Below 30 megahertz, DTMF with FM signal would take a large amount of bandwidth as well as spread spectrum. The only other options left are Pulse modulation, and FSK.

Commonly, Frequency shift keying is used on these bands, which modulates the frequency a bit with data.

Silly memory aid: The question asks about modulation below 30 MHz. FSK is the shortest/smallest answer.

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Keyboard-to-keyboard operation refers to the situation where you type characters and they're sent as a simple sequence of characters, one after the other. RTTY, PSK31 and MFSK all operate in this manner, which can be thought of almost like alternate forms of machine CW.

PACTOR, on the other hand, is a packet protocol that requires information to be divided up into packets which have header and footer information for features like data compression, error detection and acknowledgement messages for automatic repeat request (ARQ). Therefore it does not support "keyboard-to-keyboard" operation.

Just remember that PACTOR is for packets, not for keyboards typing to each other.

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PACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. Of the choices presented it is the best mode to transfer binary files. Not to say it cannot be done in other modes.

http://en.wikipedia.org/wiki/PACTOR

Sill Hint: An pACTOR can be either a male or female (Binary)

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Similar to the way Morse Code has different lengths for different characters (E is one dot) PSK31 uses variable length coding where the most used characters are given shorter lengths to save bandwidth.

It was originally called varicode by its inventor, an amateur radio operator.

SILLY HINT: Question has "length" in it. Only two answers have numbers in them that you could use to measure a length. The correct "ONE," has "1" in it - D

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PSK31 operates with a very narrow bandwidth of only 31Hz, with contacts spaced as close as 100Hz apart. As many as 20 conversations can be realistically squeezed into the bandwidth of a single 2.5KHz SSB transmission. [PSK31 Spectrum efficiency on Wikipedia]

MFSK16 uses about 316 Hz bandwidth.

Hint: (PSK) Pretty SKinny

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Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.

Note that only one answer addresses both Direct FSK and AFSK as presented in the question.

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ALE is Automatic Link Establishment, the standard for digitally initiating and sustaining HF radio communications in an automated manner. You can think of it as an automated way of calling for a specific station and answering that call.

When attempting to call another station, an operator enters an ALE address into the radio. The calling radio selects an idle channel (frequency in its list) and transmits call messages. On the radio waiting for a call, ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received. "The designated call sign" means the radio's own call sign.

There's a decent article about ALE on Wikipedia for those wanting more information.

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