In this tutorial, we discuss the radiation from a Hertzian dipole into uniform isotropic lossy media of infinite extent. If the medium is lossless, the radiated power propagates to infinity, and the apparent dissipation is measured by the radiation resistance of the dipole. If the medium is lossy, the power exponentially decays, and the physical meaning of radiation resistance needs clarification. Here, we present explicit calculations of the power absorbed in the infinite lossy host space and discuss the limit of zero losses. We show that the input impedance of dipole antennas contains a radiation-resistance contribution that does not depend on the imaginary part of the refractive index. This means that the power delivered by dipole antennas to surrounding space always contains a contribution from far fields unless the real part of the refractive index is zero. Based on this understanding, we discuss the fundamental limitations of power coupling between two antennas and possibilities of removing the limit imposed by radiation damping.

中文翻译：

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无损和有损介质中偶极子辐射电阻的物理意义：天线在有损介质中的辐射电阻是多少？

在本教程中，我们讨论从赫兹偶极子到无限范围的均匀各向同性有损介质的辐射。如果介质是无损的，则辐射功率会传播到无穷远处，而视在耗散是通过偶极子的辐射电阻来衡量的。如果介质是有损的，则功率呈指数衰减，抗辐射的物理意义需要澄清。在这里，我们提出了在无限有损宿主空间中吸收的功率的明确计算，并讨论了零损耗的极限。我们表明偶极天线的输入阻抗包含不依赖于折射率的虚部的辐射电阻贡献。这意味着偶极天线向周围空间提供的功率始终包含来自远场的贡献，除非折射率的实部为零。基于这种理解，我们讨论了两个天线之间功率耦合的基本限制以及消除辐射阻尼施加的限制的可能性。