1. Vertical antennas Vertical antennas are widely used in radio monitoring equipment.
The vertical antenna is actually a dipole antenna. The dipole antenna consists of two conductors, each of which is 1/4 wavelength, that is, the total length of the antenna is half a wavelength. So the even sub-antenna is called a half-wave oscillator.
The vibrator of the dipole antenna can be positioned horizontally or vertically. Its directional pattern is symmetrical about the feeding point. The feeding point is at the center of the half-wave oscillator. The impedance of the feeding point is pure resistance, approximately 75Ω (about 73Ω). If two 1/4-wavelength dipoles are extended and then folded back to the center and connected together, it becomes a folded dipole antenna, referred to as a folded dipole.
The impedance of the folded dipole antenna is also a pure resistance of approximately 300Ω (approximately 290Ω), showing a high input impedance. The high-impedance transmission antenna formed with a parallel feeder is used in many occasions.
Straighten the dipole antenna perpendicular to the ground to form a vertical antenna. If the lower 1/4λ vibrator is "removed", it becomes an asymmetric vertical antenna. This situation is based on two assumptions: ①The ground is a "mirror surface", and there is a "mirror" of the 1/4λ vibrator under the ground; ②The vibrator leaves the ground There is sufficient height h. Usually h>3λ. Commonly used vertical antennas are asymmetric antennas, which are isotropic in the horizontal direction.
A special vertical antenna, there are four radial elements under the 1/4λ dipole radiator. It has good ionospheric reflection effect when used in the 40-meter and 80-meter frequency bands. This kind of antenna has a special name called Marconi antenna. The antenna configured by the receiver such as R7000 belongs to this kind of antenna.
2. Yagi antenna A simple directional antenna, it has an excitation unit, and the other units are used for guiding and reflection respectively, it can gather energy in the guiding direction to form a beam. This kind of antenna is called a beam antenna. A typical beam antenna is a Yagi antenna.
The reflector and director of the Yagi antenna absorb some of the energy radiated by the exciting vibrator, and then radiate it out to form a certain directivity. A folded dipole is commonly used to excite the vibrator, and the size of the reflector is slightly longer than 1/2λ; the size of the bow is slightly shorter than 1/2λ.
The interval is about 0.15λ. The more elements of the Yagi antenna, the greater the forward gain, and the gain of the three elements is about 7dB. Before the popularization of cable TV, most of the antenna arrays on the roofs of urban and rural areas were Yagi antennas.
3. Cassegrain antenna The reflection unit and the guiding unit of the Yagi antenna have limited guiding effects, and it is only on one surface. To form a real beam in a three-dimensional space, a concave reflector is required. The effective shape is a truncated parabolic reflector.
If at the focal point of the parabolic reflector, set a hyperboloid convex reflector (sub-reflector), let the feed source radiate to the sub-reflector first, reflect on the parabola through the sub-reflector, and then reflect twice When you go out, you will get a cylindrical beam. Moving the position of the sub-reflector will change the focus. This kind of antenna has a special name, called Cassegrain antenna.
4-phase eye control array antenna If many identical dipole antennas are arranged in a row. By sequentially controlling the initial phase of each dipole fed high-frequency signal by a computer, the beam can be swept in the specified direction. In the mid-1970s, China built a phased-array radar with a front surface of 20*5m2 in North China. Its antenna is fixed on the side of the mountain, but the beam can be scanned under control.
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