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Subelement T8
SIGNALS AND EMISSIONS
Section T8A
Basic characteristics of FM and SSB; Bandwidth of various modulation modes: CW, SSB, FM, fast-scan TV; Choice of emission type: selection of USB vs LSB, use of SSB for weak signal work, use of FM for VHF packet and repeaters
Which of the following is a form of amplitude modulation?
  • Spread spectrum
  • Packet radio
  • Correct Answer
    Single sideband
  • Phase shift keying (PSK)

Single Sideband is commonly used for voice on HF. It is amplitude modulation (AM) with the carrier and one sideband removed. This halves the required bandwidth and saves the large amount of power that would otherwise go into the carrier. This makes SSB much more effective than conventional AM over long distances on crowded bands with limited transmitter power.

Spread-spectrum is not itself a form of modulation, but a family of techniques that spread a (usually already modulated) signal over a wide range of frequencies to reduce interference, tolerate multipath, avoid detection, etc.

Packet Radio uses various types of modulation, but is not itself a form of modulation.

Phase Shift Keying is a digital modulation mode that selects one of a set of carrier phases. One specific form of PSK, binary PSK, can be seen as a special form of amplitude modulation with amplitudes +1 and -1, but SSB is the better answer to this question.

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Tags: ssb amplitude modulation arrl chapter 6 arrl module 10

What type of modulation is commonly used for VHF packet radio transmissions?
  • Correct Answer
    FM or PM
  • SSB
  • AM
  • PSK

Since packet radio benefits from good bandwidth and minimal audio amplitude variations, both of which are advantages of FM; and because FM is an allowed modulation method on VHF, FM is the most commonly used modulation for packet.

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Tags: frequency modulation digital modes vhf arrl chapter 5 arrl module 10

Which type of voice mode is often used for long-distance (weak signal) contacts on the VHF and UHF bands?
  • FM
  • DRM
  • Correct Answer
    SSB
  • PM

The listed choices are: Frequency Modulation (FM), Single Side-Band (SSB), Digital Radio Mondiale (DRM), and Phase Modulation (PM). Each has its advantages and disadvantages, but SSB usually requires the least bandwidth and power for long-distance and weak signal contacts.

While FM is widely used for local simplex and repeated VHF/UHF voice, the same "capture" or "quieting" effect that works so well on strong signals causes it to perform worse than SSB on weak signals.

Phase Modulation (PM) and Frequency Modulation (FM) are closely related; in fact, what hams call FM is really PM.

Digital Radio Mondiale (DRM) is used for shortwave broadcasting. Similar methods are now being used experimentally for voice on the HF and other amateur bands, and may gain wider use in the future.

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Tags: ssb uhf vhf arrl chapter 5 arrl module 10

Which type of modulation is commonly used for VHF and UHF voice repeaters?
  • AM
  • SSB
  • PSK
  • Correct Answer
    FM or PM

FM (Frequency Modulation) is the same modulation used by the FM radio in your car, which makes it easier to remember. Note that regular FM radio broadcast stations (music, talk radio, etc) use what is commonly known as Wide FM with a 200 kHz bandwidth, whereas ham radio generally uses Narrow FM which uses about 15-20kHz.

Strictly speaking, what hams call FM is actually PM (Phase Modulation), which is very closely related to FM, differing only in their audio frequency responses. PM is FM with +6dB/octave pre-emphasis and -6 dB/octave de-emphasis.

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Tags: vhf uhf frequency modulation arrl chapter 5 arrl module 10

Which of the following types of signal has the narrowest bandwidth?
  • FM voice
  • SSB voice
  • Correct Answer
    CW
  • Slow-scan TV

CW or "Continuous Wave" is also known as "Morse Code". This consists of turning the RF carrier "on" and "off" with Morse code. Since the information transmitted is nothing more than "on" vs "off" it requires very little bandwidth.

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Tags: morse code bandwidth radio operation arrl chapter 5 arrl module 10

Which sideband is normally used for 10 meter HF, VHF, and UHF single-sideband communications?
  • Correct Answer
    Upper sideband
  • Lower sideband
  • Suppressed sideband
  • Inverted sideband

By convention, Upper Sideband (USB) is used for SSB on 10MHz HF and up -- including VHF and UHF bands. You will need to just remember upper sideband for the upper-frequency bands, 10MHz and up.

The only exception is that LSB is transmitted to amateur satellites with frequency inverting linear transponders. The transponder converts LSB on the uplink to USB on the downlink.

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Tags: 10 meter ssb vhf hf uhf arrl chapter 5 arrl module 10

What is a characteristic of single sideband (SSB) compared to FM?
  • SSB signals are easier to tune in correctly
  • SSB signals are less susceptible to interference
  • Correct Answer
    SSB signals have narrower bandwidth
  • All these choices are correct

AM has two mirror-image sidebands, upper and lower, and a carrier that contains a lot of power but no information. Single sideband (SSB) omits the carrier and one sideband, putting all of its power into the remaining (single) sideband. This halves the signal bandwidth and makes much more efficient use of transmitted power.

The narrower bandwidth does make SSB less susceptible to interference in the sense that it's a smaller target. But it is still susceptible to interference that lands within its smaller bandwidth. What's more important is that its narrower bandwidth means it causes less interference to other users of the band.

SSB is harder to tune than either AM or FM, whose carriers provide a frequency "reference" to the receiver. Without a carrier to go on, the SSB receiver must provide an accurate one of its own by careful local tuning.

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Tags: ssb frequency modulation bandwidth arrl chapter 5 arrl module 10

What is the approximate bandwidth of a typical single sideband (SSB) voice signal?
  • 1 kHz
  • Correct Answer
    3 kHz
  • 6 kHz
  • 15 kHz

Single Sideband is a modified form of Amplitude Modulation (AM). Because AM has two mirror-image sidebands, it normally uses twice the bandwidth of the modulating signal. Because single sideband only transmits one sideband, its radio frequency bandwidth is equal to the bandwidth of the modulating signal. SSB also omits the AM carrier, but it does not consume any radio bandwidth.

A traditional communications-grade voice signal extends from about 300 to 3300 Hz, with newer transceivers using digital signal processing (DSP) extending the low end to about 100 Hz to improve fidelity. So the radio bandwidth of a SSB signal is very approximately 3 kHz.

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Tags: bandwidth ssb hf arrl chapter 5 arrl module 10

What is the approximate bandwidth of a VHF repeater FM voice signal?
  • Less than 500 Hz
  • About 150 kHz
  • Correct Answer
    Between 10 and 15 kHz
  • Between 50 and 125 kHz

According to Carson's Rule, the approximate bandwidth of an FM signal is equal to twice the sum of the highest modulating frequency and the deviation. For standard ham FM, this is 2 * (3 kHz + 5 kHz) = 16 kHz. If you transmit on 146.520MHz and listen on 146.525MHz you will likely still hear the signal. If you can't (or barely can) then the bandwidth is closer to 5kHz -- standard for FRS radios, for example). The further away you get the wider the bandwidth would need to be for you to still be able to hear it.

Remember that if the bandwidth is 5kHz you will only hear them at the transmit frequency +/- 2.5Khz, because the transmit frequency is the middle so half will be above and half below. Most amateur FM uses +/- 5 kHz deviation but some support "half deviation" mode which uses , +/- 2.5 kHz.

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Tags: frequency modulation vhf repeater arrl chapter 5 arrl module 10

What is the approximate bandwidth of AM fast-scan TV transmissions?
  • More than 10 MHz
  • Correct Answer
    About 6 MHz
  • About 3 MHz
  • About 1 MHz

Analog fast-scan TV transmissions use a lot of bandwidth compared to voice and other modes. When US amateurs use the (now obsolete) US NTSC analog TV broadcast standard, the bandwidth is the same as a US TV channel: 6 MHz.

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Tags: amateur television (atv) bandwidth 70 cm arrl chapter 5 arrl module 10

What is the approximate bandwidth required to transmit a CW signal?
  • 2.4 kHz
  • Correct Answer
    150 Hz
  • 1000 Hz
  • 15 kHz

CW uses the least bandwidth of all commonly used modes; all you really need to get across clearly is a single tone so that the receiving station can hear when it is "on" or "off".

A good rule of thumb for CW (Continuous Wave, Morse Code) is that when it asks about bandwidth it's probably the smallest value listed.

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Tags: morse code arrl chapter 5 arrl module 10

Which of the following is a disadvantage of FM compared with single sideband?
  • Voice quality is poorer
  • Correct Answer
    Only one signal can be received at a time
  • FM signals are harder to tune
  • All these choices are correct

With Single Sideband signals not only is the second sideband omitted, there is no "carrier". The carrier is the silence that goes along with your transmission -- if you transmit on FM you can hear the silence even if you don't send any audio. Single Sideband transmissions don't have that -- if you are transmitting but not talking then (basically) nothing actually gets transmitted. Similarly, if two people are talking at the same time on a Single Sideband frequency you'll at least mostly hear both of them, whereas with FM two simultaneous transmissions will tend to clobber each other and you'll either only hear the stronger one or you won't be able to hear either one clearly.

Looking at the distractors:

  • FM signals are actually easier to tune because they have a clear carrier, so you tune to the "quietest" frequency, meaning the one with the least noise --hence the term "full quieting" which is used to describe a clear FM signal.
  • Voice quality is usually significantly better on FM signals vs sideband signals.

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