Fundamental Directivity and Efficiency Limitations of Single- and Multi-Port Antennas

The paper considers the fundamental limitations on directivity and radiation efficiency of antennas with both single and multiple ports. The treatment is limited to antennas made of lossless materials. The maximum available directivity measured on a single port of both small antennas and large antennas is considered in terms of the diameter of the smallest sphere that can surround the antenna. The maximum available directivity of large antennas is given by the common aperture directivity formula, and that of small antennas is 4.67 dBi (same as for a Huygen source). These two expressions are combined in a heuristic manner to a continuous maximum directivity limitation. The theoretical limitation is plotted together with directivities of known antennas which are known to have large directivities. The paper discusses also fundamental limitations of dense arrays. Dense linear arrays can be used to provide superdirectivity, but their superdirectivity is lower than what can be obtained with normal antennas under the present definition of maximum available directivity. Dense arrays also suffer from severe gain reduction due to strong coupling to the ports of the neighbouring elements. This gain reduction can be characterized in terms of a coupling efficiency, also referred to as embedded element efficiency, and appears as a reduction of the total radiated power. The fundamental property of this coupling efficiency is known from classical papers, and it has recently been verified by measurements in a reverberation chamber.