Here’s another chapter from my updated study guide…..Dan
Modulation modes and signal bandwidth
Modulation is the process of adding information to a radio wave. For example, to send a voice signal, you modulate the radio wave with your voice. There are several different types of modulation, but when you get your Technician license, chances are that frequency modulation, or FM, is the type of modulation that you’ll use first. When you frequency modulate a radio, the audio actually changes the frequency of the radio wave a little in proportion to the volume of the audio signal. An FM receiver detects this frequency change and recreates the audio.
QUESTION: Which type of modulation is most commonly used for VHF and UHF voice repeaters? (T8A04)
ANSWER: FM
QUESTION: What type of modulation is most commonly used for VHF packet radio transmissions? (T8A02)
ANSWER: FM
Amplitude modulation, or AM, is another type of modulation. To amplitude modulate a signal, you vary the amplitude of the signal in proportion to the audio level. Single-sideband, or SSB, is a form of amplitude modulation.
QUESTION: Which of the following is a form of amplitude modulation? (T8A01)
ANSWER: Single sideband
QUESTION: Which type of voice mode is most often used for long-distance (weak signal) contacts on the VHF and UHF bands? (T8A03)
ANSWER: SSB
QUESTION: Where may SSB phone be used in amateur bands above 50 MHz? (T2B13)
ANSWER: In at least some portion of all these bands
A single-sideband signal may be upper-sideband (USB) or lower-sideband (LSB).
QUESTION: Which sideband is normally used for 10 meter HF, VHF, and UHF single-sideband communications? (T8A06)
ANSWER: Upper sideband
The audio of an FM signal sounds better than an AM signal, but it occupies a wider bandwidth than an AM or single sideband signal. This means that you can have fewer FM signals in a given frequency range than SSB signals.
QUESTION: What is an advantage of single sideband (SSB) over FM for voice transmissions? (T8A07)
ANSWER: SSB signals have narrower bandwidth
QUESTION: What is the approximate bandwidth of a single sideband (SSB) voice signal? (T8A08)
ANSWER: 3 kHz
QUESTION: What is the approximate bandwidth of a VHF repeater FM phone signal? (T8A09)
ANSWER: Between 10 and 15 kHz
Some modes have very wide bandwidths, such as analog, fast-scan TV.
QUESTION: What is the typical bandwidth of analog fast-scan TV transmissions on the 70 centimeter band? (T8A10)
ANSWER: About 6 MHz
QUESTION: What type of transmission is indicated by the term “NTSC?” (T8D04)
ANSWER: An analog fast scan color TV signal
NTSC stands for National Television Standards Committee.
Morse Code, or CW signals have a narrower bandwidth than either AM or FM.
QUESTION: Which of the following types of emission has the narrowest bandwidth? (T8A05)
ANSWER: CW
QUESTION: What is the approximate maximum bandwidth required to transmit a CW signal? (T8A11)
ANSWER: 150 Hz
Technicians have CW privileges on the 80m, 40m, 15m, and 10m bands, as well as all the VHF, UHF, and microwave bands.
QUESTION: What code is used when sending CW in the amateur bands? (T8D09)
ANSWER: International Morse
QUESTION: What is an electronic keyer? (T8D14)
ANSWER: A device that assists in manual sending of Morse code
Digital modes: packet, PSK31
When hams talk about “digital modes,” we are talking about modes that send digital data rather than voice or other types of analog signals, such as television. Usually, we connect our transceivers to a computer to modulate and demodulate the digital signals, but some transceivers can do this internally.
QUESTION: Which of the following is a digital communications mode? (T8D01)
ANSWER: All of these choices are correct
Packet radio
IEEE 802.11
JT65
Packet radio was one of the first digital modes. It is called packet radio because the data to be sent from station to station are separated into a number of packets which are then sent separately by the transmitting station and received and re-assembled by the receiving station.
QUESTION: Which of the following may be included in packet transmissions? (T8D08)
ANSWER: All of these choices are correct
A check sum which permits error detection
A header that contains the call sign of the station to which the information is being sent
Automatic repeat request in case of error
Some amateur radio digital communications systems use protocols which ensure error-free communications. One such system is called an automatic repeat request, or ARQ, transmission system.
QUESTION: What is an ARQ transmission system? (T8D11)
ANSWER: A digital scheme whereby the receiving station detects errors and sends a request to the sending station to retransmit the information
APRS is one service that uses packet radio. The major application of APRS is to send location reports while operating a mobile amateur radio station.
QUESTION: What does the term “APRS” mean? (T8D02)
ANSWER: Automatic Packet Reporting System
QUESTION: Which of the following is an application of APRS (Automatic Packet Reporting System)? (T8D05)
ANSWER: Providing real-time tactical digital communications in conjunction with a map showing the locations of stations
QUESTION: Which of the following devices is used to provide data to the transmitter when sending automatic position reports from a mobile amateur radio station? (T8D03)
ANSWER: A Global Positioning System receiver
A popular digital mode on the HF bands is PSK31. PSK31 signals are modulated by shifting the phase of the tone being sent.
QUESTION: What does the abbreviation “PSK” mean? (T8D06)
ANSWER: Phase Shift Keying
The “31” in PSK31 comes from the fact that data is transmitted and received at about 31 baud and that the bandwidth of a PSK31 signal is only about 31 Hz. Fortunately, that is about how fast most people can type.
Digital Mobile Radio, or DMR, is an FM digital communications system that’s become quite popular in amateur radio. It allows amateur radio operators to use the spectrum more efficiently by multiplexing two signals in a single 12.5 kHz repeater channel. It also allows amateur radio operators to connect with one another over the internet.
QUESTION: Which of the following best describes DMR (Digital Mobile Radio)? (T8D07)
ANSWER: A technique for time-multiplexing two digital voice signals on a single 12.5 kHz repeater channel
One of the interesting things about operating DMR is the ability to join a “talk group.” A talk group is a virtual channel that connects radio users locally over a repeater or state-wide, nationally, or internationally over the internet.
QUESTION: What is a “talk group” on a DMR digital repeater? (T2B09)
ANSWER: A way for groups of users to share a channel at different times without being heard by other users on the channel
QUESTION: How can you join a digital repeater’s “talk group”? (T2B07)
ANSWER: Program your radio with the group’s ID or code
WSJT software is software that provides weak-signal digital communication modes on amateur radio frequencies. To communicate with one of the digital modes that WSJT software provides, you need a single sideband transceiver and a personal computer with a sound card.
QUESTION: Which of the following operating activities is supported by digital mode software in the WSJT suite? (T8D10)
ANSWER: All of these choices are correct
Moonbounce or Earth-Moon-Earth
Weak-signal propagation beacons
Meteor scatter
FT8 is one of the modes supported by WSJT.
QUESTION: What is FT8? (T8D13)
ANSWER: A digital mode capable of operating in low signal-to-noise conditions that transmits on 15-second intervals
Mesh networking is another amateur radio activity that’s becoming popular. Mesh networks use WiFi frequencies that just happen to be part of the 2.4 GHz, 3.4 GHz, and 5.8 GHz amateur radio bands. Mesh networks allow amateur radio stations to establish digital communications links for supporting high-speed emergency communications and internet connectivity. Broadband-Hamnet and Amateur Radio Emergency Data Network (AREDN) are two projects that use mesh networking to provide digital communications.
QUESTION: Which of the following best describes Broadband-Hamnet(TM), also referred to as a high-speed multi-media network? (T8D12)
ANSWER: An amateur-radio-based data network using commercial Wi-Fi gear with modified firmware
Dave New, N8SBE says
Didn’t you post this on 4/23?
Dan KB6NU says
Opppps. This one has a few changes. I’ve deleted the other one.