I was told a car antenna has a wire wrapped around a supporting shaft. The windings get closer and closer as you advance from bottom to top. Thus, as I was taught, the long windings are spaced such that they receive (by induction) the longer wave length waves (i.e.: low frequency) and the short windings receive the shorter wave length waves (i.e.: high frequency).
However, an EE who knows some radio engineering claims any whip antenna such as this is sized to accomodate a range of wavelenghts along its WHOLE effective length.
Could his explanation be the short story, and my explanation be the finer details as to what is specifically meant by “across its WHOLE effective length”?
Typically, this type of antenna is a quarter-wave vertical, designed to be a quarter-wavelength at some specific frequency. Since a quarter-wavelength at AM frequencies is ridiculously large and impractical, an inductor (coiled wire) is added to make the antenna electrically equivalent to a quarter-wavelength at the design frequency. The inductor can be at the top, middle, or bottom of the antenna. It can also be stretched out, like a slinky, so that the whole antenna is an inductor. As usual, you don’t get something for nothing, so the physically short antenna isn’t as sensitive or as broadband as a true quarter-wavelength antenna. You can also lose signal strength due to resistive losses in the antenna if the antenna is too thin or the inductor uses thin wire.
Well, he’s right but not entirely clear. He’s correct that any size antenna will pick up any frequency, but the maximum gain (i.e. best reception) of a given frequency occurs when the antenna is an odd quarter length of the wavelength of that frequency. Up to a point, the longer the antenna, the better it will receive across a range of frequencies. If you shorten the antenna it may still receive across the range with less gain.
The windings on a car antenna, as you suspect, are there to improve reception through induction. I’m willing to bet that the wider-spaced windings correspond to the bottom end of the 4 meter band (i.e. FM broadcast), and the shorter windings correspond to the top end, giving the antenna more gain for that frequency range.
Most car antennas are just a hollow stainless steel tube. There are no wire windings. You can buy adaptors that claim to help with reception, they are just a coil of copper wire that can help reception. I had a Ford Fairmont that had a 3 foot long piece of welding rod glued into the broken antenna stub and recieved excellent reception, both AM and FM. The OP appears to be describing a short wave or police band antenna that is used for both transmitting and recieving, not an average car radio aerial.
I’m aware of two types of wound antennea:
There is a fiberglass or carbon fiber core, the wire is a conductor. The length of wire and dimensions of the coil are pretty much insignificant in the AM broadcast band, (MF) but are useful for FM broadcast (VHF).
The reason for varying the pitch is that most of the effectivness of the antenna comes from the current flowing near the base. The upper section serves to tune the structure to resonance. Putting the coil toward the top is known as top loading, and increases effiency somewhat, by increasing the length of the high current section, and minimizing resistive losses in that area.
The second type is rubberized, and has a larger diameter coil, much more tightly wound.These are known as “rubber duck” antennea by hams and public safety responders. These are extreamly convienient on handheld radios, and may deter vandalism on cars, but these are margionally efficient.
