Antenna

The antenna is the transducer that turns guided electrical signals into radiating radio waves and back again, and despite being one of the oldest parts of the stack it is anything but solved. Its physical size scales with wavelength, which is why a 700 MHz antenna element is large and a 28 GHz element is tiny — small enough that you can fit dozens or hundreds into a single panel.

That size relationship is the quiet enabler behind modern radio. Because mmWave elements are so compact, you can build large arrays that beamform tightly, which is the only way mmWave coverage works at all. Key parameters an RF engineer actually tunes are gain (how focused the radiation is), the radiation pattern, polarisation, and tilt — both mechanical and the electrical down-tilt used to control how far a sector reaches and to manage interference into neighbouring cells.