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As described above, radar units have a maximum range which is dependent on the height of the antenna mounting and the height of the target. At extreme range, it is the curvature of the earth that gets in the way - radar is line-of-sight. However, this also means that there is an area immediately around your boat where the there NO coverage.
The radar beam is usually focused within 10 degrees of horizontal. This means that there is a "cone" around your boat which is not covered by the radar because no signal reaches into that space. You can calculate the minimum distance at which the radar is effective by multiplying 4.5 by the mounting height (assuming a 10 degree below horizontal transmission limit, sea level-height object).
Our 25 ft high installation means that any object at sea level closer than 140 feet will not show up. This means that "instrument landings" in narrow harbor entrances will more likely find a happy ending with properly functioning DGPS than Radar on our vessel. Any future vessels would have a lower mounted randome on them.
Even though the radar randome is not transmitting 100% of the time even when on, it''s a good idea to keep the transmitter well above the usual "head-levels" on board. Remember, microwave exposure can be harmful!