電基本振子輻射出的電場 (藍色) 與磁場 (紅色)
電基本振子(Electric Short Dipole)又稱電流元,指一段理想高頻電流直導線,其長度l遠小于波長λ並且其半徑r遠小於l,同時振子沿線的電流I處處等幅並且同相。這樣的電流元可構成現實當中更複雜的天線,因而電基本振子的輻射特性是研究更複雜天線輻射特性的基礎。[1]
在球坐標當中,電基本振子在無限大自由空間中場強為[2]
![{\displaystyle E_{r}={\frac {Z\,I_{0}\delta \ell }{2\pi }}\left({\frac {1}{r^{2}}}-{\frac {i}{kr^{3}}}\right)e^{i(\omega t-k\,r)}\,\cos(\theta )}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ab04d1b1a7aa5d2a284c767421dfc80272521d1a)
![{\displaystyle E_{\theta }=i{\frac {Z\,I_{0}\delta \ell }{4\pi }}\left({\frac {k}{r}}-{\frac {i}{r^{2}}}-{\frac {1}{kr^{3}}}\right)e^{i(\omega t-k\,r)}\,\sin(\theta )}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5092ef78235fd3407e7a14e3d740d6fd56682697)
![{\displaystyle H_{\phi }=i{\frac {I_{0}\delta \ell }{4\pi }}\left({\frac {k}{r}}-{\frac {i}{r^{2}}}\right)e^{i(\omega t-k\,r)}\,\sin(\theta )}](https://wikimedia.org/api/rest_v1/media/math/render/svg/411ac30450b99ee5a61fee2e58e524fea73c94fd)
![{\displaystyle E_{\phi }=H_{r}=H_{\theta }=0,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/07abff4477872534bca501da284f6bdecaa813d6)
在近區場坡印廷向量的均值
電基本振子的遠區場表達式為:
![{\displaystyle H_{\phi }=j{\frac {Il}{2\lambda r}}sin\theta e^{-jkr}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c62687641e552b863b185df1f746039603f4a90f)
![{\displaystyle E_{\theta }=j{\frac {60\pi Il}{\lambda r}}sin\theta e^{-jkr}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/9b9d2f807df2e3e7be6db7c652b1342f994836bb)
![{\displaystyle H_{r}=H_{\theta }=E_{r}=E_{\phi }=0}](https://wikimedia.org/api/rest_v1/media/math/render/svg/70655ffc823236c9c5e4c5fa4684e5a507be4493)
電偶極子向自由空間輻射的總功率為
![{\displaystyle P_{r}=\oint _{S}{\frac {1}{2}}Re[\mathbf {E\times H^{*}} ]\cdot d\mathbf {s} =40\pi ^{2}I^{2}({\frac {l}{\lambda }})^{2}\ W}](https://wikimedia.org/api/rest_v1/media/math/render/svg/76192eeeea79e6becde86168ba3fca6f0026ac3a)
參考文獻[編輯]
- ^ [1] 宋崢. 天線與電波傳播[M]. 2. 西安:西安電子科技大學出版社, 2011.
- ^ Silver, Samuel. Microwave Antenna Theory and Design. 1949: 92–94.