(B). A vertical (whip) antenna has a radiation pattern best described by a circle.

A whip antenna has a circular 360 degree donut-shaped radiation pattern. Best used for not a specific direction communication.

An excellent article on Wikipedia Whip Antenna

For more info on long wire antennas, please see Electronic Desk: Long wire antenna

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Why is an antenna tuner required for MF-HF transmissions?

(C). The length of the physically-fixed antenna must be electrically matched to the intended frequency of operation.

The length of the antenna should be matched to frequency when selecting it before installation.

But, the answer is kind of correct. What the antenna tuner does is to match the impedance (resistance) of the transmitter cable with the impedance of the antenna. WHY?

If there is more resistance in the antenna, then part of the energy will flow back to the transmitter, and they have protection against that energy damaging their parts, so the transmitter will send less energy.

The antenna tuner is like an agent that takes in energy at whatever resistance and feeds the antenna at the antenna's resistance. No power is lost, transmitter thinks that there is a match, so it does not lower the emitted energy.

A good pdf presentation that explains more is on How Does Antenna Tuner Work?

Even more info for this important topic is on Electronics Notes: What is an Antenna Tuner

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A vertical quarter wave antenna with a good ground connection will
(D). Radiate omni-directionally.

A vertical antenna of any length will radiate in all directions. The shape of the emitted radio waves looks like a doughnut around the vertical antenna.

The condition of the ground is important because when the waves bounce from the ground, depending on the condition, the ground may absorb more or less energy.

If it absorbs more, than there is less for the next bounce, and that limits broadcasting range.

The very best ground is salt water, and so marine antennas have better ranges than antennas on soil, rock, etc.

For more info, please see Wikipedia article Radiation Pattern

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What is the most common type of antenna for GMDSS MF-HF?

(A). Vertically polarized whip antenna

The most effective of antenna types for marine GMDSS VHF is a vertically polarized whip antenna.

Other types of antennas (and there are tons of them) do not provide the vertical radiation that uses salt water to bounce without much loss of radiation energy (not like on land).

A marine-focused article about antennas is on
VHF Antennas

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What is the purpose or function of the antenna coupler?

(D). All of these answers are functions of the antenna coupler.

• Electrically matching the antenna system to the transmit frequency.

• Changing the overall wavelength of the antenna system (in addition to the fixed length whip.)

• Enabling maximum transmitted signal on the band chosen by the operator.

An antenna coupler helps to combine radiation of one or more antennas to synchronize and propagate a number of wavelengths. It matches at a time one transmitter frequency wave with one antenna.

For more info, please see What is an antenna coupler

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(A). Advantage does a vertical whip have over a long wire-- It radiates equally well in all directions.

A whip antenna has a circular 360 degree donut-shaped radiation pattern. Best used for not a specific direction communication.

A long wire antenna may have unidirectional or bidirectional radiation pattern. The patterns as a main lobes and side lobs, which means that they concentrate waves in specific directions.

To reach whoever is in the area, the vertically polarized whip antenna is far more effective. The waves bounce off salt water without much energy loss, unlike when on the ground, and the ground absorbs some energy with every bounce off.

An excellent article on Wikipedia Whip Antenna

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