An antenna is an element capable of radiating and intercepting radio waves. The radiation and reception of radio waves is most effective when the antenna is in resonance. Various resonant configurations can be achieved by antennas with dimensions of 1 or 1 wavelength, or multiples thereof. It is more important for a transmitting antenna to be in resonance than for a receiving antenna since transmitter performance can be badly degraded by a mismatched antenna. Older types of transmitter could be damaged by feeding into a poor antenna but modern designs usually incorporate automatic protection circuitry to shut down the transmitter or reduce power to a safe level if necessary.

VHF ANTENNAS

As the wavelength in the maritime VHF band (154-162 MHz) is around 2 metres, it is possible to use 1 and 1 wavelength antennas. The most basic design is the dipole, which consists of a split 1-wavelength element connected at the centre to a balanced feeder cable. Figure 1 shows some simple examples of VHF antennas, including the artificial ground-plane antenna and the VHF rod antenna - typically a 1.5 m fibreglass pole contains a dipole antenna. As noted in section 3, it is important that VHF antennas are mounted as high as possible and in a position free from obstruction by the ship's superstructure ( Figure 2) .

VHF Antenna

Figure 1. CX 3 - typical VHF rod antenna

VHF antenna

Figure 2. VHF antenna with artificial ground plane

La antena de VHF es una antena vertical con una longitud de 1 m aproximadamente. Se coloca en una posición tan alta como sea posible. Para la conexión entre la radio y la antena de VHF se suele utilizar un cable coaxial.

antena cable
Cable coaxial
A: envoltura de plástica externa
B: pantalla de cobre
C: aislador del dieléctrico interno
D: base de cobre

El cable coaxial tiene una impedancia (resistencia para la corriente alterna CA) de 50 Ω (ohmio). Esta impedancia tiene que ser igual en el conector de antena de la radio VHF así como en el conector en la antena. Cuando esta conexión no es de 50 Ω no solo habrá energía irradiada en el cable, sino que también habrá una cierta energía reflejada. Cuando el nivel de energía reflejada es demasiado alto, reducirá la emisión de la señal de radio disminuyendo su alcance habitual. Se podría dañar también al equipo radio VHF cuando la reflexión es muy alta en un período largo de transmisión. Y a la vez este problema de impedancia causará una pérdida de la intensidad de señal (en el cable de la antena) de todas las señales recibidas.

Un cambio de impedancia se puede producir en el cable de antena debido al agua. Cuando el cable se daña en el exterior (de la envoltura plástica externa) podría entrar agua en el cable (en la pantalla de cobre). El agua cambiará la impedancia del cable y esto llevará a una reflexión de la energía. También la conexión del cable a la antena podría causar este problema (que entre agua en el cable de antena). Esta conexión se debe hacer impermeable; esto se puede hacer usando una cinta de vulcanización.

MF/HF ANTENNAS

In the MF/HF bands, however, wavelengths vary from 180 metres (1650 kHz) to about 12 metres (25 MHz). Resonant 1- or 1-wavelength antennas covering this entire frequency range are therefore not possible. The problem can be eased by using a number of separate antennas, each covering a single band or several harmonically related bands.
An antenna tuning unit (ATU) is usually used to "match" the transmitter output to the antenna over a wide range of frequencies. In effect, the ATU uses electrical components, i.e. coils (inductors) and capacitors, to achieve a resonant electrical length in combination with the actual physical length of the antenna. Nevertheless, it must be noted that the efficiency will vary over the frequency range used because the radiating efficiency is still determined by the physical length of the antenna. Even if the ATU can match a very short antenna to the transmitter, for example, the overall efficiency will be poor. Connections between the transceiver, the ATU and the main antenna should be kept as short as possible to ensure the efficient transfer of energy to the antenna.

If there is ample space between existing masts or to erect special antenna masts, then the main or emergency antenna may be a wire antenna. A wire antenna may be stretched between masts or between a mast and another elevated part of the ship's superstructure. An example is shown in Figure 3 of a T-type antenna, although inverted-L types may also be found.

MF/HF wire antenna

Figure 3. T-type MF/HF wire antenna

However, because of lack of space on board many modern ships, most GMDSS fittings use vertical whip antennas for MF/HF transmissions. For example, the main HF transceiver may use an 8-12 m whip (Figure 4 ), the MF/HF DSC watchkeeping receiver may use a 3-6 m ( Figure 5 ) whip and the NAVTEX receiver may use a 1 m whip.

AT100D antenna

Figure 4. AT100D is a high quality glassfibre transmitting antenna for marine HF telephony bands.
Frequency range: 1,6 – 30 MHz. Power rating: 1,5 kW.


AR42

Figure 5. AR42 is an efficient fibreglass receiving antenna for the marine coastal and HF communication frequencies ( HF DSC ).
Frequency range: 0,15 – 30 MHz.

straznik5

Figure 5. Example antenna installation on STRAŻNIK 5

1. MF/HF SSB Tx/Rx antenna.
2. GPS antenna.
3. VHF antenna.
4. VHF antenna.
5. VHF antenna.
6. VHF DF antenna.

straznik5

Figure 6. Example antenna installation on STRAŻNIK 5

1. MF/HF SSB Tx/Rx antenna.
2. Antenna tuning unit (ATU).

Scandinavia

Figure 7. Example antenna installation on M/F SCANDYNAVIA

1. MF/HF SSB Tx/Rx antenna.
2. VHF & VHF DSC antenna.
3. MF/HF DSC Rx antenna.
4. VHF antenna.

ANTENNA MAINTENANCE

All antennas should be kept clean, salt deposits removed, and feeders and brackets checked regularly.
The various insulators must also be checked for cracks and must be cleaned regularly. The safety loop on a wire antenna prevents the antenna falling if undue strain (e.g., from high winds or build-up of ice) is placed upon it; the weak link should break in the first instance rather than the antenna.
A spare wire antenna has to be carried and should be stored in an easily accessible place so that it can rapidly be erected in an emergency.
It should be remembered that dangerously high voltages and RF currents are present close to the main antenna. Ideally, the ATU and the link to the main antenna should be protected to prevent anyone touching the feeder. Before doing any maintenance work on any antenna, ensure that power is removed from the equipment and that the main fuses are removed and kept in a safe place (a pocket is often the simplest and safest place).
As a further precaution, the antenna should be also be grounded, since RF energy can still be induced in the antenna from other antennas on board or on nearby ships. Even though a shock from an induced RF voltage may only startle rather than cause direct injury, an accident may still result through, for example, falling from a ladder, or dropping tools from a height.
An antenna rigging plan should be available showing the positions of the various antennas.
Last modified: Saturday, 25 April 2020, 7:50 PM