter band): 300 divided by 146 comes out close to 2. So roughly a 2-meter wavelength — which is exactly why hams call it the 2-meter band.
• At 440 MHz (the 70-centimeter band): 300 divided by 440 is about 0.7. That is about 70 centimeters, since 1 meter equals 100 centimeters.
• At 3 MHz: 300 divided by 3 = 100. That is a very long 100-meter wavelength — see how a low frequency gives you a long wave?

Read the test wording carefully here, because the wrong answers are designed to trip you. Some say "multiply by 300," some flip it to "frequency divided by 300," and some use hertz instead of MHz. The one correct version is always 300, divided by the frequency in MHz, in that exact order.

Why bands have "meter" nicknames

Because of that simple math, every frequency has a matching wavelength, so hams hand out friendly nicknames to their bands using the wavelength. That is why, in addition to its frequency, an amateur band is also identified by its approximate wavelength in meters. You will constantly hear people say "the 2-meter band" or "the 70-centimeter band" instead of rattling off the exact numbers in MHz. It is just an easier, friendlier shorthand for the same thing.

The three frequency ranges: HF, VHF, and UHF

The whole radio spectrum is chopped up into named chunks. For the Technician exam you only need to know three of them, and there is a lovely pattern that makes them easy. Notice that each range is exactly ten times bigger than the one before it (3, then 30, then 300, then 3000):

• HF (High Frequency): 3 to 30 MHz
• VHF (Very High Frequency): 30 MHz to 300 MHz
• UHF (Ultra High Frequency): 300 to 3000 MHz

See the trick? The top number of one range is the bottom number of the next (30, then 300, then 3000), and you simply multiply by 10 each time you step up. A common trap on the test quietly swaps kHz (thousands) for MHz (millions). All three of these ranges are measured in MHz. If an answer says something like "30 to 300 kHz," that is the wrong unit and the wrong answer for VHF.

Line-of-sight and the radio horizon

VHF and UHF signals usually travel in straight lines, the way a flashlight beam shoots straight ahead. So most of the time they reach about as far as you could "see" if your eyes were sitting right up on the antenna — out to a line called the radio horizon. The horizon is that faraway line where the ground seems to meet the sky. Because the Earth is round, the surface eventually curves downward and falls away, so a straight beam keeps going straight and shoots off above the ground, which is why the signal cannot reach much past that point.

But here is a neat little detail. The radio horizon is more distant than the visual horizon (the one your eyes can see). Why? Because the atmosphere refracts radio waves slightly. To "refract" means "to bend a little." The air gently bends the radio wave so it hugs the curve of the Earth just a touch longer than a perfectly straight beam would, handing you a bit of bonus distance. (It is not because radio is faster than light, and it has nothing to do with dust in the air — it is purely that gentle bending by the atmosphere.)

Once a UHF signal gets past that radio horizon, it usually just vanishes. That is why simplex UHF signals are rarely heard beyond their radio horizon: because UHF signals are usually not propagated by the ionosphere. In plain words, the sky-mirror does not catch them. A UHF wave punches right through the ionosphere and keeps climbing up into space instead of bouncing back down. So once UHF crosses the horizon, it is generally gone for good. (The word "simplex" here just means the two stations talk directly to each other, not through a repeater.)

Why HF is the long-distance champion

Remember the ionosphere, our mirror in the sky? It reflects the lower-frequency waves far better than the high ones. That difference gives us the big headline of this group: one characteristic of HF communication, compared with VHF and higher frequencies, is that long-distance ionospheric propagation is far more common on HF. HF waves leap up, bounce off the sky, and come back down to Earth hundreds or even thousands of miles away — that long sky-bounce is what hams call "skip." VHF and UHF usually do not get that bounce, so they normally stay close to home. (A couple of common myths to ignore: HF antennas are actually bigger, not smaller, and HF tends to have more crackly static, not less.)

F-region skip: bouncing off the highest layer

The ionosphere is not just one sheet; it has a few layers stacked up. The highest one is named the F region, and it does most of the heavy lifting for long-distance HF skip. Sunlight is what charges it up, and the Sun runs through an 11-year cycle in which it produces more or fewer sunspots. More sunspots means a stronger, more energetic F region, which means better skip.

• The best time for long-distance 10-meter band propagation via the F region is from dawn to shortly after sunset during periods of high sunspot activity. The F region needs daylight to do its job, and a busy, sunspot-rich Sun pushes the skip all the way up to the 10-meter band.
• At the very peak of the sunspot cycle, the F region gets strong enough to give long-distance contacts on the 6 and 10 meter bands — the two Technician-friendly bands that benefit most. (Higher bands such as 70 centimeters are simply too high in frequency; the F region cannot grab and reflect them.)

Sporadic E: surprise band openings

Sporadic E (often written "Es") is one of the fun ones. The word "sporadic" means "happens now and then, unpredictably, by surprise." Patches of unusually strong charge suddenly form down in a lower layer called the E region, and for a little while they act like a mirror — even for VHF, which normally never bounces. This is the type of propagation most commonly behind occasional strong signals on the 10-, 6-, and 2-meter bands from beyond the radio horizon. Out of nowhere, a station hundreds or even a thousand miles away comes booming in loud and clear. On the 6-meter band this is the famous "the band is open!" moment that gets hams excited, and it shows up most often in the summer months.

Tropospheric ducting: a tunnel made of air

Down low, in the layer of air we actually live and breathe in (called the troposphere), warm and cool air can stack up in a way that traps signals. Normally the air gets steadily cooler as you climb higher. But every so often, a layer of warm air ends up sitting on top of cooler air instead. That upside-down stacking is called a temperature inversion, and a temperature inversion in the atmosphere is what causes tropospheric ducting. The trapped layer behaves like a tunnel or a pipe, guiding VHF and UHF signals far past their normal horizon. Tropospheric ducting is the type of propagation responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis. Unlike the surprise pop of sporadic E, ducting tends to show up fairly predictably, especially during calm, stable weather.

Meteor scatter: bouncing off shooting stars

When a meteor (a "shooting star") zips into our atmosphere and burns up, it leaves a brief streak of electrified gas glowing behind it. For just a second or two, that streak can reflect a radio signal before it fades away. Hams use quick bursts of signal to catch these fleeting flashes, a technique called meteor scatter. The band best suited for communicating via meteor scatter is the 6-meter band. Think of it like skipping a stone off a splash that lasts only a single heartbeat — you have to be quick.

Auroral backscatter: bouncing off the northern lights

When the Sun hurls a storm of charged particles at the Earth, we get those beautiful glowing curtains in the sky called the aurora, better known as the northern lights. That shimmering, glowing region can bounce VHF signals back down to Earth. But because the aurora is always dancing, rippling, and shifting, it smears the signal as it reflects. So a key characteristic of VHF signals received via auroral backscatter is that they are distorted, with a characteristic raspy sound. Voices and Morse code come through rough, buzzy, and scratchy — almost growly — rather than clean.

Knife-edge diffraction: peeking over the top of an obstacle

Radio waves can bend a little as they pass a sharp edge, such as the top of a mountain ridge or the corner of a tall building. This particular bending is called knife-edge diffraction, and it is one of the effects that may allow radio signals to travel beyond obstructions sitting between the transmitting and receiving stations. Picture light sneaking just over the top edge of a tall fence and casting a soft glow down into the shadow on the far side. Radio does the same clever trick, dribbling a little bit of usable signal into a spot where you would otherwise expect to hear nothing at all.