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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Having a PC clock that is in sync will ensure transmission times are accurate and the other stations are decoding your transmission during the receiving 15 seconds. There are many ways to ensure your PC clock is in sync like using WWV broadcasts or software such as Dimension 4.

https://www.nist.gov/pml/time-and-frequency-division/time-distribution/radio-station-wwv

http://www.thinkman.com/dimension4/

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FT4, like its sibling FT8, is a weak signal condition digital protocol. FT4 was a followup to FT8 and is designed for speed to benefit contesting. Using four-tone CPFSK (hence the 4 in FT4), the bandwidth is 90hz, compared to the 50hz of FT8, and its speed is potentially 2.5 times that of FT8.

FT4 is included in the popular WSJT-X software as well as others. FT4 also has additional useful contesting features such as the ability to potentially determine the best QSO partner for the next contact.

Memory aid: FSK, which FT4 is based on is a system that uses specific frequencies or "Tones". Therefore the number in the name is the number of tones. FT8 - 8 tones FT4 - 4 tones FT2 (or binary FSK) - 2 tones

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Memory Aid: FST4 = Four answer choices are correct.

The FST4 mode has several characteristics that make it unique and effective for LF and MF band communications. Here’s a breakdown of each characteristic mentioned in the question:

1. Four-tone Gaussian frequency shift keying (4-GFSK): FST4 uses a modulation scheme called 4-GFSK, which stands for Four-tone Gaussian Frequency Shift Keying. This involves four distinct frequency tones, and Gaussian filtering is applied to minimize bandwidth and reduce interference, enhancing the efficiency of the signal transmission.

2. Variable transmit/receive periods: FST4 offers a variety of transmit/receive (T/R) periods, allowing users to choose from multiple sequence lengths depending on their needs. The available periods include 15, 30, 60, 120, 300, 900, and 1800 seconds, providing flexibility for different communication scenarios and signal conditions.

3. Seven different tone spacings: Corresponding to the various T/R periods, FST4 also has seven different tone spacings. These tone spacings are adjusted to fit the duration of the transmission sequence, ensuring optimal performance and sensitivity for each specific period.

Since these are all characteristics, all of these choices are correct.

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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.

WSPR, pronounced "whisper", stands for Weak Signal Propagation Reporter. It's a protocol that was designed just for testing propagation paths and the messages passed only include the station call sign, grid locator, and how much power is transmitted. In essence, it's just sending out a beacon so that other stations can report that they received it and thus build a map of who can or cannot receive it.

Thus WSPR does not support "keyboard-to-keyboard" operation because the messages are carefully structured and have a lot of error correction built in. It's similar in that way to many other protocols such as FT8 or JT65, though with less information that can be passed.

RTTY, PSK31, and MFSK16 are all protocols which directly translate keys sent to sound and transmit them, so the message is sent sequentially and directly -- so they literally can send keys as you type them on the keyboard (though it's usually better to type the message ahead of time and then send).

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FT8 transmissions are 15 seconds in length. A complete QSO without any retransmissions will take 90 seconds.

Calling CQ - 15 seconds

Receive a reply with callsign and grid sqaure - 15 seconds

Send Signal Strength - 15 seconds

Receive Signal Strength - 15 seconds

RR73/RRR - 15 seconds

Receive 73 - 15 seconds

Memory Aid: “F T” sounds like “Fifteen,” which is the correct answer.

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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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FT8 is a mode optimized for extremely weak digital signals, using as little as 50 Hz of bandwidth. It is extremely limited in how much information can be passed, but because it uses so little bandwidth it can be successfully decoded even if the signal is only barely audible (as low as -24dB).

The other modes here may not use a lot of bandwidth, but all of them use more than 50 Hz!

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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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