Radar arches are becoming more common on cruising boats because they provide a convenient place to mount not only the  radar, but a wide variety of other devices.  On Spice, the radar arch is fitted with a pair of dingy davits, a flag pole, TV antenna, loran antenna, transom light, and attach points for a sun shade.  Other options include wind generators, solar panels, backup vhf antenna, and GPS antenna.  Truly a practical addition for the cruiser.

(yes, the dinghy's name is Sugar)

    In the above picture, the sun shade is "reefed" and tied down to the boom gallows, where it shelters the helmsman and allows normal boom motion under sail.  At anchor, the shade is unrolled over the boom, providing cover for the entire cockpit.  The davits are built up from 4 layers of 3/4" plywood and bolted to the arch sides.  They can be removed for offshore passages.

Some "lessons learned" in designing and building a radar arch:

    Blend the structure into the boat by curving the top and adjusting the side angles to complement the transom and cabin lines.  Make it look like it belongs on the boat.  Use scale drawings or mock up an arch using wood and PVC pipe.  Then look at it closely from all angles. 

    Anchor the arch supports into the stern pulpit for additional stiffness.  Ideally, the stern pulpit and radar arch should be combined into a single rigid structure.

    With a little welding skill, you can do this job yourself.  A cheap AC stick welder will do a good job on stainless steel if you use special stainless rod.  Smooth the welds with a disc sander and finish with a round file, emery paper, and metal polish.

Or:  Check out the following manufactures:

http://www.gate.net/~radrarch/.