The RF signal power output by the radio transmitter is transmitted to the antenna through the feeder (cable) and radiated by the antenna in the form of electromagnetic waves. After the electromagnetic wave arrives at the receiving location, it is followed by the antenna (only a small fraction of the power is received) and sent to the radio receiver via the feeder. Obviously, an antenna is an important radio device that emits and receives electromagnetic waves. Without an antenna, there is no radio communication.
There are many varieties of antennas for use under different conditions such as different frequencies, different uses, different occasions, and different requirements.
For many varieties of antennas, it is necessary to properly classify them:
* Classification by application: It can be divided into communication antenna,
TV Antenna, radar antenna, etc.
* Classified by working frequency band: can be divided into short-wave antenna, ultra-short wave antenna, microwave antenna, etc.;
* Classified according to directionality: can be divided into omnidirectional antennas, directional antennas, etc.;
* Classification by shape: can be divided into linear antenna, surface antenna and so on.
Radiation of electromagnetic waves
When alternating current flows on the wire, electromagnetic wave radiation can occur, and the ability to radiate depends on the length and shape of the wire. As shown in Fig. 1.1a, if the two wires are close together, the electric field is trapped between the two wires and the radiation is very weak; Increased radiation.
It must be pointed out that when the length L of the wire is much smaller than the wavelength λ, the radiation is very weak; when the length L of the wire is increased to be comparable to the wavelength, the current on the wire will greatly increase, and thus stronger radiation can be formed.
Symmetrical oscillator
The symmetric oscillator is a classical and most widely used antenna. A single half-wave symmetric oscillator can be simply used independently or as a feed for a parabolic antenna, or multiple half-wave symmetric oscillators can be used to form an antenna array.
Vibrators with equal length of both arms are called symmetric vibrators. An oscillator with a length of one quarter wavelength and one half wavelength per arm is called a half-wave symmetric oscillator, see Fig. 1.2a.
In addition, there is a special-shaped half-wave symmetric oscillator, which can be seen as a system that folds a full-wave symmetric oscillator into a narrow rectangular box and stacks the two ends of a full-wave symmetric oscillator. This narrow and long rectangular box is called Converted to a transducer, note that the length of the folded transducer is also one-half wavelength, so it is called a half-wave folded transducer, see Figure 1.2b.
Antenna directionality
One of the basic functions of the transmitting antenna is to radiate the energy obtained from the feeder to the surrounding space. The second basic function is to radiate most of the energy in the desired direction. The vertically placed half-wave symmetrical vibrator has a flat "donut ring"
