Where To Mount A Radome For Best PerformanceBy Lenny Rudow
Published: June/July 2014
No matter how big, how powerful, or how expensive your radar is, its performance may be limited by the way you mount the antenna.
Most fast powerboats run with a slight bow-up angle, which can be compensated for
by placing a wedge under the dome or array during installation. In the case of some custom
or semi-custom boats, this angle may already be built into the radar-mounting platform.
The radome or open-array antenna sitting atop your boat is your radar's eyes and ears, and like the eyes and ears upon your own head, the way it's situated can make a world of difference in what it sees and hears. When it comes to radar, of course, the way it sees is the way it hears. Think of your radar antenna as a loudspeaker and microphone, all in one. The loudspeaker shouts — with a microwave pulse — and the antenna listens for an echo. Then the processor crunches a few numbers and, voila, a blip appears on your screen.
The radar antennas on most boats today transmit microwave pulses with a magnetron, then listen for those reflections that bounce back from solid targets. A mathematical calculation using the amount of time the pulse takes to be reflected determines the exact distance to the target. The latest technology, Broadband radar, broadcasts a continuous transmission wave that increases in frequency as it moves away from the radome, hits a target, and is reflected back. The unit's brain then uses the difference between frequencies of the transmitted and returned waves to determine target distance.
When it comes to mounting a radar antenna, higher is better, to a point (see sidebar). First off, getting the dome above head level is a must because you don't want to bake yourself and your crew with microwave pulses every time you use the radar. (This is less of an issue with Broadband, which emits a much lower burst of power.) Beyond the safety concerns, height is important because the biggest limiting factor relating to radar's performance is the Earth's curvature. Those microwave beams can't be bent to follow that curvature, so the height of your radar antennas and the height of the target you're looking for will always limit the distance your radar can see. Ready for some basic math?
(1.22NM x square root of the Height of Target) +
(1.22NM x square root of the Height of Radar Antenna) = range
For example, a radome mounted 12 feet off the water might first spot a small powerboat, with a maximum height of 9 feet, at a distance of just under 8 nautical miles. No matter how big and powerful your radar may be, no matter how much it cost, you can never expect it to see beyond that. Nor will it commonly see all the way out to this range; this is merely the maximum you can hope for. Target shape and density, atmospheric conditions, and other variables usually will have an effect on range as well.
Sailboaters have a natural advantage regarding height because they have a tall mast to work with. But in some cases, back-stay or post-mounting options make more sense. This is a judgment call: Is it more important to you to gain range, or to have easy access to the radar antenna? Do you need to use the radar often when heeling? If so, you need a mount that matches your needs, and this may affect whether you use the mast or go a different route (see sidebar).
Powerboaters, on the other hand, may need to add a mast or pedestal to gain elevation if they want to maximize range. How much is necessary? The above equation holds the answer, but for those of us who aren't math whizzes, let's use the example of a 35-foot cruiser with a hardtop that's elevated 10 feet over the water. If you mount the radome right on that hardtop, you'll be able to see a 300-foot-tall building or water tower on shore from just over 25 miles off. You'd see a 100-foot-tall freighter at around 16 miles. And you'd see other boats of the same size as yours at around 8 miles. Raising the dome 10 feet above the hardtop on a mast does add range; however, not as much as you'd think. That tall building might now be visible at just over 26 miles. The freighter could appear onscreen at 18 miles. And a boat like yours would come into radar range at around 9 miles. So is an extra nautical mile of collision warning worth it?
Tips for accurately interpreting the range and bearing of landmarks and boats on your radar screen.
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Higher Is Not Always Better
On a sailboat, the greater range of a radar mounted on the mast must be weighed against these advantages of mounting it lower:
- The scanner is less vulnerable to damage from an errant halyard or sail.
- The "cone of silence" is reduced so that targets close to the boat can still be "seen" by the radar.
- The motion in a seaway is reduced, enhancing radar accuracy.
- The radar is easier to service.