Size of the antenna or aerial

Size of the Antenna or Aerial

Transmitting a signal efficiently requires an antenna whose physical size is comparable to the signal's wavelength - at least about $\lambda/4$. A signal of frequency 20 kHz has a wavelength of 15 km, so an antenna sized to match it would be physically impossible to build and operate. This means a low-frequency baseband signal cannot realistically be radiated directly. …