天线原理

     A radio antenna is the structure associated with the region of transition between a guided wave and a free-space wave, or vice versa.  A guided wave traveling along a transmission line which opens out, will radiate as a free-space wave. The guided wave is a plane wave while the free-space wave is a spherically expanding wave.  Thus, an antenna is also a transition device, or transducer, between a guided wave and a free-space wave, or vice versa. The antenna, like the eye, is a transformation device converting electromagnetic photons into circuit currents.

       The transmitter connected to an antenna could be an alternating current (AC) source. The AC source produces a varying potential difference in the antenna that alternates at the frequency of the AC source. This varying potential difference generates a corresponding varying electric field that propagates away from the antenna. The changing electric field also generates a varying magnetic field perpendicular to the electric field.  The combined electric and magnetic fields are electromagnetic waves that spread out into space, moving at the speed of light.

 

       An electromagnetic wave produced by an antenna is polarized; that is, its electric field is parallel to the antenna's conductor. The frequency of the wave transmitted by an AC source is equal to the frequency of the rotating AC generator and is limited to about 1 kHz. 

       A common method of generating high-frequency electromagnetic waves is to use a coil and a capacitor connected in a series circuit. 

 

       Just as a pendulum eventually stops swinging if it is left alone, the oscillations in a coil and capacitor die out over time due to resistance in the circuit. The oscillations of both systems can be made to continue by adding energy. Gentle pushes, applied at the correct times, will keep a pendulum swinging. The largest amplitude swings occur when the frequency of pushes matches the frequency of swinging motion. This is the condition of resonance. Similarly, voltage pulses applied to the coil-and-capacitor circuit at the right frequency keep the oscillations in the circuit going. One way of doing this is to add a second coil to the circuit, to form a transformer. The alternating current induced in the secondary coil is increased by an amplifier and added back to the coil and capacitor. This type of circuit can produce frequencies up to approximately 400 MHz.

 

       The oscillation frequency produced by a coil-and-capacitor circuit can be increased by decreasing the size of the coil and capacitor used. However, above frequencies of 1 GHz, individual coils and capacitors can no longer be used. High frequency microwaves, with frequencies from 1 GHz to 100 GHz, are produced using a resonant cavity. The resonant cavity is a rectangular box that acts as both a coil and a capacitor. The size of the box determines the frequency of oscillation.

 

       To produce even higher frequency infrared waves, the size of the resonant cavity would have to be reduced to molecular size. The oscillating electrons that produce infrared waves are, in fact, within the molecules. Visible and ultraviolet waves are generated by electrons within atoms. X rays and gamma rays are the result of accelerating charges in the nuclei of atoms. All electromagnetic waves arise from accelerated charges, and all travel at the speed of light.

 

       How will the waves be detected? Reception involves an antenna. The wave’s electric fields accelerate the electrons of the material making up the antenna. The acceleration is largest when the antenna is positioned in the same direction as the wave polarization; that is, when it is parallel to the direction of the wave’s electric fields. A potential difference across the terminals of the antenna oscillates at the frequency of the electromagnetic wave. This voltage is largest when the length of the antenna is one-half the wavelength of the wave it is to detect. Thus, an antenna’s length is designed to be one-half of the wavelength of the wave it is supposed to receive. For this reason, an antenna designed to receive radio and television waves is much longer than one designed to receive microwaves.

 

       While a simple wire antenna can detect electromagnetic waves, several wires are more effective. A television antenna often consists of two or more wires spaced about one-quarter wavelength apart. Electric fields that are generated in the individual wires form constructive interference patterns that increase the strength of the signal.