隨著無線多媒體網際網路應用的日益頻繁，地面微波通訊的應用上日趨重要，由於頻譜之擁擠，世界各國均傾向採用Ka(26~40GHz)波段。不過此一波段信號在地表大氣通道傳播時，容易受到自然環境因素影響，其中植被遮蔽造成的信號衰減情況甚為嚴重，所以本文將針對植被遮蔽的效應進行討論。
在植被遮蔽效應實驗，以闊葉榕樹進行量測訊號穿透的功率衰減和功率機率分佈。當植被遮蔽效應時，藉由不同極化情形下所能得知入射功率、輸出功率和植被遮蔽路徑長度之關係。經由實驗後發現在不同發射、接收極化方向情況下所得到的接收功率會有所差異，其植被消散係數也會有所差異，實驗結果中得知在垂直發射、垂直接收情況下訊號的衰減和推算出植被之消散係數為26.80dB和4.85dB/m。水平發射、水平接收情況為29.44dB和5.10dB/m。而垂直發射、水平接收情況為18.85dB和4.70dB/m。水平發射、垂直接收情況為20.67dB和4.82dB/m。

With more and more applications in the wireless internet multimedia, the importance of terrestrial microwave communication should be studied and investigated. However, a lot of people tend to apply the frequency in Ka-band (26-40GHz) for the lack of the useful spectra. But the region of Ka-band is sensitive to be influenced by the environment, when it transferred in the atmosphere, especially for the attenuation by the vegetations. In this study, we will discuss some effects of that.
In the experiment of vegetations effect, the experiment is operated in a transmit-through mode. And the kind of tree is banyan which is used to measure the attenuation and made the probability distribution pass through it. And we show the expression relating the power of the received power to the incident power and the thickness of the random-medium layer of different polarization under vegetations effect. It showed that the power and the extinction coefficient change due to the different polarization of transmit and receive modules. The attenuation of receive power and the extinction coefficient for both horizontally polarized form are 26.80 dB and 4.84 dB/m. The attenuation of receive power and the extinction coefficient for both vertically polarized form are 29.44 dB and 5.09 dB/m. The attenuation of receive power and the extinction coefficient for horizontally and vertically polarized form are 18.85 dB and 4.58 dB/m. The attenuation of receive power and the extinction coefficient for vertically and horizontally polarized form are 20.67 dB and 4.79 dB/m.

[1] S. T. Wu, “Preliminary Report on Measurement of Forest Canopies with C Band Radar Scatterometer at NASA/NSLT,” IEEE Trans Geosci. Remote Sensing, vol. GE-24, pp. 894-899, 1986.
[2] H. Hirosawa, Y. Matsuzaka, and O. Kobayashi, “Measurement of Microwave Backscatter from a Cypress With and Without Leaves,” IEEE Trans. Geosci. Remote Sensing, vol. 27, no. 6, pp.698-701, 1989.
[3]Felix K. Schwering,Edmond J. Violette and Richard H. Espeland,”Millimeter-Wave Propagation in Vegetation: Experiments and Theory,”IEEE Trans. Geosci. Remote Sensing,vol.26, no. 3,MAY, 1988
[4]Adib Y. Nashashibi ,Fawwaz T. Ulaby, and Panayiotis Frantzis,R. D. De Roo,”Measurement of the Propagation Parameters of Tree Canopies at MMW Frequencies,”IEEE Trans. Geosci. Remote Sensing,vol.40,no. 2,February,2002
[5]K. S. Chen and Chih-Yuan Chu, "Statistical Characterization of Scattering Signals from Vegetation at Frequency Above X-band by Numerical Simulations," JCIEE,vol.10, pp.11-20, 2003 .
[6]Djuric,Dusan,”Weather analysis,”pp70-72 ,1994
[7]John Toftgard and Partcik C.F. Eggers,”Experimental Characterization of the Polarization State Dynamics of Personal Communication Radio Channels,” Prco. IEEE VTC’93, pp65-69
[8]謝光龍, “ Ka波段地面鏈路降雨效應與植被遮蔽效應之研究,” 國立中央大學太空科學研究所碩士論文, 2002
[9]郭志輝,”中華衛星一號Ka波段傳播實驗診斷分析,” 國立中央大學太空科學研究所碩士論文, 2003
[10]陳仁杰,”雨滴粒徑與植被效應對Ka波段電波衰減影響之探討,” 國立中央大學太空科學研究所碩士論文,2005
[11]鞠志遠, “Ka波段台灣地區降雨及地面環境傳播特性研究,” 國立中央大學太空科學研究所博士論文, 2003.