How Does an Antenna Work? Turning Current Into Radiation

An accelerating charge radiates, so alternating current in a wire produces electromagnetic radiation. Whether that radiation escapes efficiently depends on the relationship between the length of the conductor and the wavelength of the signal. When a conductor is about half a wavelength long, the current distribution along it sets up a standing wave that radiates strongly and the antenna presents a convenient impedance to the transmitter, which is why the half-wave dipole is the reference design against which others are measured. A quarter-wave element above a conducting ground plane behaves similarly, using the reflection as the missing half, which is why car and handset antennas are quarter-wave structures. That relationship explains the enormous range of antenna sizes: long wave broadcasting at low frequency requires masts hundreds of metres tall, mobile phone bands around a couple of gigahertz need elements a few centimetres long that disappear inside the case, and millimetre wave systems use arrays small enough to sit behind a panel.

Antenna designs solve different combinations of frequency, direction and size:

• •The dipole, two conductors fed in the middle, which radiates equally in all directions perpendicular to itself and is the baseline for comparison
• •The monopole over a ground plane, effectively half a dipole, compact and used everywhere from vehicles to handsets
• •The Yagi-Uda array, familiar as a rooftop television antenna, which adds parasitic elements in front and behind a driven dipole so that reflections reinforce in one direction and cancel in others, producing substantial gain
• •The loop antenna, which responds to the magnetic component of the field and is used for direction finding and in compact receivers
• •The parabolic dish, which collects a wide area of wavefront and focuses it, giving very high gain and a narrow beam for satellite and radio astronomy work
• •The patch and microstrip antenna, flat and printed onto a circuit board, which is what sits inside most modern devices
• •The phased array, many small elements whose relative timing is controlled electronically so the beam can be steered with no moving parts, which is how modern radar and satellite constellations work

Antenna gain does not mean amplification, since a passive antenna adds no energy. It means concentration: radiating more strongly in some directions by radiating less in others, measured against a theoretical isotropic radiator that would radiate equally everywhere. A high gain antenna therefore trades coverage for reach, which is why a dish must be aimed accurately and a broadcast antenna is deliberately omnidirectional. The radiation pattern describes that distribution and typically shows a main lobe plus unwanted side lobes. Polarisation matters practically, since an antenna responds best to waves whose electric field is oriented as it is, and a vertical antenna receives a horizontally polarised signal very poorly, which is why broadcast and receiving antennas must match and why some satellite systems use circular polarisation to avoid the problem. Impedance matching is the other decisive factor, since a mismatch between transmitter, feed line and antenna reflects power back down the cable instead of radiating it, which wastes energy and can damage the transmitter.

Physics imposes a trade that marketing frequently obscures: an antenna substantially smaller than a wavelength can be made to work and becomes inefficient and narrowband as it shrinks, so a very small antenna covering many bands is always a compromise. Surroundings matter enormously, since nearby metal, the human body, buildings and the ground all absorb, reflect and detune, which is why phone performance changes with grip and why installation height is the single most valuable improvement to most fixed antennas. Multipath, in which a signal arrives by several routes with different delays, causes fading and was a serious problem before receivers learned to exploit it, and modern systems use multiple antennas to turn multipath from a defect into extra capacity by sending independent streams over the different paths. Noise sets the floor for reception, and at lower frequencies the dominant noise is external, coming from the atmosphere and from human electrical activity, which means a bigger antenna does not always help.

An antenna radiates because alternating current means accelerating charge, and it works efficiently when its length is related to the wavelength, with the half-wave dipole as the reference and the quarter-wave monopole over a ground plane as its compact equivalent. Gain means concentrating radiation in one direction rather than amplifying, so reach is traded against coverage. Polarisation must match and impedance mismatch reflects power back down the cable. Antennas much smaller than a wavelength are always a compromise.