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T8D

SIGNALS AND EMISSIONS

Non-voice and digital communications: image signals and definition of NTSC, CW, packet radio, PSK, APRS, error detection and correction, amateur radio networking, DMR, WSJT modes, Broadband-Hamnet

T8D011 of 12

Which of the following is a digital communications mode?

Why All three listed modes send discrete data bits rather than an analog voice or image waveform, so all qualify as digital. Packet radio breaks data into addressed frames (usually AX.25) sent over the air, IEEE 802.11 is the WiFi standard that hams can run at higher power on overlapping 2.4 GHz and 5 GHz amateur allocations (as in Broadband-Hamnet mesh networks), and FT8 is one of the WSJT-X weak-signal modes that encodes short structured messages for decoding below the noise floor.

Watch out Each single-mode choice looks right on its own, which is the trap: the question asks which is a digital mode, not which is the only one, and none of the three is analog.

Packets, WiFi, FT8: all bits, no analog. When every choice moves data, pick 'all of these.'

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D022 of 12

What is FT8?

Why FT8 is one of the WSJT-X weak-signal digital modes developed by Joe Taylor (K1JT), designed to complete a minimal contact (callsigns, grid squares, signal reports) in structured 15-second transmit/receive sequences. It uses 8-tone FSK in a bandwidth of only about 50 Hz, which concentrates the transmitted energy and lets the decoder pull signals out of the noise roughly 20 dB below what the ear can hear. That is why it is popular for HF DX and weak-signal VHF work with modest antennas and power.

Watch out The slow-scan TV description is a distractor built from real-sounding jargon; FT8 sends no images, just short structured data messages. The 8 in the name refers to the eight FSK tones, not to eight channels or a wide FM signal.

FT8 = Franke-Taylor, 8 tones, 15-second slots, decodes far below the noise floor.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D033 of 12

What kind of data can be transmitted by APRS?

Why APRS (Automatic Packet Reporting System) is a packet data network, usually running AX.25 at 1200 baud on 144.390 MHz in the US, and its packets can carry any short data payload. Position reports from a GPS receiver are the best known use, but the same format supports short text messages between stations and automated weather station reports of temperature, wind, and rainfall. Because all three are standard APRS packet types, the all-inclusive choice is correct.

Watch out Picking GPS position data alone is the common trap, since that is APRS's most visible use on map websites, but it is only one of several defined packet types.

APRS = Automatic Packet Reporting System: position, messages, weather, telemetry. If a choice says 'all', take it.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D044 of 12

What is meant by the term "NTSC?"

Why NTSC stands for the National Television System Committee, the group that defined the analog color television standard used in North America for decades. In amateur radio it refers to analog fast-scan color TV, which fills a roughly 6 MHz channel and so is only permitted on the 70 cm band and higher. "Fast-scan" distinguishes it from SSTV (slow-scan TV), which squeezes still images into a voice-width channel on HF.

Watch out The compression-scheme answer is tempting because modern digital TV does use compression, but NTSC predates that and is purely analog with no compression or encryption.

NTSC = National Television System Committee: old-school analog color TV, needs 6 MHz so 70 cm and up.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D055 of 12

Which of the following is an application of APRS?

Why APRS stands for Automatic Packet Reporting System. Stations beacon short packets containing GPS position, status, weather data or brief messages, and receiving software plots those positions on a map in real time. That makes it a tactical tool for public service events and emergencies, where net control can see where mobile stations actually are. The usual VHF channel for APRS in North America is 144.39 MHz.

Watch out Voice over internet between repeaters describes linking systems like EchoLink or IRLP, not APRS, and PACTOR is a separate HF digital protocol used mainly for message forwarding.

APRS = position packets on a map. Think "Automatic Position Reporting" on 144.39.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D066 of 12

What does the abbreviation "PSK" mean?

Why PSK stands for Phase Shift Keying, a digital modulation method that carries data by shifting the phase of the carrier rather than changing its amplitude or frequency. In amateur PSK31, for example, a phase reversal represents one data state and no reversal represents the other, which lets a very narrow 31 Hz signal carry keyboard-to-keyboard text. Because the amplitude stays essentially constant, PSK works well with modest power and in weak-signal conditions.

Watch out Pulse Shift Keying sounds plausible but is not a real mode name, and the choices mentioning packet or power confuse PSK with packet radio or with amplitude-based keying.

The P in PSK is Phase, the same P as in PSK31, the phase-shifting keyboard mode.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D077 of 12

Which of the following describes DMR?

Why DMR (Digital Mobile Radio) uses TDMA, time-division multiple access, splitting a 12.5 kHz channel into two alternating time slots so two separate digital voice conversations share the same frequency pair at once. Each radio transmits in short bursts during its assigned slot, which also saves battery since the transmitter is off half the time. The commercial land-mobile roots of DMR are why the 12.5 kHz channel spacing shows up in the definition.

Watch out The automatic position tracking description belongs to APRS, not DMR; DMR is a voice mode, and nothing in it involves transmitting on two repeater inputs at once.

DMR = Two slots, one 12.5 kHz channel. Remember the pair: TDMA and 12.5.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D088 of 12

Which of the following is included in packet radio transmissions?

Why Packet radio (AX.25) sends data in framed bundles, and each frame carries all three items. The header holds the destination and source call signs so stations know who the data is for, a frame check sequence (checksum) is appended so the receiver can tell if any bits were corrupted, and the protocol uses automatic repeat request (ARQ), meaning the receiver asks the sender to resend any frame that fails the check. That combination is what makes packet a reliable, error-free data mode.

Watch out Each individual choice is a true statement about packet, so picking just one is incomplete; the question is testing whether you know the whole frame structure.

Packet frame = address + checksum + resend: header, error check, ARQ. All three, so pick 'all'.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D099 of 12

What is CW?

Why CW stands for "continuous wave," the oldest amateur mode, in which an unmodulated carrier is switched on and off in the dot-and-dash patterns of Morse code. The FCC emission designator is A1A, and because the signal occupies only a very narrow bandwidth (roughly 100-150 Hz) it gets through noise and weak-signal conditions well. In casual amateur usage, saying "CW" and saying "Morse code" mean the same thing.

Watch out Error correction using code words describes forward error correction schemes in digital modes, and CW has no built-in error correction; the operator's ear does the decoding. Nothing about CW involves a propagation mechanism.

CW = Continuous Wave keyed on and off = Morse. Think "carrier, wiggled" by a straight key.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D1010 of 12

Which of the following operating activities is supported by digital mode software in the WSJT-X software suite?

Why WSJT-X was written by Joe Taylor, K1JT, specifically for weak-signal work where signals are at or below the noise floor. Its mode list includes JT65 and Q65 for Earth-Moon-Earth (moonbounce), WSPR for very low power propagation beacon reporting, and MSK144 for the very short bursts reflected off ionized meteor trails. FT8 and FT4 for ordinary weak-signal HF and VHF contacts are in the same suite, so all three listed activities are supported.

Watch out Each single-activity choice names a real WSJT-X capability, but picking just one ignores the others; the suite is a collection of modes, not one mode.

WSJT-X = weak signal toolbox: moon, meteors, and WSPR beacons all in one package.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D1111 of 12

What is the role of ARQ in a transmission system?

Why ARQ stands for Automatic Repeat reQuest. The receiving station checks each received block against a checksum or CRC, and if the block fails the check it asks the sender to transmit it again. This is an error correction scheme that relies on a return path, unlike forward error correction (FEC), which adds redundant bits so the receiver can repair errors without asking. Modes like packet radio (AX.25) and some HF data modes use ARQ to guarantee clean delivery.

Watch out The data compression choice invents an expansion of the letters; Q codes have nothing to do with it, and ARQ adds overhead for retries rather than shrinking the data. Encryption to obscure meaning is also generally prohibited on the amateur bands under Part 97.

ARQ = Automatic Repeat reQuest: "say again" in digital form.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T8D1212 of 12

Which of the following best describes an amateur radio mesh network?

Why Amateur mesh networking (Broadband-Hamnet, AREDN) takes inexpensive consumer Wi-Fi routers and reflashes them with amateur firmware so they operate on the parts of the 2.4 GHz and 5 GHz bands that overlap amateur allocations (13 cm and 5 cm). Running under Part 97 instead of Part 15, hams may use higher power and high-gain directional antennas, and each node automatically finds and routes through its neighbors to form a self-healing data network. The result is a local high-speed IP network carrying file transfer, video, VoIP and messaging without the internet.

Watch out The internet linking description fits systems such as IRLP, EchoLink or DMR talkgroups, which tie repeaters together over the public internet rather than building a radio-only routed data network.

Mesh = hacked Wi-Fi routers on ham bands. Think Broadband-Hamnet/AREDN, not DMR and not the internet.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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