Single Sideband is a modulation mode commonly used with HF operation and can be essentially described as a variant of amplitude modulation (AM) that only uses half of the signal and no carrier. Because of this, it is easier to use over a long distance where the reduced bandwidth usage results in less interference and the lack of carrier makes it more likely that the signal will be copyable despite any interference.
Spread-spectrum is not in and of itself a form of modulation, but rather is a technique (or a family of techniques, actually) that spreads a signal out on a wide range of frequencies in order to reduce interference, avoid detection, etc.
Packet Radio could probably be used with Amplitude Modulation, but itself can be used with many different modulations.
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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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The listed choices are: Frequency Modulation (FM), Single Side-Band (SSB), Digital Radio Mondiale (DRM), and Phase Modulation (PM). All of these have their own advantages and disadvantages, but of them Single Side-Band is unique in that it does not transmit a carrier and requires approximately half of the bandwidth of an FM signal; these two characteristics make it ideal for long- distance and weak signal contacts in nearly any band because less of the signal needs to make it through for the receiver to correctly copy the transmission.
For comparison, consider your AM/FM car radio; when you are too far from the tower, the signal begins to fuzz making it uncopyable. With SSB the signal would fade but you would have less "white noise" in the faded signal and more of it would be just the voice part that you are interested in. The downside to this mode is that without a carrier, even when the signal is strong it may not be as clear as an AM or FM signal.
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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, whereas ham radio generally uses Narrow FM which uses less bandwidth (about 5-15kHz).
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As a convention agreed upon, the 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.
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A rough way of understanding single sideband is to consider that AM voice transmissions have two sidebands -- upper and lower, with each sideband being approximately half of the bandwidth of the signal. With Single Sideband there is no carrier and only one of the sidebands is used, either Upper or Lower, and as a result SSB has a much narrower bandwidth.
It is true that SSB signals are more likely to be heard than FM signals when the signal is weak, but this does not imply that they are less susceptible to interference or that they are easier to tune -- they are simply still copyable if a smaller portion of the signal makes it through.
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Single Sideband is a form of modified Amplitude Modulation (AM). Whereas AM normally uses twice the bandwidth of the original carrier, Single Sideband avoids that issue and also does not waste power on a carrier.
The bandwidth used by a single sideband voice signal varies between 300 and 3400 Hz, or .3 to 3.4 kHz. The approximate bandwidth, therefore, is the rough maximum used, which is approximately 3kHz.
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This can be demonstrated on a 2 meter handheld radio if you have an interest to see it; if you transmit on 146.520MHz and listen on 146.525MHz you will likely still be able to hear the signal. If you can't (or barely can) then the bandwidth is closer to 5kHz (standard for FRS radios, for example) and if you can hear it strongly it may be closer to 10 or 15kHz. 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 handheld radios seem to use 10kHz, but some support "half deviation" mode which uses 5kHz.
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Analog fast-scan TV transmissions use a lot of bandwidth compared to voice and other modes because of how much information is attached to the minorities that need to be transmitted. The approximate bandwidth is about 6 MHz.
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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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