G8: Signals and Emissions

Start with the plain-language picture. A bare, unchanging radio wave by itself carries no information; it is just a steady tone humming away at one frequency. That steady tone is called the carrier (because it is what "carries" your message once you change it). To send anything, you have to nudge that carrier in some pattern that matches your voice or data. There are only three things about a wave you can possibly nudge: how strong it is, how fast it wiggles, or where it is in its cycle. Each one gives its own kind of modulation.

The three things you can change about a wave

• Change its strength (its height/loudness). When you vary the instantaneous power level (the moment-to-moment strength) of the wave to match your voice, that is amplitude modulation, abbreviated AM. ("Amplitude" is just a fancy word for the height, or strength, of a wave.) So the answer to "what type of modulation varies the instantaneous power level of the RF signal?" is amplitude modulation.
• Change how fast it wiggles (its frequency). When you vary the instantaneous frequency (the moment-to-moment wiggle rate) to match your voice, that is frequency modulation, abbreviated FM. So "the process that changes the instantaneous frequency of an RF wave to convey information" is frequency modulation. FM is what your local 2-meter repeater uses.
• Change where it is in its cycle (its phase). "Phase" describes the timing of the wave, whether it is a little ahead or a little behind compared to a steady reference. Shifting that timing to carry information is phase modulation, abbreviated PM. So "the process that changes the phase angle of an RF signal to convey information" is phase modulation.

Here is a neat fact the test likes. A circuit called a reactance modulator (a device that tweaks the timing of a stage) connected to a transmitter's RF amplifier stage actually produces phase modulation. ("Reactance" is the property of certain parts that lets them shift the timing of a signal; you do not need the deep theory, just the result.) FM and PM are very close cousins, the wave's frequency and its phase are tightly linked, which is why a reactance modulator can produce either depending on where and how it is connected.

Single sideband: AM on a diet

Plain old AM is wasteful. When you amplitude-modulate a carrier with your voice, you end up transmitting three things at once: the original carrier (which carries no information by itself) and two mirror-image copies of your voice called sidebands, one just above the carrier and one just below. (A "sideband" is the band of frequencies that appears beside the carrier when you modulate it.) Both sidebands carry the exact same information, so sending both, plus the carrier, is like mailing three identical letters to make sure one arrives. Wasteful.

Single sideband, abbreviated SSB, fixes that. It throws away the carrier and one of the two redundant sidebands, and transmits only the single remaining sideband. The result is that all your transmitter power goes into the one piece that actually carries your voice, and your signal takes up far less room on the band. That is why, of the voice (phone) emissions, single sideband uses the narrowest bandwidth. ("Bandwidth" means how wide a slice of frequency a signal occupies; narrower is better when a band is crowded.) SSB is the workhorse voice mode on the HF bands you just earned access to.

Digital modulation: nudging the wave with a computer

Digital modes use the same three knobs (amplitude, frequency, phase), but instead of a smoothly varying voice, the change represents bits (the 1s and 0s of computer data). A few you should recognize:

• FSK (frequency shift keying). "Keying" means switching something on, off, or between states to send data. In FSK you switch the wave between two (or more) frequencies to represent the bits. Direct binary FSK is generated by changing an oscillator's frequency directly with a digital control signal. ("Oscillator" is the circuit that produces the steady tone; the digital signal simply pushes its frequency back and forth.)
• FT8 is one of the most popular digital modes on the air today. Its modulation is 8-tone frequency shift keying, meaning it shifts among eight different tones to carry the data. It is famous for getting through when signals are extremely weak.
• PSK and QPSK use the phase knob. QPSK stands for "quadrature phase shift keying," and QPSK modulation transmits digital data using 0-, 90-, 180-, and 270-degree phase shifts to represent pairs of bits. (Four distinct phase positions let each shift stand for two bits at once.) For the variant QPSK31, the test's correct choice is "all these choices are correct" (its several listed traits are all true together).

Overmodulation: pushing too hard

If you crank your microphone gain or drive level too high, you push the modulation past what the transmitter can cleanly handle. That is overmodulation, and it is bad. An effect of overmodulation is excessive bandwidth, meaning your signal smears out and gets too wide, splattering onto your neighbors' frequencies.

For AM and SSB there is a specific name for the damage: flat-topping. Picture the smooth, rounded peaks of a clean signal getting chopped off flat across the top because the transmitter ran out of headroom. Flat-topping is signal distortion caused by excessive drive or speech levels. The fix is simply to back off the gain so your peaks stay clean.

The modulation envelope

If you watched an AM signal on a screen and traced a line connecting the tip of every peak, that traced outline is the modulation envelope. Formally, the modulation envelope of an AM signal is the waveform created by connecting the peak values of the modulated signal. It is literally the shape of your voice riding on top of the carrier, an outline you could draw around the wiggling wave.

Link budget and link margin: will my signal make it?

These two terms are about predicting whether a contact will succeed, especially on VHF/UHF and satellites. Think of it like a checkbook for signal strength.

• A link budget is the sum of transmit power and antenna gains minus system losses, as seen at the receiver. In plain words: add up everything that helps your signal (transmitter power, the gain of the antennas at both ends) and subtract everything that hurts it (cable losses, distance, and so on). The leftover number tells you how strong your signal arrives.
• Link margin is the difference between the received power level and the minimum required signal level at the input to the receiver. In plain words: it is the cushion, how much stronger your arriving signal is than the bare minimum the receiver needs to make sense of it. A bigger margin means a more reliable contact; a margin near zero means the slightest fade will drop you.

This group has three threads woven together: how radios shift a signal from one frequency to another, how much room (bandwidth) each mode needs, and the junk signals that appear when circuits are overdriven. Let's take them in order.

Mixing: shifting a signal to a new frequency

Radios almost never work directly on the frequency you tune to. Instead they slide every incoming signal to one fixed, convenient frequency where the radio can do its careful filtering and amplifying. The circuit that does the sliding is a mixer. A mixer takes two inputs: the signal you care about, and a steady tone from a tunable oscillator called the local oscillator (abbreviated LO, "local" because it lives right inside your radio).

• What comes out of a mixer? When you feed a mixer the LO and an RF input, the output contains the sum and the difference of those two frequencies. (For example, feed it 14 MHz and 5 MHz and you get 19 MHz and 9 MHz in the output.) This combining of two RF signals even has its own name: heterodyning. So "another term for the mixing of two RF signals" is heterodyning.
• Which input do you tune? You vary the local oscillator to convert signals of different frequencies down to one fixed intermediate frequency. ("Intermediate frequency," abbreviated IF, is that fixed in-between frequency where the radio does its real work, in between the antenna and the speaker.) Turning your radio's tuning knob is really just changing the LO frequency so a new station lands on the fixed IF.

Image response: an unwanted second station sneaks in

The sum-and-difference behavior has a downside. There is a second, unwanted frequency that will also mix down to your IF and barge in on top of the station you want. This unwanted intruder sits at twice the IF away from your desired signal, and the interference it causes is called image response. So "interference from a signal at twice the IF frequency from the desired signal" is image response. Good radios use filtering to reject it.

Multipliers: building up to VHF

Some VHF FM transmitters start with a lower, easy-to-stabilize frequency and then multiply it up to the operating frequency. The stage that does this is a multiplier: it generates a harmonic of a lower frequency signal to reach the desired operating frequency. (A "harmonic" is a whole-number multiple of a frequency, double, triple, and so on, which we will revisit under intermodulation.) This connects to a small calculation the test asks: in a 5 kHz deviation, 146.52 MHz FM transmitter built from a 12.21 MHz reactance modulated oscillator, what is the deviation back at that 12.21 MHz oscillator? You divide the final 5 kHz deviation by the multiplication factor. Here 146.52 ÷ 12.21 = 12, so the oscillator's deviation is 5000 Hz ÷ 12 = 416.7 Hz. (The deviation gets multiplied up by the same factor as the frequency.)

Bandwidth: how wide is each mode?

Every signal occupies a slice of frequency, its bandwidth. Two useful rules and one formula:

• FM bandwidth (Carson's rule). An FM signal's total width is roughly twice the sum of its deviation and its highest modulating frequency. ("Deviation" is how far the FM signal swings away from its center; "modulating frequency" is the pitch of the audio going in.) For 5 kHz deviation and a 3 kHz modulating frequency: 2 × (5 + 3) = 16 kHz. That is the total bandwidth.
• Symbol rate and bandwidth. A "symbol" is one chunk of data sent at a time. The rule is simple and intuitive: higher symbol rates require wider bandwidth. The faster you shove data through, the more room on the band it takes.
• Match your receiver to the mode. It is good to match receiver bandwidth to the bandwidth of the operating mode because doing so results in the best signal-to-noise ratio. ("Signal-to-noise ratio" compares how strong your wanted signal is versus the background hiss; higher is better.) Too wide a receiver lets in extra noise; too narrow chops off part of the signal. Matching is the sweet spot.

Duty cycle: do not cook your transmitter

"Duty cycle" means what fraction of the time your transmitter is putting out full power during a transmission. SSB voice rests during pauses; some digital modes blast continuously. It matters because some modes have high duty cycles that could exceed the transmitter's average power rating. In plain words, a mode that transmits flat-out the whole time can overheat a radio that is happy with the on-and-off nature of voice. When you run a high-duty-cycle digital mode, you often turn the power down to stay safe.

Intermodulation and harmonics: the junk signals

When two signals share a circuit that is not perfectly clean (a "non-linear" stage, meaning one that distorts), they mix together and create brand-new unwanted frequencies. That process is intermodulation, formally "the process that combines two signals in a non-linear circuit to produce unwanted spurious outputs." ("Spurious" just means unwanted, not supposed to be there.) Key points:

• The unwanted products created by odd-order intermodulation are the troublemakers because they land closest to the original signal frequencies, right next door where filtering cannot easily remove them. So "which intermodulation products are closest to the original signal frequencies?" is odd-order.
• You should be able to spot an odd-order product by its formula. For two frequencies F1 and F2, an example of an odd-order product is 2F1 − F2. (Count the coefficients: 2 + 1 = 3, an odd number, hence "odd-order." Something like F1 + F2 would be 1 + 1 = 2, an even-order product.)

The HF bands today are full of digital signals, and this group is your field guide to the common ones. None of it requires math; it is about knowing what each mode does and how to read it on the screen.

Reading a waterfall display

Most digital software shows a waterfall display, a scrolling picture of the band. Here is exactly how to read it: frequency runs horizontally, signal strength shows up as brightness or color intensity, and time scrolls vertically (newest at the top, sliding downward like a waterfall). So each signal appears as a colored vertical streak you can click on to tune it. That layout is the test's definition of a waterfall display.

One warning sign to recognize: if you see one or more vertical lines on either side of a data-mode or RTTY signal on the waterfall, that indicates overmodulation. Those extra side-lines are splatter from driving the signal too hard, exactly the "excessive bandwidth" idea from G8A, now shown as a picture. Back off the audio to clean it up.

FT8 and WSPR: weak-signal champions

• FT8 is the narrow-band digital mode that can receive signals with very low signal-to-noise ratios, meaning it digs contacts out of noise so deep your ears could never hear them. When FT8 gives you a signal report of +3, it means the signal-to-noise ratio is equivalent to +3 dB measured in a 2.5 kHz bandwidth. (It is a standardized way to say "this much signal above the noise.")
• WSPR (often said "whisper," and standing for Weak Signal Propagation Reporter) is used as a low-power beacon for assessing HF propagation. Hams run tiny WSPR transmitters and watch online maps to see exactly where their faint signal is being heard, a fantastic way to study which bands are open and where.

RTTY and Baudot

RTTY ("radioteletype") is one of the oldest digital modes, sending text by shifting between two tones. The two separate frequencies of a frequency shift keyed (FSK) signal are identified as mark and space (the historical names for the "1" tone and the "0" tone). RTTY traditionally carries its characters in Baudot code, which is a 5-bit code with additional start and stop bits. (Five bits per character, bracketed by little markers that tell the receiver where each character begins and ends.)

PSK31 and Varicode

PSK31 is a popular keyboard-to-keyboard chat mode that fits in an extremely narrow slice of band. It sends characters using Varicode, a clever code where common characters get short bit patterns and rare ones get longer patterns (so ordinary typing flows quickly). A consequence the test asks about: in PSK31, uppercase letters use longer Varicode bit sequences and thus slow down transmission. So TYPING IN ALL CAPS literally sends slower, another reason not to shout online.

Packet radio and ARQ: sending data reliably

Packet radio chops data into little bundles called packets. Each packet's header is the part that contains the routing and handling information (where it is going and how to deal with it), much like the address and postmark on an envelope.

Some modes guarantee delivery using ARQ (Automatic Repeat reQuest), a back-and-forth handshake where the receiver confirms each packet. Two terms:

• A NAK response ("negative acknowledgment") to a transmitted packet is a request to retransmit the packet, the receiver saying "that came through garbled, please send it again."
• If the two stations keep failing because of excessive transmission attempts, the result is that the connection is dropped. After too many failed retries, the link gives up rather than hammering away forever.

Forward error correction

A different reliability trick is forward error correction, abbreviated FEC. It lets the receiver fix errors all by itself, with no need to ask for a resend, by transmitting redundant information along with the data. The extra, redundant bits let the receiver reconstruct anything that got corrupted in flight, handy when there is no time or path for a back-and-forth handshake.

Mesh networks

Some hams build microwave mesh networks, webs of interconnected radio nodes. Their great strength: if one node fails, a packet may still reach its target station via an alternate node. Because there are many paths through the web, knocking out a single node does not break the whole network, traffic simply reroutes around the gap.

Digital voice modes

Finally, voice can be digital too. The three common amateur digital voice systems are DMR, D-STAR, and System Fusion. If the test asks which of these provide digital voice, the answer covers all three. Each one turns your voice into data, sends it, and reconstructs it on the other end, often linking through the internet for worldwide reach.