- Siting of a radar antenna needs careful consideration so that a suitable compromise is reached which takes into consideration the effect of height on range performance and sea clutter, physical integrity and the need to minimise shadow sectors and false echoes through reflection.
- Maximum radar range is dependent, amongst other factors, upon antenna height. However, whilst an increase in antenna height increases the radar range, it also increases the amplitude and extent of sea clutter. The echoes of buoys and small craft within the area of sea clutter may not always be conspicuous.
- The physical structure supporting the radar antenna must be substantial enough to prevent twist which would cause bearing errors. The supporting structure must not allow/introduce excessive vibration which may degrade performance, reduce reliability and lead to early failure.
Shadow Sectors and False Echoes
- Interaction of the antenna beam with the structure of a vessel will impact upon the performance of the radar through:
- Blockage whereby structures such as masts or funnels are directly in the path of the beam from the antenna to a target and thereby cause shadow sectors.
- Reflection where energy from the antenna is reflected off part of the ship’s structure such as a mast, funnel or the deck and thereby causes false or distorted echoes.
- It should be noted that both blockage and reflection will occur with some structures such as masts or funnels and both effects influence radar performance. These effects are not only related to metallic structures – all objects, whether they are metal, plastic, wood, etc., can cause degradation in performance.
- As a general installation guideline, obstacles should not lie within the –10dB beamwidth of the antenna. Objects such as masts, posts and funnels in the horizontal plane of the antenna may lead to shadow sectors, ghosting and smearing. If they cannot be eliminated they should be positioned at the greatest possible separation distance from the antenna. Objects within the –10dB elevation beamwidth of the antenna, i.e. 20° to 25° below the line of sight should also be avoided as they too cause degradation in performance; inclined deck areas within this region are a particular cause for concern.
- Raising the antenna so that it looks over obstructions may be an acceptable measure, particularly to avoid shadow sectors caused by the bow of the ship, provided the limitations mentioned in paragraphs above are borne in mind. In ships that frequently navigate astern, the need to avoid shadow sectors astern should not be forgotten.
- The Surveyor should ascertain that the shadow sectors have been measured and recorded and are displayed adjacent to the radar. Following installation the angular width and bearing of any shadow sectors should be determined by the Master at the first opportunity and recorded. For a new ship, this should be done during trials, and kept up to date following any changes likely to affect shadow sectors.
- The cause and effect of changes in shadow sectors arising from temporary variations such as alterations in trim, the carriage of deck cargo and the stowage of derricks and cranes in different positions, should be recorded.
Fouling
The antenna unit should be mounted where there is least danger of its being fouled by halyards, derricks, radio antennae, etc.
