Car Antenna Internet Nonsense Cleared Up - Canadian TV, Computing and Home Theatre Forums
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post #1 of 4 (permalink) Old 2016-04-04, 02:45 PM Thread Starter
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Thumbs up Car Antenna Internet Nonsense Cleared Up

While looking up replacement AM/FM Car Radio Antenna aerials online, I have come across massive amounts of nonsense and ill-informed advice (yep, such is life on the internet, of course). Hopefully this thread might come up for people doing similar web searches on this topic. AM-only car antennas don't really exist so car radio tuners simply connect electronically to the FM antenna for AM reception.

So, here in this thread we present the facts.

Theory: the ideal FM car antenna for long distance reception would theoretically be a dipole (2 opposite elements) measuring 1.54m (~60") wide from tip to tip, with director elements on a central horizontal beam, projecting high up from the vehicle and with automated aiming that makes sure the dipole always is perpendicular to the signal.

Reality: the theoretical antenna above is ridiculously impossible for a multitude of reasons having to do with keeping the darned huge thing up and running in the wind, all the while looking spectacularly ugly and stupid. The best solution for an FM car antenna is a monopole, in which the aerial represents one side of that theoretical dipole and the car's metal superstructure represents the other side. It should ideally be vertical, mounted on a horizontal part of the car's mounting surface, but a mild bit of angle will still do since FM Radio stations broadcast in a way that allows them to work too. The ideal length for such an aerial is well known thanks to the study of physics and electromagnetism: 1/2 wavelength of the middle of the FM Radio band = 1.54m (~60") but that makes for a pretty long antenna, so in the real world we can use 1/4 wavelength = .77m (~30") and not take much of a performance hit. A 1/8 wavelength antenna would be getting too small for good all-around reception.

OEM Radio Antennas: "Rubber Ducky" (small, stubby, almost always flat black, mounted at centre-rear of roof), "Shark Fin" (usually body colour, mostly on BMW, Volvo, Acura, a few others, mounted at centre-rear of roof), pillar-mounted (1970s-2000s Asian imports), and embedded windshield (General Motors 1970s-2000s) FM car antennas. These OEM (factory installed) radio antennas are technical compromises that cheat (1/8 or 1/16 wavelength for the Rubber Ducky and Shark), and have been brought in by car designers for esthetic and not radio performance reasons. Their reception capability is usually good for urban areas with strong local stations, and the car manufacturers often use tweaks within the radio tuner itself to adjust for technical shortcomings of the designs. In Canada's rural and wilderness areas and long stretches of countryside highway their performance is generally poorer than a proper aerial. Even in hilly urban areas like Metro Vancouver (for Americans, think Seattle or San Francisco) such small antennas may be to blame for fluttery or inconsistent reception of even local FM radio stations. In some car washes the smaller antennas will fare better, but most car washes are now touchless so it hardly matters anymore. WARNING: some Rubber Ducky and Shark Fin antennas are not for AM/FM radio use, so be sure to check first before you mess up your Satellite, GPS, OnStar, Mobile Phone, or WiFi antenna! Also, an OEM antenna that is body coloured must never be repainted with metallic paint or the antenna's performance will be harmed.

Best Aftermarket Solution: if you are dissatisfied with the FM reception on your newer car with its Rubber Ducky or Shark Fin antenna, it may be possible to unscrew it from the base and replace it with a proper 1/4 wavelength aerial, and there are manufactured replacements that come with a variety of different thread adaptors for different brands of car. Older cars with pillar or windshield antennas can simply have a replacement monopole installed on a flat metal fender with the antenna lead going right to the radio to replace the original antenna, and there are motorized antenna options too. Used monopole antennas are commonplace at auto wreckers, and a DIYer could fabricate one from a stainless steel or aluminum rod and 50ohm coaxial car antenna cable, obeying all the rules of electronics. Anyone using a shabbily straightened coat hanger should be arrested by the fashion police.

Did you leave anything out? Yep, tons of technical discussion has been skipped that would probably make most peoples' eyes glaze over, so this is as close to an end-user, consumer level document as you'll find. I've saved you a lot of reading.

But I'm a geek and I need to know it all! Step right up to our world famous Antenna Research & Development sub-forum, but don't post anything there until you've done lots of reading beforehand.

Your results may vary: As I have said many, many times here over the years, I am delighted when someone gets reception that they find satisfactory, even if it is with questionable solutions, but I will not recommend things that will make life difficult for the majority of our readers.
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post #2 of 4 (permalink) Old 2016-04-04, 03:05 PM Thread Starter
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My reason for looking up replacement aerials is that during all the times that I've driven a certain route through Metro Vancouver's abruptly hilly landscape I've always had great FM Radio reception from my truck's standard, vertical aerial, but recently I drove the same route in my mother-in-law's new car with its Rubber Ducky antenna and had long periods of "picket fence" fluttering on a very strong local station. Yes, I acknowledge that the 2 radios are not the same, but I replaced the Rubber Ducky on her car with a store-bought 1/4 wavelength (30") aerial and the reception problems went away. Solved.

I tried the same test on mrs. stampeder's car, replacing the Rubber Ducky with a proper 30" aerial and it had the same superior outcome, with Victoria, BC and Bellingham, WA stations becoming much more stable while the audio quality of local stations was more punchy and clear.

On a recent road trip through Southern Ontario my rental car had a Rubber Ducky and I have to say it was way too limited in long distance reception for my satisfaction. A standard aerial would probably have done much better.
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post #3 of 4 (permalink) Old 2016-04-04, 03:07 PM
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Theory: the ideal AM/FM car antenna would theoretically be a dipole
Unless attached to a rotor, this would be too directional to be practical for a car.

The ideal antenna for a car is a ground plane. That would consist of 1/4 wavelength monopole antenna mounted in the center of the roof (assuming it is metal.) Not only is this type of antenna omnidirectional, it also has higher gain than a dipole antenna.

The commonly used side mounted monopole used on many cars is simply a ground plane antenna that has been bastardized for practical reasons. A 1/4 wavelength monopole mounted on the center of the trunk would be a better compromise.
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post #4 of 4 (permalink) Old 2016-04-04, 03:15 PM Thread Starter
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Yep, that's what I've said.

The centre of the car's roof is good if there is lots of metal up there, otherwise a metal fender is appropriate.

A fibreglass, composite, or plastic car is pretty well useless as an antenna ground plane. A roof or body part made of non-metallic material is also useless for that.

An RV with a metal roof is a spectacular place to put up a 1/2 wavelength 1.54m (~60") aerial for best FM Radio reception in wilderness areas, but it would have a lot of wind force on it while on the highway so it would be best if the aerial was removeable, collapsible, or able to be lowered or laid flat.
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