T8: Signals & Emissions

First, what does "modulation" really mean?

Every transmitter starts by making a plain, steady radio wave called the carrier. Imagine the carrier as a blank moving sidewalk that runs at a perfectly even speed and carries nothing. By itself it says nothing at all. To send a message, we have to change the wave a tiny bit, over and over, in time with our voice or our data. Changing the wave like that is modulation — we are loading the moving sidewalk with our message.

There are really only two things about a wave we can change: how tall it is (its strength, which engineers call its amplitude) and how fast it wiggles (its frequency). Which one we choose to change is what gives each mode its name.

The everyday voice modes

• FM (frequency modulation) changes the speed of the wiggle to match your voice. Talk louder or higher and the wave wiggles a touch faster; the height stays the same. PM (phase modulation) is a very close cousin that ends up sounding the same, so the exam almost always offers them together as "FM or PM." FM comes through clean and clear, the way an FM music station does, which is why it is the standard mode for VHF and UHF voice repeaters and for VHF packet radio (sending computer data over the air). FM does have one quirk we will meet below.
• AM (amplitude modulation) changes the height of the wave to match your voice. As you talk, the wave grows taller and shorter. Plain AM is what old-time broadcast radio uses, and a few hams still enjoy it.
• SSB (single sideband) is the surprising one. The exam wants you to know that single sideband is a form of amplitude modulation. It is a clever, trimmed-down version of AM: we throw away the parts of an AM signal we do not really need and keep just one slim slice (one "sideband"). Because it is so slim and efficient, SSB is the favorite mode for long-distance, weak-signal voice contacts on the VHF and UHF bands. It takes a steadier hand to tune than FM, but it carries your voice much farther on the same amount of power.

The FM quirk: only one at a time

Here is FM's weak spot. An FM receiver grabs onto the strongest signal it hears and ignores everything weaker. So if two stations transmit on the same channel, you hear only the loud one. The exam says it plainly: a disadvantage of FM compared with single sideband is that only one signal can be received at a time. With SSB you can often hear several faint stations mixed together; with FM the strongest one wins and the rest vanish. Engineers call this the "capture effect," but you only need the plain idea: FM lets one signal capture the receiver.

Why "narrow" wins for distance

Picture pouring the same cup of water through a wide funnel versus a skinny straw. The skinny straw shoots out a stronger, more focused stream. A narrow radio signal works the same way: it squeezes all your power into a tiny slice of the dial, so a faint, far-away station has a better chance of hearing you. That is the whole reason weak-signal operators love SSB and CW — both are narrow, so both punch farther. The exam puts it directly: compared with FM, an SSB signal has a narrower bandwidth.

USB vs LSB

SSB keeps just one sideband, and there are two to pick from: the upper sideband (USB) and the lower sideband (LSB). If two stations picked different ones, their voices would sound garbled to each other, so hams agreed on a rule. For 10-meter HF, VHF, and UHF single sideband, everyone uses the upper sideband (USB). (Down on the lower HF bands the custom flips to LSB, but for every band a Technician cares about, the answer is USB.)

CW — the tiniest signal of all

CW stands for "continuous wave," and it is plain old Morse code. The transmitter simply flicks the carrier on and off — dit, dit, dah — like blinking a flashlight in patterns. Because it carries so little at any one instant, CW has the narrowest bandwidth of any common signal, only about 150 Hz (that is 150 wiggles-per-second worth of room). That itty-bitty footprint is exactly why a Morse signal can sneak through when every other mode is buried in noise.

Bandwidth: how much "room" a signal takes

Bandwidth is just how wide a chunk of the radio dial a signal needs to do its job. Picture the dial as a parking lot: a bicycle (CW) needs almost no space, a car (SSB) needs a normal spot, a delivery truck (FM) needs a big spot, and a moving TV picture (fast-scan TV) closes off a whole row. Here are the four numbers the exam loves, from skinniest to widest:

Mode	About how wide
CW (Morse code)	150 Hz — the narrowest of all
SSB voice	3 kHz
FM voice on VHF repeaters	10 to 15 kHz — widest of the everyday voice modes
AM fast-scan TV (moving video)	about 6 MHz — gigantic

The pattern could not be simpler: the more stuff a mode has to carry, the more room it eats up. A blinking flashlight (CW) hardly needs any. A live, moving TV picture holds a flood of information every second, so it gulps a huge slice of spectrum — about 6 MHz, which is roughly two thousand times wider than a single SSB voice signal.

Pick-the-mode cheat sheet

What you want to do	Mode to use
Talk on a VHF or UHF voice repeater	FM (or PM)
Send data with VHF packet radio	FM (or PM)
Reach far / work weak signals on VHF or UHF	SSB
Use SSB on 10 m, VHF, or UHF	Upper sideband (USB)
Get the narrowest signal possible	CW (Morse)

Wait — I can really talk through space?

Yes, you really can. There are small amateur radio satellites circling the Earth right this minute, and a Technician license lets you use almost all of them. Many can be worked with nothing fancier than a handheld radio and a little homemade beam antenna you hold up toward the sky like an old rabbit-ear TV antenna. You aim it overhead, your voice rides up into space, and it comes back down to someone hundreds of miles away. It is one of the most jaw-dropping things a brand-new ham can do.

Uplink and downlink — the up escalator and the down escalator

A satellite cannot listen and talk on the exact same frequency at the very same instant, so it uses two different frequencies. You transmit up to the satellite on the uplink, and you listen to the satellite on the downlink. Think of a shopping mall with one escalator going up and a separate one coming down. The two are usually on different bands so they never bump into each other.

"Mode" letters like U/V

To say which bands a satellite uses, hams write two letters: the uplink band first, the downlink band second. The most common is U/V mode. That means the uplink is in the 70-centimeter band (UHF) and the downlink is in the 2-meter band (VHF). So you talk up on UHF and listen down on VHF. (You might also see V/U mode, which is just the reverse.) The exam loves to see if you read the letters in the right order, so remember: first letter up, second letter down.

What modes do satellites use?

Pretty much all of them. Amateur satellites carry FM, SSB, and CW or data depending on the bird, so when the exam asks what mode satellites use, the answer is all of these. There is no single "satellite mode" to memorize — different satellites use different modes, and many use more than one.

Doppler shift — the ambulance-siren effect

You know how an ambulance siren sounds higher-pitched as it races toward you, then lower as it speeds away? Radio waves do the exact same thing. A satellite zips across the sky at thousands of miles per hour, so to your radio its frequency seems to slide as it rushes toward you and then away again. That slide is Doppler shift, which the exam defines as an observed change in signal frequency caused by the relative motion between the satellite and the Earth station. During a pass you gently re-tune your radio to keep chasing the signal, and tracking software can predict the slide for you ahead of time.

Beacons and telemetry — the satellite's status light

A satellite beacon is a steady little signal the satellite sends out on its own; the exam defines it as a transmission from a satellite that contains status information. It is the satellite quietly saying "I'm alive, here's how I'm doing." The status data it sends is called telemetry, and it reports the health and status of the satellite — things like battery voltage and how hot or cold it is up there. Best of all, telemetry is open to everybody: anyone is permitted to receive amateur satellite telemetry, even someone with no license at all. Listening is always free.

Tracking programs — your map of the sky

Because a satellite never stops moving, you need to know when it will fly over and where to point your antenna. A satellite tracking program handles all of this: it draws a live map showing the satellite's path over the Earth, it tells you the time and direction of the start, the highest point, and the end of each pass, and it even shows the Doppler-shifted frequency so you tune to the right spot. To do its math, a tracking program needs one special input: the Keplerian elements. These are a small set of orbit numbers that describe the satellite's path through space, and hams just call them "keps." Feed the program fresh keps and it knows exactly where the bird will be.

LEO — Low Earth Orbit

LEO stands for Low Earth Orbit, an orbit close to the planet with a period (the time for one full trip around the Earth) of around 100 minutes. Most ham satellites live here, so they whip overhead quickly — a usable pass lasts only a few minutes — but the good news is they swing back around again before long.

Spin fading — like a slow strobe light

Satellites often spin or tumble slowly as they orbit. As the spacecraft and its antennas turn, the signal aimed at you keeps shifting direction, so it fades up and down like the slowly sweeping beam of a lighthouse. That rise-and-fall is spin fading, and the exam says it is caused by the rotation of the satellite and its antennas. Nothing is wrong with your gear; the satellite is just turning.

How loud should you be? Don't be a bully

A satellite has only a tiny bit of power on board and is shared by hams all over the world. If you blast it with too much, the exam warns that the result is blocking access by other users — you drown everyone else out and hog the satellite for yourself. Some satellites carry a linear transponder (a relay that repeats a whole slice of signals at once), and there is a simple way to set your power just right on one: listen to your own voice coming back on the downlink and compare it to the satellite's steady beacon. Your downlink signal should sound about as strong as the beacon — not louder. Match the beacon and everyone gets a fair turn.

Radio direction finding — playing detective

Sometimes a mystery signal causes interference, or someone is deliberately jamming a frequency, and we need to figure out where it is coming from. The skill of sniffing out a signal's location is radio direction finding, and the exam names it as the method used to locate sources of noise interference or jamming. It is also the heart of a popular game called a hidden transmitter hunt (or "fox hunt"), where one person hides a small transmitter somewhere and everybody else races to find it.

The key tool for the chase is a directional antenna — a small beam that hears best in one direction, like cupping your hand behind your ear. You swing it slowly around like a flashlight in a dark room; whichever direction makes the signal loudest is pointing toward the "fox." Take a reading from one spot, then drive to a different spot and take another. Draw both lines on a map and they cross right where the transmitter is hiding. It is real-life treasure hunting with radio.

Contesting — the radio race

Contesting is the activity that involves contacting as many stations as possible during a specified period of time. Think of it as a friendly race to see who can make the most contacts before the clock runs out. Because every second counts and the band gets crowded, the smart and polite move is to keep each contact short: send only the minimum information needed for proper identification and the contest exchange. No long chit-chat — just the essentials, then move on to the next station so everyone keeps moving.

Grid locators — a worldwide address

A grid locator (sometimes called a Maidenhead locator, or just a "grid square") is a letter-number designator assigned to a geographic location — codes like EN70 or FM18. It is a fast way to tell another ham roughly where you are, a bit like a super-simple zip code that works for the whole planet. Hams trade grid squares constantly on VHF and UHF and especially when working satellites, since knowing each other's grid helps figure out how far the contact reached.

Linking radios through the internet

Here is a neat trick: we can connect a local radio to the internet so that even a small handheld can reach across the world. A few different systems do this, and the exam asks about each one:

• VoIP (Voice Over Internet Protocol) is the basic idea underneath all of them. The exam defines it as a method of delivering voice communications over the internet using digital techniques. It is the very same technology that lets you make a phone call over Wi-Fi.
• IRLP (Internet Radio Linking Project) is a technique to connect amateur radio systems, such as repeaters, via the internet. You control an IRLP link right from your radio: over the air, access to IRLP nodes is accomplished by using DTMF (Dual-Tone Multi-Frequency) signals — the beep-boop touch-tones a telephone keypad makes when you press the buttons.
• EchoLink links stations over the internet too, but with a twist. It is the system that lets an amateur station transmit through a repeater without using a radio to initiate the transmission — you can connect straight from a computer or a phone app. Before you are allowed to use EchoLink, though, the exam says you must register your call sign and provide proof of license, so the system can be sure you are a real, licensed ham.

Gateways — the on-ramp to the internet

An amateur radio station that connects other amateur stations to the internet is called a gateway. Picture it as a highway on-ramp: local radio traffic rolls up to the gateway and merges onto the worldwide internet, then comes back down to radio somewhere else.

What is a "digital" mode?

Instead of sending your actual voice as sound, a digital mode turns your message into computer data — little bursts of tones and beeps that a computer on the other end decodes back into text or sound. The exam wants you to know that packet radio, IEEE 802.11 (that is plain Wi-Fi), and FT8 are all digital communications modes. So whenever a question lists several digital modes and offers "all of these," that is the answer.

FT8 — hearing the unhearable

FT8 is one of the most popular digital modes ever made, and the exam describes it as a digital mode capable of low signal-to-noise operation. In plain words, "low signal-to-noise" means the signal is barely stronger than the static around it. FT8 can decode a signal so faint your ears would hear nothing but hiss — the computer reaches right into the noise and pulls the message out. That superpower lets people reach far-off stations using only a small antenna and modest power. FT8 lives inside a free program called the WSJT-X software suite, which also supports, according to the exam, all of these weak-signal adventures: Earth-Moon-Earth (called "moonbounce," where you literally bounce your signal off the Moon), meteor scatter (bouncing off the glowing trails of shooting stars), and weak-signal propagation beacons. Genuine space-age stuff from your own desk.

PSK

PSK stands for Phase Shift Keying. It sends data by nudging the timing of the wave (its "phase") back and forth — one little nudge means one thing, a different nudge means another, and the computer reads the pattern of nudges as letters.

Packet radio — mailing data in labeled envelopes

Packet radio chops your data into small chunks called packets and mails each one like a labeled envelope. The exam asks what is included in a packet transmission, and the answer is all of these:

• A checksum that permits error detection — a quick math fingerprint of the data, so the receiver can instantly tell if anything got scrambled along the way.
• A header that contains the call sign of the station to which the information is being sent — basically the address written on the front of the envelope.
• Automatic repeat request in case of error — a built-in way to ask for a do-over if the data arrives garbled.

That last one has its own name. ARQ stands for Automatic Repeat reQuest, and the exam defines its role as an error correction method in which the receiving station detects errors and sends a request for retransmission. It works just like a friend saying "wait, say that last word again?" until the message comes through perfectly. That is how digital data arrives clean even when the radio path is messy.

APRS — a live map of who's where

APRS (Automatic Packet Reporting System) is a digital network that can carry all of these kinds of data: GPS position data, text messages, and weather data. Its most famous job, the one the exam calls out, is providing real-time tactical digital communications in conjunction with a map showing the locations of stations. During a parade, a bike race, or an emergency, the people in charge can watch little icons creep across a map and see exactly where each radio operator is, second by second. Very handy when you need to know where everyone is.

CW and NTSC — two old-school terms

CW is simply another name for a Morse code transmission — the same on-off keying you met in the bandwidth lesson. NTSC is the name of an analog fast-scan color TV signal, the old over-the-air television standard, still used by hams who like to send live moving video over the air.

DMR — two conversations on one channel

DMR (Digital Mobile Radio) is a digital voice mode with a clever trick. The exam describes it as a technique for time-multiplexing two digital voice signals on a single 12.5 kHz repeater channel. "Time-multiplexing" sounds scary but just means it rapidly takes turns: a sliver of one conversation, then a sliver of the other, switching so fast that two separate chats (called "time slots") fit on one channel at the same time. (You may also hear about D-STAR and System Fusion, two other digital voice systems, but DMR is the one the exam names.)

Mesh networks — hams building their own internet

An amateur radio mesh network (you may hear names like Broadband-Hamnet or AREDN) is, in the exam's words, an amateur-radio data network using commercial Wi-Fi equipment with modified firmware. In other words, hams take ordinary store-bought Wi-Fi gear, change its built-in software (its "firmware") so it runs on amateur frequencies, and link lots of these devices together. Data then hops from one device to the next — like a chain of friends passing a note hand to hand — which lets hams build their own fast local network with no internet company involved at all.