tailieunhanh - Fundamentals of Digital Television Transmission phần 10

Ngoài ra mối quan hệ của nó với các mô hình định hướng ăng-ten, sự lựa chọn những gì thu được ăng-ten liên quan đến một số thương mại-off. Như đã thảo luận trước đó, Beamwidth ăng-ten là tỉ lệ nghịch với chiều dài khẩu độ. Kể từ khi giảm Beamwidth nghĩa định hướng ngày càng tăng, sau được đó là tỷ lệ chiều dài. | 242 RADIO-WAVE PROPAGATION Raleigh R085 -2 ----------------------------------------1---------------------------------------1----------------------------------------1 -4 6 -8 10 -12 14 16 18 20 22 Peak to peak frequency response dB Filed strength dBu Figure 8-33. Tap energy versus response. Raleigh R275 Distance km Calculated Measured Figure 8-34. Field strength versus distance. SUMMARY 243 Distance km - - FCC 50 10 FCC 50 50 - - FCC 50 90 Calculated R65 Figure 8-35. Comparison with FCC. The computed field strength is plotted along with predictions from FCC curves in Figure 8-35. The computed curve matches the FCC 50 50 curve best at 30 km and at long range. Up to dB should be subtracted from the FCC curve to treat the Raleigh terrain properly. Measured data for R065 are repeated for comparison. Indoor antenna tests were performed at 36 sites. Three types of indoor antenna was tested a loop a single bowtie and a dual bowtie over a ground plane. A usable signal with the indoor antennas was observed at all but three sites. At these sites the median signal strength on the indoor antennas was lower than the outdoor measurements by and dB respectively. The loss in signal strength included the effect of height loss building penetration loss and a less directive receiving antenna. The equalizer tap energy was significantly higher than for the outdoor measurements. The average tap energy on the indoor antennas was about 6 dB compared to 15 dB on the outside antennas. This would indicated significantly higher multipath indoors. SUMMARY The factors that affect the propagation of digital television signals at VHF and UHF have been considered along with various means of estimating signal strength and frequency response. It is evident that the means do not exist to predict with 244 RADIO-WAVE PROPAGATION precision the field strength or frequency response at any location and time. This is due to the nature of the propagation .