What is a Digital TV Antenna?

In cable television systems, the task of the antenna is to receive open-air television signals or satellite television signals in the air and send them to the system for processing, transmission, and distribution. The antenna that receives open-circuit television signals in the VHF and UHF bands is called a television receiving antenna.

We know that when the charged capacitor C and inductor L in Figure 3.1 (1) are connected, the frequency of constant excitation in the inductor and capacitor is

The phenomenon of alternating electric and magnetic fields is called electromagnetic oscillation, and this circuit is called an oscillating circuit. At this time, the electric field energy and magnetic field energy are mainly concentrated inside the capacitor and the inductor coil. The electromagnetic waves radiated to space are very small. In order to increase the radiation power of the electromagnetic waves, we must increase the oscillation frequency f and change the shape of the oscillation circuit so that the electric and magnetic fields Open as much as possible to the surrounding space. Open the two plates of capacitor C gradually, gradually reduce the number of turns of the self-inductive coil L, and gradually increase the distance between the coils. The process in Figure 3.1 (2) becomes as shown in Figure 3.1 (3). A straight line, as the capacitance C and inductance L decrease, the oscillation frequency continues to increase; as the system continues to open, the energy of the radiated electromagnetic waves becomes larger and larger. Figure 3.1 (3) is a typical line antenna that can radiate electromagnetic waves into space. Of course, as the electromagnetic waves continue to radiate outward, the energy in the antenna continues to decrease, and there must be a high-frequency current to continuously deliver energy to the antenna. If the length of the antenna in Figure 3.1 (3) is 1/2 of the wavelength of the radiated electromagnetic wave, and it is disconnected from the middle and connected to both ends of the high-frequency power supply, the basic half-wave dipole antenna shown in Figure 3.1 (4) is formed. . ^[2]

1.Receive the energy of high-frequency electromagnetic waves from TV stations

This is the main role of the TV receiving antenna. The TV station modulates the full-color TV signals and sound signals to be transmitted to the high-frequency carrier, radiates them from the antenna through the TV transmitter, and spreads in all directions in free space. After these electromagnetic waves reach the receiving antenna, they will induce induced electromotive force and induced current in the antenna conductor, and guide the electromagnetic waves to be transmitted from the antenna to the receiving system. In this way, the cable television system obtains a part of the energy of the radio waves emitted by the television station, and then processes, amplifies, etc., enters the television set, and converts it into the original video and audio signals. ^[2]

TV receiving antennas are generally

(1) Determine the location of the antenna. For safety, TV receiving antennas should be kept away from power lines, especially high-voltage lines. To reduce losses, keep the antenna away from large metal objects. If possible, it is best to make the antenna higher than nearby trees and buildings.

(2) Find the best place to receive TV signals. You can use a temporary, lightweight pole to put the antenna up. One person slowly changes the position, height, and orientation of the antenna. The other person observes the image of the TV until the image quality is the best, interference and ghosting are minimized, and sound The best so far. The best position is selected.

Schematic diagram of TV receiving antenna device ^[3]

(3) Set up the antenna. The erection must be firm, it must be able to withstand the storms, and it must be considered for easy maintenance. If the pole is tall, use three iron wires to tighten it in three different directions. The upper part of the wire must be cut into smaller sections with less than half a wavelength, and then connected with insulators, as shown in Figure 1-104. In order to ensure safety, in addition to installing lightning rods on the antenna poles, lightning arresters should also be installed on the antenna feeder, especially in open areas, lightning arresters must be installed.

(4) Before fixing the antenna, carefully rotate the antenna until it has the best receiving effect, and then fix the antenna.