Auto fins signal a new (electronic) era

By Jeffrey Krasner, Globe Staff | November 7, 2004

Fins are back.

But instead of the garish, chrome-encrusteed tail fins that adorned cars in the 1950s and early '60s, today's fins are small, unobtrusive, and practical.
High-end cars from a number of manufacturers -- including Audi, BMW, and Acura -- are sprouting single antenna housings at the rear of their roofs that look like shark fins.

The shape of the new antennas has a lot more to do with electromagnetic reception and the proliferation of on-board electronics than designers' efforts to give cars a racy look.

Today, radio is just the start for vehicle communication needs. Besides AM and FM reception, many cars come prewired for satellite radio services.

Then there are navigation systems, which use signals from global positioning system satellites to track the vehicle's precise location.

And more cars are being built with emergency communications systems, such as OnStar by General Motors, that provide a variety of services, such as automatically notifying emergency responders if an air bag deploys and unlocking doors from a remote location should you leave your keys in the car.

How to handle all these different signals? Many carmakers find the rear of the car's roof to be the ideal location to place an antenna. It's unlikely to be damaged in all but the most serious collisions, and the flat surface helps to gather the signal.

"Height is a key factor in obtaining the best signal," said Jim Geier, accessory engineering manager for BMW of North America. "If the antenna is on top of the car, the signal is not easily blocked."

The plastic housing that gives the shark fins their predatory appeal covers the electronic components underneath and was designed as a fin simply because it needs to be aerodynamic and resistant to damage in a car wash, unlike a traditional mast antenna.

While the fin design may be universal, manufacturers are taking diverse approaches to what swims beneath it.

BMW puts the GPS antenna, two cellular phone antennas, and two satellite radio antennas underneath the fin.

Audi puts GPS and antennas for both satellite radio providers, but places antennas for the prewired cellphone inside the front and rear bumpers.

The fin on the new Acura RL sedan is strictly for OnStar. The carmaker conceals the XM satellite radio antenna within the cover for the center high-mount stop lamp.

What about plain old AM and FM reception? Turns out nothing brings in the tunes and the talk better than an old-fashioned mast antenna, Geier said.

But carmakers say the stainless steel rods spoil the lines of the cars and often malfunction. More of them are embedding radio antennas inside the rear window glass, sometimes with wiring that looks like part of the defogger. Often, there are several antennas within the rear glass, aimed in different directions. The radio automatically chooses whichever is delivering the strongest signal.

But just as quickly as they have become popular, shark fins may be headed for extinction. The components underneath continue to shrink. The newest innovation is the patch antenna, a small one-inch square that barely sticks above the surface of the roof.

"The antenna will continue to get smaller," Geier said.

Boston.com / Cars