The field of this invention relates to antennas, and more particularly to an antenna for picking up a radio signal.
An antenna is a device for transmitting or receiving radio waves. The transmitting antenna converts the electrical signals from a transmitter (radio, television or radar) into an electromagnetic wave which spreads out from the transmitter. A receiving antenna intercepts this wave and converts it back into electrical signals; that can be amplified and decoded by a receiver such as a radio, television or radar set.
A radio transmitter produces its signal in the form of an alternating electric current, that is, one which oscillates rapidly back and forth along its wire. The rate of this oscillation can be anything from tens of thousands of times a second to thousands of millions times a second. The rate is known as a frequency and is measured in kilohertz or kilocycles and for higher frequencies in MegaHertz or Megacycles.
The oscillating current in the transmitting antenna produces an electromagnetic wave around it, which spreads out from it like ripples in a pond. This wave sets up electric and magnetic fields. The lines of the electric field run along the antenna and those of the magnetic field around it. Both the electric and magnetic fields oscillate in time with the electric current.
Whenever this wave comes into contact with the receiving antenna, it induces a small electric current in the antenna. This small electric current alternates back and forth along the antenna in time with the oscillations of the wave.
The air is full of radio waves at all frequencies which the antenna picks up indiscriminately. Each radio set has a means of selecting a narrow band of frequencies at any one time. This is what happens when a particular signal is tuned in. Each radio set can be tuned within a certain frequency range and will respond to signals only in that range.
Electricity travels along a wire at a speed close to the speed of light. It will greatly increase the efficiency of an antenna if its length is correctly correlated to the wavelength of the signal it received or transmits. Ideally, antennas are normally selected to be one-half or one-quarter of the wavelength that they are designed to receive with the addition of a small amount of length to compensate for loss within the antenna itself. An AM radio signal is over one thousand feet in length. An FM radio signal is substantially shorter and is approximately one hundred eight inches in length. Therefore, within conventional radio sets, it is difficult to design an antenna which is any significant percentage in length of either AM or FM. Clearly, AM would be more difficult since the quarter wave length in AM would be over two hundred fifty feet in length. A quarter wavelength for FM would require an antenna approaching thirty inches which is still too large in size for most radio sets which would result in a rather unattractive appearance. A typical antenna for a radio set will usually take the form of some form of coil or wound wire which is mounted on some form of a stand which is placed on or near the radio set.