Have you ever wondered about the length of car antennae? Of course you have—you’re a human being (or advanced AI, or a brain-augmented dolphin) reading this, and as such you’re full of curiosity and wonder and almost crippling physical desires. As such, you’ve likely noticed that many modern cars have dispensed with the tall, thin “whip” antennae of older cars and now have stubby little antennae, or even little shark fin things. Why is this? Why did we use to use such long antennae? Were they all the same length? What changed? Why am I a dolphin reading the internet? Relax. I’m here to help.
The antennae I’m going to talk about are the ones most common, still, in cars—radio antennae, designed to pick up (primarily) frequency-modulated (FM) radio signals. Sure, I know you kids with your Spotify and podcasts and Friendster likely don’t listen to the old over-the-air radio as much, but it’s still a thing, and cars still have antennae.
Now, pretty much anything that conducts will act as a sort of antenna—tinfoil, a coathanger, some crutches—but in order to optimally capture that electromagnetic radiation and send it into a little box that converts it into electrical impulses that your speakers convert into physical compressions of the surrounding air and your eardrums convert back into other bioelectric signals and your brain converts into the sweet, sweet strains of Radar Love, then that conductive whatever should be a certain length.
In the case of FM radio, the conductive thing can simply be a metal rod, and, ideally, its length should match the wavelength of an FM signal, which, at about 100 Mhz, comes to around a wavelength of roughly ten feet.
Now, I bet you’re thinking, wait a minute there, Jayjay Torch—I’ve sure as hell never had a car with a freaking ten foot antenna! And, of course, you’re right. A ten foot antenna would be ridiculous. Luckily, antennas work almost (longer antennae do work better, but still) as good if you make them some reasonable fraction of the wavelength you want to get—in the case of most car radio antennae, that’s about a quarter the wavelength.
So, the standard antenna size on most cars, for decades, was this 1/4 wave antenna size, which comes to about 2.34 feet, or around 31 inches. If your car has a telescoping antenna, you can get a good sense of the difference proper length makes by noting how much better reception is when the antenna is fully extended to the 1/4 wavelength size as opposed to shorter.
So what’s going on with modern car antennae that are often little fat stumpy things or even shark fin-like projections? Mazda was even so pleased about moving to shark fin-style antennae that they made a whole web page just about that.
In their page about their shark fin antennae, they give a hint about what’s going on inside that fin that allows them to get comparatively good reception even when the antenna is clearly much, much smaller than the 1/4 wavelength size of around 31 inches:
Because the antenna cannot be removed and it affects the car’s overall height, Mazda’s engineers worked on lowering the antenna’s height without sacrificing reception. They came up with an idea to change bar-shaped parts to spiral coil, and used electronic substrate to make a receiver.
Many technical challenges were conquered by antenna expert engineers with 25 years of experience in radio wave technology development, and Mazda’s very first shark fin antenna was successfully mounted on the CX-5.
The key there is where they talk about a coil; that’s also basically the same solution that’s employed on most modern, rubber-coated, stumpy roof-mounted radio antennae.
Inside modern shark-fin-style radio antennae on modern cars, you’ll actually find multiple antennae—ones for cell signals, satellite radio, and old AM/FM broadcasts. You can see what the inside of one of these units looks like here, in this Reddit post from some poor guy whose antenna cover flew off his Sonata:
Inside those boxes are coiled antenna wires, length optimized for whatever EM band they’re intended to pick up. Some are simpler inside, like this aftermarket antenna for a Cadilllac:
You see the “coil” of conductive copper right in there, and I expect if you were to stretch it out, it’d be about the size of a 1/4 wavelength car antenna, around 31 inches or so, or maybe some close fraction thereof.
Some shark fins don’t bother with FM antennae, which are instead integrated into the rear window glass, along with the defroster, or sometimes hidden in places like rear spoilers.
Most shark fin antennae actually kind of compromise FM signals—an old school 1/4 wavelength whip antenna likely will get better reception due to its better gain—but the other antennae it contains (satellite radio, etc) are now considered more important, as are the improved aesthetics, so we just deal with it, and for the most part, it’s fine.
Antenna quality has been sacrificed for aesthetics before, even when AM and FM were important—remember GM’s windshield antennae? They were never great, but they were good enough, and arguably gave the cars a cleaner look.
So, I hope this helps. There actually is a proper, ideal length for a radio antenna, and modern cars do all they can to hide that from you.