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G8C

SIGNALS AND EMISSIONS

- Digital emission modes

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G8C021 of 15

Which digital mode is used as a low-power beacon for assessing HF propagation?

Why WSPR stands for Weak Signal Propagation Reporter, a mode designed specifically for propagation beaconing. Stations transmit a short, highly compressed message (callsign, grid square, power level) using very narrow 4-tone FSK over a two-minute window, then listen and upload decodes to a central database. Because the mode works far below the noise floor, operators can run a watt or less and still see which paths are open worldwide.
Watch out MFSK16 and PSK31 are keyboard-to-keyboard conversational modes, not automated beacons, and SSB-SC (suppressed carrier single sideband) is an analog voice technique, not a digital mode at all.
WSPR sounds like 'whisper': a tiny signal whispering around the world to map propagation.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C032 of 15

What part of a packet radio frame contains the routing and handling information?

Why Packet radio (AX.25) frames are built like an envelope around the data: the header carries the addressing and control fields, meaning the source and destination callsigns, any digipeater path, and the control/protocol bytes that tell the receiving station how to handle the frame. The payload follows, and the frame closes with a checksum (FCS) and flag. So the routing and handling information all lives in the header.
Watch out The preamble is just the sync pattern or flag that lets the receiver lock on to the start of the frame, and the trailer holds the error-check sequence and closing flag, not addresses.
Like a letter: the header is the address on the envelope, the trailer is just the seal.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C043 of 15

Which of the following describes Baudot code?

Why Baudot (the ITA2 code used in traditional amateur RTTY) represents each character with just 5 data bits, framed by a start bit and a stop bit for asynchronous timing. Five bits only give 32 combinations, so Baudot uses LTRS and FIGS shift characters to switch between letters and figures. It carries no parity and no error correction, so a hit by noise simply prints the wrong character.
Watch out The 7-bit code with start, stop and parity bits describes ASCII, and the error detecting/correcting mode with SELCAL is AMTOR, not Baudot.
Baudot = 5 bits (B-a-u-d-o-t, five letters after the B? just remember 5-bit RTTY); ASCII = 7-bit.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C054 of 15

In an ARQ mode, what is meant by a NAK response to a transmitted packet?

Why ARQ stands for Automatic Repeat reQuest: the receiving station checks each packet's error-detecting code and reports back. A positive acknowledgment (ACK) tells the sender the packet arrived intact and it may move on, while a negative acknowledgment (NAK) says the check failed, so the sender must send that packet again. This handshake is what lets modes like AMTOR, PACTOR and packet radio deliver error-free data over noisy HF paths.
Watch out The choice saying the packet was received without error describes an ACK, the opposite response; NAK is the error report, not the all-clear.
N in NAK = No good, send it again. ACK = All Correct, Keep going.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C065 of 15

What action results from a failure to exchange information due to excessive transmission attempts when using an ARQ mode?

Why ARQ stands for Automatic Repeat reQuest: the receiving station acknowledges good blocks and asks for a resend of any block that fails its error check. Each block has a retry limit, and if the sending station exhausts those attempts without a valid acknowledgment, the protocol gives up and the link times out, dropping the connection. This is what keeps a dead or hopeless path from tying up the channel with endless repeats.
Watch out Checksums are simply compared, not accumulated, so nothing overflows, and misrouted packets describe an addressing or network problem rather than a failed handshake on a point-to-point ARQ link.
ARQ = Automatic Repeat reQuest. Too many repeats with no ACK and the link just hangs up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C076 of 15

Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?

Why FT8 is part of the WSJT-X weak-signal family, using 8-tone FSK in about 50 Hz of bandwidth with 15-second transmit/receive sequences and heavy forward error correction. That combination of narrow bandwidth, long integration time and coding lets the decoder pull callsigns and grid squares out of signals around -21 dB signal-to-noise ratio referenced to a 2500 Hz bandwidth, well below what the ear can hear. That is why FT8 works routinely with low power and compromise antennas.
Watch out MSK144 is also a WSJT-X mode but it is optimized for meteor scatter, sending very fast repeated messages to catch brief, relatively strong pings rather than digging into the noise; AMTOR and MFSK32 are keyboard/text modes that need a usable signal-to-noise ratio to copy.
FT8 = "Faint Transmissions": decodes about 21 dB below the noise. If the question says weakest signals, answer FT8.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C087 of 15

Which of the following statements is true about PSK31?

Why PSK31 encodes characters with Varicode, a variable-length code like Morse where the most common characters get the shortest bit patterns. Lowercase letters, which dominate normal English text, get short sequences, while uppercase letters are assigned longer ones. Since the symbol rate is fixed at 31.25 baud, typing in all caps means more bits per character and therefore slower effective throughput.
Watch out Standard PSK31 has no forward error correction at all, which is why a noisy signal produces garbled characters rather than corrected ones; and PSK31's narrow bandwidth means it needs less power than RTTY, not more, for comparable results.
Varicode = common letters short. SHOUTING IN CAPS costs extra bits, so it slows you down.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C098 of 15

Which is true of mesh network microwave nodes?

Why A mesh network is built from many nodes that each relay traffic, and the routing protocol constantly maps which neighbors are reachable. Because there is usually more than one path between any two stations, the network can route a packet around a node that goes off the air or loses its link. That self-healing, multi-path routing is the defining feature of mesh networking, as opposed to a single fixed repeater or point-to-point link.
Watch out The idea that links between nodes can use different frequencies and bandwidths sounds plausible and is possible in some hardware, but it is not the characteristic the pool is testing; adding nodes also does not raise signal strength or reduce out-of-band interference, since each node is an independent low-power transmitter.
Mesh = many paths. Lose a node, the packet just takes a detour.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C109 of 15

How does forward error correction (FEC) allow the receiver to correct data errors?

Why Forward error correction works by sending extra, mathematically derived redundant bits along with the message data. The receiver uses that redundancy to detect and actually reconstruct bits that were corrupted in transit, without asking for a retransmission. That is why FEC modes like FT8 and some PSK/MFSK modes can decode cleanly on weak or noisy paths where an ARQ system would have to keep repeating itself.
Watch out A parity bit is tempting because it also adds a bit, but parity only flags that an error occurred, it cannot locate or repair it. Varicode is just the variable-length character encoding used by PSK31, unrelated to error correction.
Forward = fix it here, no asking back. The fix costs redundant bits.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1110 of 15

How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified?

Why FSK shifts the carrier between two discrete frequencies to send binary data, and those two states carry the traditional teleprinter names: mark (the idle/1 state) and space (the 0 state). The terminology comes from mechanical telegraph and RTTY practice, where the mark condition kept current flowing and the space interrupted it. On the ham bands RTTY typically uses a 170 Hz shift between the mark and space tones.
Watch out Dot and dash belong to Morse code (CW), and on/off describes on-off keying, where the carrier is interrupted rather than shifted between two frequencies.
FSK = two tones = Mark and Space. Think RTTY: the mark is the resting tone, the space is the other one.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1211 of 15

Which type of code is used for sending characters in a PSK31 signal?

Why PSK31 encodes text with Varicode, a variable-length character code in which the most common letters get the shortest bit patterns, much like Morse code assigns short symbols to E and T. This keeps the average typing throughput near 31 baud while fitting the whole character set into a very narrow bandwidth. Characters are separated by two consecutive zero bits, and no valid Varicode character contains two zeros in a row, so the receiver can find character boundaries.
Watch out Viterbi is tempting because it does appear in PSK31: it is the decoding algorithm used for the forward error correction in the QPSK mode, but it is not the character code itself.
PSK31 = Varicode: VARIable length, like Morse, common letters get short codes.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1312 of 15

What is indicated on a waterfall display by one or more vertical lines on either side of a data mode or RTTY signal?

Why A clean FSK or PSK data signal shows only its intended tones on a waterfall. When the audio drive into an SSB transmitter is too high, the final amplifier is driven into nonlinearity and intermodulation products appear as extra tones spaced on either side of the wanted signal, drawing as additional vertical lines. The cure is to reduce the soundcard or microphone gain and the ALC action until those extra lines disappear.
Watch out Insufficient modulation is tempting because it is the opposite fault, but too little drive simply makes the trace faint or absent; it never adds spurious tones. Long path and backscatter are propagation effects that may smear or echo a signal, not create symmetric sidebands.
Extra lines means extra drive: turn the audio down until the sidebands vanish.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1413 of 15

Which of the following describes a waterfall display?

Why A waterfall is a scrolling spectrogram: the horizontal axis is frequency, just like a spectrum analyzer or panadapter, and each new line of data is painted at the top (or bottom) and scrolls, so the vertical axis represents time. Signal strength is not a height but a color or brightness, so a strong signal shows as a bright or hot-colored vertical streak. That is why it is called a waterfall, the display flows downward as time passes.
Watch out The choice with frequency horizontal and signal strength vertical describes an ordinary spectrum scope or panadapter trace, which shows amplitude versus frequency at one instant with no time history.
Waterfalls fall down: time runs vertically, frequency across, strength = color.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1514 of 15

What does an FT8 signal report of +3 mean?

Why FT8 and other WSJT-X modes report signal strength as a signal-to-noise ratio in decibels, referenced to a standard 2.5 kHz bandwidth (about the width of an SSB channel). A report of +3 means the decoded signal measured 3 dB above the noise in that reference bandwidth. Because FT8 occupies only about 50 Hz, decodes are routine at negative numbers, typically down to around -21 dB, and reports above about +5 are rare.
Watch out The S-meter style answers are the trap: FT8 reports are not S units at all, and nothing like "S3" or "3 dB over S9" is exchanged. Those belong to voice and CW RST reporting.
FT8 numbers are dB SNR in 2.5 kHz, not S units. Negative is normal; +3 is a strong one.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8C1615 of 15

Which of the following provide digital voice modes?

Why DMR, D-STAR and System Fusion are the three common amateur digital voice systems, mostly used on VHF/UHF repeaters. Each digitizes speech with a vocoder (typically AMBE/AMBE+2) and sends the resulting bit stream, usually with call sign and other data riding along. They are not interoperable with each other, but all three carry voice rather than keyboard text.
Watch out FT8, FT4 and FST4 are tempting because they are the best known modern digital modes, but they are weak-signal data modes that exchange only tiny structured messages, no voice. Winlink and the PACTOR modes carry email and file data, WSPR is a beacon mode, MFSK16 is text chat, and EasyPAL sends digital images.
Voice modes are the repeater trio: DMR, D-STAR, Fusion. Anything with numbers in the name (FT8, PACTOR II) is data.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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