Loading…
Electromagnetic waves
Electric and magnetic fields travelling together at the speed of light.
Electric and magnetic fields travelling together at the speed of light.
A radio wave is an electric field and a magnetic field travelling together through space. Once it leaves the antenna it needs no wire and no medium, and it carries energy and information with it at the speed of light. Everything in amateur radio, from a handheld's chirp to a signal bounced off the Moon, is this one thing.
Electric charges that speed up, slow down or change direction radiate. An alternating current in an antenna does exactly that: it shoves charge back and forth, and the fields around the wire change as it does.
Here is the trick that lets the wave leave home. A changing magnetic field creates an electric field, and a changing electric field creates a magnetic field. Each keeps regenerating the other, so the disturbance moves outward by itself and keeps going after the antenna stops.
Far from the antenna the two fields are in step: both peak at the same instant, E and H at right angles. The direction of E is the wave's polarization. Close to the antenna the fields are messier; see Near field and far field.
Every radio wave, whatever its frequency, travels at the same speed in free space, the speed of light. Frequency decides how fast the fields oscillate and so how long one cycle is; see Frequency and wavelength. In air the speed is almost the same. In glass, water or coax insulation it is lower, and the wavelength shortens in proportion while the frequency stays put.
Light, infrared, X-rays and radio are all the same kind of wave at different frequencies:
Radio sits at the low-frequency end, where each unit of energy is far too small to break chemical bonds. That is why radio is called non-ionizing; its real hazard is heating, covered under RF exposure.
A wave leaving an antenna spreads over a sphere that grows with distance, so the power through each square metre falls with the square of the distance. Double the distance and the power density drops to a quarter, a loss of 6 dB (see Decibels). This spreading loss happens even in perfect vacuum; real paths add absorption, reflection and bending on top, which is what the propagation articles are about.