Short answers, pulled from the story.
An antenna, called an aerial in British English, is a structure that converts alternating electric currents into radio waves for transmission and converts radio waves back into electric currents for reception. It is the interface between radio waves propagating through space and electric currents moving in metal conductors, and it is an essential component of all radio equipment.
The first antennas were built in 1886 by German physicist Heinrich Hertz during his experiments to prove the existence of electromagnetic waves predicted by James Clerk Maxwell's 1867 theory. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving.
The word antenna is attributed to Italian radio pioneer Guglielmo Marconi, who in the summer of 1895 experimented with long wire aerials suspended from a pole near Bologna. In Italian a tent pole is called l'antenna centrale, and the pole with the wire was simply called l'antenna; before then the radiating elements were known only as terminals.
A high-gain antenna radiates most of its power in a particular direction, offering longer range and better signal quality but requiring careful aiming, such as a parabolic satellite television dish. A low-gain antenna radiates over a wide angle with shorter range, so its orientation matters little, such as the whip antenna on portable radios and cordless phones.
A half-wave dipole consists of two quarter-wavelength elements arranged end-to-end along the same axis, each feeding one side of a two-conductor transmission line. The arrangement places the elements 180 degrees out of phase, so at any instant one element drives current into the line while the other pulls it out, creating a standing wave with maximum current at the feed point.
Resonant antennas efficiently feed a signal into the transmission line only when the source frequency is close to the antenna's design frequency or one of its resonant multiples. Away from that frequency the induced current arrives out of phase, the impedance no longer matches, and the signal reflects back into the antenna, so the design is useful only over a small range of frequencies centered on resonance.
The ground can influence both the radiation pattern and the driving point impedance of a terrestrial antenna through its conductivity, permittivity, and the antenna's height above it. When a wave strikes the ground, part is reflected according to the Fresnel coefficients, and at frequencies between 3 and 30 megahertz a phase-reversed reflection can either cancel or reinforce the direct wave depending on antenna height and elevation angle.