How to choose the medium frequency antenna for offshore Non Directional Beacon
Efficient operation of offshore Non Directional Beacon (NDB) for helicopters homing that uses the frequency range 315.0–526.5 kHz is possible only if the antenna is selected and located at site properly. To avoid mistakes the specific operational conditions of the NDB on board of the ships, floating drilling rigs, and fixed offshore platforms should be taken into account when designing and engineering the site.
Technomarine offers the following NDB antennas: Koltso-2 Helipad Antenna, AC-15 (AC-15D), and 100MF.
Each of these antennas has different structure and has certain advantages and disadvantages. Type of antenna should be chosen while keeping in mind the efficient exploitation of the advantages, and the mitigation of the negative factors, depending on the antenna site specificity. The following main features should be considered when choosing an antenna:
Personnel safety requirements, antenna operation and maintenance safety requirements;
Limited space on board for antenna installation (limited above-deck space, limited options for antenna positioning and fastening with regard of the helicopter deck structure, ship superstructures and masts, influence of metal structures, mutual interferences from other antennas, distance between the antenna and antenna tuning unit from the NDB transmitter);
Electric properties of the antenna to guarantee the required coverage of the NDB (antenna performance and minimization of the high-frequency voltage in the base of the antenna in order to avoid the insulators breakdown).
It is necessary to make the compromise decisions for efficient application of the ship's radio equipment when designing and engineering. Often, after preliminary selection of the antenna type after approval of the equipment installation location, the antenna type needs to be changed because the project does not comply with some requirements for the NDB antenna or other near-by equipment.
Table 1 illustrates the main advantages and disadvantages of the previously discussed marine antennas.
Table 1 — Features of the NDB marine antennas manufactured by Technomarine
The 100MF is base mounted 10 meters heavy duty top loaded (inductance and capacitance) antenna system. Top hat contains 6 capacity top radials and the loading coil. The loading coil provides more efficient current distribution over the mast. 100MF antenna is shown on Figure 1.
Figure 1 — 100MF antenna
Antenna may be installed using the antenna tilting device making the antenna lifting and lowering easy, but in this case enough free space is required on the deck. Operational position of the installed antenna requires just a little of space and has small footprint, but when installing the antenna, antenna’s top deflection caused by wind and / or oscillations should be taken into consideration. The top deflection is up to 3 meters. The main disadvantage of the 100MF antenna is related to high reactance, and consequently to the limited permissible antenna current and poor stability of the antenna parameters under the influence of weather conditions. AC-15 antenna consists of 6-wire cylinder (length 13 m and diameter 0.55 m) and downlead wire. The length of the cylinder may be changed by request to meet the specific project requirements. Typically, the antenna cylinder is located horizontally between two masts or other tower-like or pole structures. Inclined position of the cylinder is also possible, with the higher position of the non-powered end. The cylinder should be strained at height of 6 to 10 m. AC-15 antenna is shown in Figure 2.
If the height the cylinder installation is insufficient, consider the of AC-15D antenna. This antenna type is similar to AC-15, but there are two cylinders used, the length of each of them is 9.4 m. The cylinders should be positioned in the azimuth plane, 20 to 50° to each other, and 0 to 20° to horizon. Upper ends of the cylinders should be positioned at the most possible height. In certain situations, the inclination angle to the horizon may be increased, if there is no way to fasten the bottom parts of the cylinders or antenna tuning unit installation as recommended.
Figure 2 — AC-15 antenna
Koltso-2 antenna is a long wire antenna installed on insulators horizontally along the perimeter of the helicopter deck. This long wire provides capacity load of the antenna, and the radiating elements are the metal structures of the ship, floating drilling rig, or fixed offshore platform. The length of the wire the installation kit is about 100 m. Koltso-2 antenna is shown in Figure 3. Koltso-2 antenna is preferable for use on floating drilling rigs and fixed offshore platforms, as well as on the ships, where the helicopter deck is at least 10 m above the sea level. It is also necessary to provide free space between the wire and sea surface, not shielded by lower decks or other metal structures.
Dimensions and configuration of the helicopter deck shall guarantee the installation of at least 40 m of the wire. Usually the long wire goes along the free part of the helicopter deck perimeter in the form of semi-ring, powered from one end.
Figure 3 – Koltso-2 antenna
The main advantages of the Koltso-2 antenna are better (in comparison to other marine antennas) electric parameters, good electromagnetic compatibility with other marine radio-frequency equipment thanks to spatial separation, and the fact that the antenna is not an obstacle for helicopters.
When installing the marine antennas special attention should be paid to the requirements governing the minimum distance between the NDB antenna and metal structures (such as masts, yards, stacks, and others), and distance to other antennas.
Additional requirements pertinent to the installing of the antennas on the floating drilling rigs and fixed offshore platforms are provided in the “Rules for classification, construction, and outfitting of the floating drilling rigs and fixed offshore platforms” of the Russian Maritime Register of Shipping:
Transmitting antennas shall be positioned beyond the explosion hazardous zones;
All transmitting and receiving antennas shall not be closer than 9 m to drilling derrick, crane booms, and other high-rise structures that may cause the shielding effect.
The article is for your information only, and non-regulatory. In order to avoid mistakes it is recommended to come to agreement with Technomarine about the site layout of the radio beacon equipment. To agree the project or to get in touch with us for technical support, send us an E-mail to: support.ndb@Technomarine.ru.