Radar Horizon & Range
Radar signals are line of sight, just like light, if your eye height was the same as your radar height, your radar horizon would be the same as your visual horizon. We need to know our radar horizon, reasons will become clear later in this section, and in order to calculate it we only need the height of the vessels radome.
The formula for radar horizon we are going to use gives our answer in Nautical Miles, you may have seen other formulas that give answers in meters.
Radar Horizon in NM = (√ Ant height m ) X 2.2
In the animation you can see the higher the antenna, the further away the radar horizon is, note that you can still see targets beyond the horizon but they must contribute to this range with some height of their own. If you know the height of the reflector, say for example if it is a charted object such as a mountain, you can work out the maximum range you should be able to pick it up - simply do the same formula above for the mountain height and add the answer to your own radar horizon. This can be useful for navigation.
In practice for collision avoidance we have more trust in the radar image right the way up to the radar horizon range. You will be able to see targets with higher radar reflectors beyond your horizon but not the smaller vessels that are below your radars line of sight.
The area in green is the coverage of your radar beam. It covers the area all the way up to the radar horizon, and a zone beyond your radar horizon, above the surface, and above your radars line of sight.
The area in red is the dead zone, you can still detect ships that are in the dead zone if they have a radar reflector high enough to reach into your green zone above.
As an example if you have a radar at 9m, the maths is √9 = 3 x 2.2= 6.6NM so your radar horizon is 6.6 NM
To further this example, if you were hoping to find a known vessel on your radar with a reflector height of 12m you can work out the maximum range you would be able to detect the other vessel by simply doing the same formula for the other vessels reflector and adding the answer from this to your own radar horizon, so this would be:
√12 = 3.46 x 2.2 = 7.6 NM + 6.6 NM = 14.22 NM
From a range perspective the higher the radar the better, there are however some practical limitations to this, on sailing yachts anything beyond around 1 third the way up the mast produces too much lateral movement, and the shock loads and inertia beyond that point in the mast are too high. On motor vessels the radar would be best suited to be as high as the structure will permit.