從無線電波中獲取能量

　　電力的無線傳輸常常帶來一個世紀前尼古拉特斯拉及其無線輸電技術的圖像。William C. Brown，微波功率傳輸的知名整流天線的發明者，寫了一本極好的歷史回顧的技術可以追溯到海因里希赫茲，實際上早于特斯拉的工作。1雖然特斯拉的電網愿景是從來沒有通過，其他應用程序更適合有用功率無線傳輸和收集軍事和航天現在正在實施的。

　　對于能量收集，人們可以考慮兩種方法——寬或窄波段。任何熟悉RF技術的人都會明白，在盡可能窄的頻帶范圍內進行大量的調整通常會消耗大量的精力。對于無線電通信信號的典型使用，主要的好處是減少噪聲和拒絕其他信號以外的通信信道。窄帶設計有兩個好處，特別是在能源收獲的好處。聚焦在一個小范圍的頻率通常會產生增益的天線或電路，以及改進的功率傳輸通過匹配調諧所有的電路元件。

　　However， narrowing the band of collected frequencies has the deleterious effect of rejecting other ambient energy that could otherwise be scavenged. Ultra-wideband （UWB） is a communications protocol that exploits a larger range of frequency to improve performance for certain applications. Wideband techniques are also employed for energy-harvesting applications to collect the widest possible array of frequencies in the bands with the greatest energy density available.

　　As one may expect， most off-the-shelf antennas are designed with specific tuning for narrow band reception of RF signals. In some cases， especially for mobile phone applications， antennas are available that cover several bands. For antennas in the cellular GSM band， designers should consider the Taoglas 931-1052-ND Penta-band antenna （Figure 1） covering five major cellular bands near both 900 MHz and 1.9 GHz. Another pentaband antenna is available from Ethertronics. The 939-1022-1-ND（Figure 2） offers high gain of 2.5 dBi for the 2-GHz band and is shorter than the Taoglas design.