An antenna is a transducer between a guided wave and a free-space wave. On one side of the connector, the energy is bottled up inside copper, neatly defined by voltages and currents on a transmission line. On the other side, that same energy is loose in space, propagating as an electromagnetic field that obeys Maxwell's equations and travels at the speed of light. The job of an antenna is to make that handoff smooth, with as little reflection as possible and with a chosen pattern in space. It is the most physical, most analog, most nineteenth-century-feeling component in modern electronics, and yet it is what makes Wi-Fi, GPS, 5G, radar, satellite TV, deep-space exploration, and every wireless attack possible.
In Chapter 9 we built up plane waves out of Maxwell's equations and learned how guided waves propagate on transmission lines. We watched signals reflect, set up standing waves, and transform through quarter-wave sections. That chapter ended at a connector. This chapter is what happens after the connector, when energy is intentionally let go into open space. Some structures are very good at that release (and at the reverse, capturing energy out of space). Those structures are antennas.
A surprising amount of antenna theory is just three ideas applied recursively: an accelerating charge radiates, the radiation pattern of an extended source is the interference pattern of all its tiny charges, and a structure receives exactly as well as it transmits. Once those three sit in your bones, dipoles, dishes, Yagis, phased arrays, and even fractal antennas all become variations on a theme.