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.
Figure 2. VHF antenna with artificial ground plane
The connection between the VHF radio and VHF antenna is made by coaxial cable.
This coaxial cable will have an impedance (resistance for alternating current AC) of 50 Ω (Ohm). This impedance will be the same on the VHF radio antenna cable connector as well as on the cable connector on the antenna. When this connection is not 50 Ω there will not only be radiated power in the cable but also some reflected power. When the level of reflected power is too high this will reduce your emission of radio signal and this will lead to a shorter range of maximum reach of your signal. It could also damage your VHF radio when the reflection is very high for a longer period of transmission. This above mentioned problem will also cause a loss of signal strength (in the antenna cable) of received signals.
The change in the impedance of the antenna cable could be caused by water in the antenna cable. When the cable is damaged on the outside (outer plastic sheath) water could enter the cable (in the copper screen). This water will change the impedance of the cable and this will lead to a reflection of power. Also the cable connection to the antenna could cause this problem (water entering the antenna cable). This connection should be made waterproof; this could be done by using vulcanizing tape.
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.
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.
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.
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.
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.
Figure 6. Example antenna installation on STRAŻNIK 5
1. MF/HF SSB Tx/Rx antenna.
2. Antenna tuning unit (ATU).
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.
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.
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) .Figure 2. VHF antenna with artificial ground plane
The connection between the VHF radio and VHF antenna is made by coaxial cable.
Coaxial cable
A: outer plastic sheath
B: copper screen
C: inner dielectric insulator
D: copper core
A: outer plastic sheath
B: copper screen
C: inner dielectric insulator
D: copper core
This coaxial cable will have an impedance (resistance for alternating current AC) of 50 Ω (Ohm). This impedance will be the same on the VHF radio antenna cable connector as well as on the cable connector on the antenna. When this connection is not 50 Ω there will not only be radiated power in the cable but also some reflected power. When the level of reflected power is too high this will reduce your emission of radio signal and this will lead to a shorter range of maximum reach of your signal. It could also damage your VHF radio when the reflection is very high for a longer period of transmission. This above mentioned problem will also cause a loss of signal strength (in the antenna cable) of received signals.
The change in the impedance of the antenna cable could be caused by water in the antenna cable. When the cable is damaged on the outside (outer plastic sheath) water could enter the cable (in the copper screen). This water will change the impedance of the cable and this will lead to a reflection of power. Also the cable connection to the antenna could cause this problem (water entering the antenna cable). This connection should be made waterproof; this could be done by using vulcanizing tape.
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.
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.
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.
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.
Figure 5. Example antenna installation on STRAŻNIK 5
2. GPS antenna.
3. VHF antenna.
4. VHF antenna.
5. VHF antenna.
6. VHF DF antenna.
Figure 6. Example antenna installation on STRAŻNIK 5
2. Antenna tuning unit (ATU).
Figure 7. Example antenna installation on M/F SCANDYNAVIA
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