Radome vs Open Array Radar

This is one of the few marine electronics decisions where the physics is simple, the trade-off is real, and the right answer is usually obvious once you say the boat out loud.

#The physics: beam width

A radar antenna's horizontal beam width determines whether two objects at the same range but slightly different bearings appear as two targets or one.

A longer antenna produces a narrower beam. That is the entire advantage, and it is not marketing — it is aperture. An open array is simply a longer antenna than will fit inside a dome.

What a narrower beam buys you in practice:

• Picking a gap in a line of shipping rather than seeing a single smear
• Separating a vessel from the coastline behind it
• Reading a fishing fleet as individual boats
• Better long-range performance, because energy is concentrated

#The seamanship: everything else

On a sailing yacht, the right-hand column is a list of problems. Weight aloft affects stability and motion. Windage affects performance and, in a blow, loads. A rotating open antenna near a sail plan is an unhappy arrangement. And as the radar guide notes, serviceability matters more than most buyers expect — a mast-mounted array is the least accessible equipment on the boat.

#The limit that applies to both

Neither antenna can see what is below the radar horizon. Antenna height, target height and target reflectivity set detection range before beam width gets a say, which is why the radar guide argues that advertised maximum range is close to meaningless and that the valuable range for a cruising yacht is a fraction of a mile out to about 12 NM.

An owner agonising over beam width while mounting the antenna two metres lower than necessary has the priorities backwards.

#By vessel

#Cruising sailing yacht, 30 to 50 ft

Radome. Almost without exception. A 20 to 24-inch solid-state Doppler dome — HALO20+/24, Quantum 2, a Fantom dome or a Furuno NXT dome depending on ecosystem. See sailing yacht navigation.

#Cruising catamaran

Radome, usually on an arch or hardtop where the horizon is good and servicing is possible from the deck.

#Offshore fishing boat, 8 to 12 m

Either. A premium dome is entirely adequate; an open array is justified if you regularly work around dense traffic or want maximum separation. Refresh rate matters here because the boat is fast.

#Motor yacht and large sportfisher

Open array is strongly worth comparing. This is the case it exists for — see motor yacht navigation. At 25 knots a vessel covers 0.42 NM a minute, and target separation plus refresh rate buy back decision time.

#Trailer boat

Radome if anything at all, and only if the boat genuinely runs at night or in poor visibility. See trailer boat electronics.

#What to spend the difference on

If you were considering an open array for a cruising yacht and choose a good radome instead, the saving is substantial. The cost guide ranks where it should go: a better heading sensor, correctly sited; autopilot drive margin; a SOTDMA AIS transceiver; and proper commissioning including radar-to-chart alignment.

A brilliant radar with a poor heading source will still disappoint. That is not a slogan — misaligned heading rotates the overlay and corrupts every MARPA solution.

#Bottom line

Buy a radome for a sailing yacht, a catamaran or a trailer boat. Compare open arrays seriously on a motor yacht, a large sportfisher, or anywhere a narrower beam earns back its windage and weight. And in either case, spend equal attention on antenna height, heading quality, mounting and training.