subject: Terrestrial Digital Tv Broadcasting Works In Practice Several Problems - Terrestrial Digital [print this page] Of this article MrOf this article Mr. Pan Changyong, Tsinghua University, State Key Microwave and Digital Communications Laboratory Lecturer, Master; Mr. Dong Xin and Changsha Radio Digital Mobile Media Ltd chief engineer.
Key words: digital terrestrial television Launch Antenna frequency coverage GPS C / N
Digital TV Television technology trends. Some countries have begun to digital TV from analog TV Broadcasting And to phase out analog television broadcast. The United States has in many TV station Carried out high-definition terrestrial television broadcasting; Britain has in many areas of the wireless digital TV broadcasting business; in Berlin, Germany, part of the wireless analog TV channels have been shut down and shift to digital broadcasting. In China, currently has more than 10 provinces and cities during the broadcast terrestrial digital television broadcasting trial. Changsha, 2003, SVA Group project for digital Mobile TV Test, the past two years we have been exploring, with the number of harvest, discuss this with their peers.
Transmitting antenna Transmitting antenna is the role of electromagnetic radiation to space, electromagnetic waves by the alternating electric field and magnetic field form. To facilitate the analysis, projects are usually required to: Electricity Field direction that the antenna polarization. General use of the antenna are unipolar. Photo shows two basic linear polarized antenna polarization: The vertical polarization and horizontal polarization.
Transmission of electromagnetic wave polarization In the traditional analog TV wireless coverage, considering the line of sight radio waves (no block) modes of transmission, the antenna with tower (mountain) suspension. Horizontally polarized antenna as a good far field intensity distribution, under the same transmission power can cover a larger area, so the level of transmission antennas are essentially the antenna polarization. However, the levels of polarization antenna near field intensity distribution is inferior to vertical polarization, especially in the wet, more water, more trees and other environmental conditions, levels of polarization antenna polarization deflection as the reason, nearly vertical polarization antenna obvious advantages in reception; addition, since the vertical polarization antenna is easy to achieve the horizontal plane 360 reception, so when the mobile receiver antenna vertical polarization is more suitable. Technical Research Centre of Tsinghua University Digital TV technology of the actual test results also show that the use of vertical polarization in the transmitting antenna, receiving antenna has vertical polarization antenna, mobile reception is better than other combinations. Recommended that the TV transmitters in the installation of a new antenna for mobile reception, giving priority to vertical polarization. This structure can be used at the receiver side simple whip antenna suction cup car, its benefits, in addition to rain and snow can be solved under the receiver performance degradation issues, but also easy car installation, lower cost and so on the receiving end.
Antenna site Present, most of the TV transmitting antenna site location, there are two situations: low mountains tower in peace (building) high tower. Practice site in Changsha City, we found that the antenna location is very important. UHF band for wireless transmission, rain attenuation is not a transmission loss of the most important part, when the transmitting antenna using horizontal polarization, in the spring and summer storm season, some from the tower close, usually a very good reception where the receiver significantly worse results, while the field strength than the fine Shihai increased slightly. By the received wireless signals Test Analysis, we can see a marked deterioration in multi-path rain. The reason for this result, we believe that with the location of the transmitting antenna. Changsha, radio and television transmission tower in Yuelu Mountain (295m) at an elevation of 270m in places, tower 88m, mobile television transmission antenna is located 70m away from the tower base position, is a typical low mountain towers. Spring and summer, the Yuelu mountain broad-leaved Mao-Sheng Lin; in Yuxue Tian, due to changes in water content of the mountain, making the dielectric constant between the antenna and the earth changes the transmission characteristics of electromagnetic waves are also changes; Canada on broad-leaved forest after leaf attached mirror reflection of water and other factors. Although the use of vertical polarization could improve the situation, but can not fundamentally solve the problem. Proposed more rain in the south city location during terrestrial digital TV antenna as close as possible to avoid the low mountains tower, and give priority to the tower model.
As traditional analog TV to consider the wide range of coverage, and mobile TV at this stage the main consideration is the Metro cover, so the choice of mobile television transmission antenna, the focus of specific service areas according to the topographical features, with uniform for the purpose of coverage. Location does not necessarily require a very high antenna, the antenna gain is not high priority goal. General to select no more than 4 layers board about the dipole antenna is suitable.
Firing frequency
Currently carried out throughout the terrestrial digital television experiment, the general scope of the working frequency is not selected. However, the terrestrial digital television technology, the frequency of work or have a certain requirement. First of all, and now digital terrestrial TV Transmitter Generally used LDMOS Amplifier Module, in order to obtain a larger linear dynamic range, basically have to choose their operating frequency in the UHF band. Second, the current digital television channel FFT demodulation chips rely on a limited speed to complete its processing capacity is limited, the delay of the multipath signal the shorter the treatment sooner; while for the same speed in terms of mobile reception , the lower the frequency, the smaller the Doppler frequency shift, the relatively low frequency of some of the high-speed mobile reception is favorable. Third, water, wetlands, forests and other wireless digital signal absorption, with the frequency Higher Increases. Fourthly, the current low-end UHF band, electromagnetic interference seriously. According to the relevant foreign Expert Views, carried out in order to move the main business of receiving terrestrial digital television broadcasting, its frequency in the 550 ~ 700MHz usually choose between the more appropriate. Especially in the water, wetlands, forest areas of the high side, to carry out digital mobile TV broadcasting to the low frequency side is appropriate.
