far-field

The far field is the region of space where an antenna's radiated electromagnetic wave has become a plane wave traveling outward at the speed of light. It begins at a distance typically several wavelengths away from the antenna, beyond the transition zone called the near field. In this region, the electric and magnetic fields are perpendicular to each other and to the direction of propagation, with their amplitudes related by the impedance of free space, about 377 ohms.

The boundary between near and far field depends on the antenna's dimensions and operating frequency. For electrically small antennas, the far field may begin at just a few wavelengths out. For large arrays or microwave dishes, it can extend much farther. A rough rule is that far field begins around 2D squared divided by the wavelength, where D is the largest antenna dimension. At 1 GHz with a half-meter antenna, you might not reach true far field until several meters away.

Measuring and modeling in practice

Far-field measurements and calculations are simpler than near-field analysis because the wave structure is predictable. Engineers use far-field patterns to specify antenna gain, directivity, and radiation efficiency. Test ranges for antenna measurement are designed so that equipment under test sits far enough from the source or measurement probe that plane-wave conditions hold. Anechoic chambers with absorbing walls often house these ranges to eliminate reflections.

Numerical modeling also benefits from far-field assumptions. Codes using the moment method or finite-difference time-domain can often neglect the near field entirely if you only care about how the antenna radiates. The far field is where the antenna's response to incoming signals also becomes clean and predictable, which matters for receiver design and communications links.

Near the antenna, stored energy and reactive coupling dominate. Move out into the far field and you see only the traveling wave. This distinction matters in practice: a transmit antenna's near field can couple unwanted energy into nearby conductors, while its far field is what reaches the receiver kilometers away. Ignorance of this boundary has caused crosstalk problems in dense antenna arrays and false predictions of system performance.