電視頻段二維場強預測傳播模式之建立

Abstract

本論文旨在建立一套二維傳播模式用以預測 VHF/UHF 頻段之電波場強分 佈. 這套模式考慮由發射站到接受點間的二維地形對電波的繞射及遮蔽效 應. 在處理時首先將二維地形近似成一連串不同大小及不同角度的楔形體 組合, 每一楔形體所產生的繞射效應由"統一繞射理論" (UTD) 算出,最後 再利用"射線傳送矩陣" (RTM) 求出多個楔形體所造成的多路徑多次繞射 結果. 此電波傳播模式在台灣中部地區三條不同地形的路徑上所預測出來 的場強結果, 經與實際量測結果比較, 相當吻合, 証明此模式的正確性及 可用性. 在本論文的最後, 我們利用這套模式預測台灣中部芬園發射站周 圍 80*80 Km*Km 內的場強分佈, 並解釋此預測結果與實際地形間的關係. In this thesis, a two-dimensional (2-D) propagation model for field strength forecasting and coverage prediction in VHF/UHF frequency range is presented. Based on given digital terrain data, this model considers multiple diffractions of waves over the 2-D natural terrain cut by the vertical transmitter- receiver plane. The scatterings from other obstacles not associated with this plane are ignored. To tackle the problem, the 2-D natural terrain is first approximated by suitable wedges of various angles and heights. The uniform geometrical theory of diffraction (UTD) is then used to compute the diffraction of waves by a single wedge. Finally the ray transmission matrix (RTM) method is implemented to deal with the multiple diffractions between these wedges. The diffraction of waves by one and two obstacle wedges with or without rough surfaces is fully investigates. It is found that the surface roughnesses have little influence on the diffracted waves. The propagation effects of a wave of freqency 205.25 MHz along three different paths in the Taiwan middle region are calculated and compared with the measurement. Good agreement between the calculation and measurement is observed. Finally, as an example, the field strength forecasting and coverage prediction associated with a VHF TV transmitter is presented and discussed.