The electromagnetic wave: (Scroll down)
The Vector representation of a Radio Wave
First consider the diagram on the top left, drawn to appear to be three-dimensional. Three arrows are shown and they are all mutually perpendicular. The NZART logo is shown here too to make your eye adopt the correct perspective.
The electric and magnetic field strengths are shown as coloured arrows. This is a vector representation. The direction of each arrow shows the direction of the field at that point. The length of the arrow shows the strength of the field. Both fields are at right-angles to one-another.
The direction in which the wave is travelling is shown in black. This direction is perpendicular to the plane of the two field vectors. This is a transverse wave. The direction of motion of this wave is also the direction of energy propagation.
Not shown, but what we must clearly understand, is that the fields are changing in intensity. Both fields are changing sinusoidally. They pass through zero together, reach a maximum together, decrease and pass through zero again, reach a maximum in the opposite direction before decreasing again to pass back through zero. The frequency of these changes is set by the signal source.
Neither an electric field nor a magnetic field will go anywhere by itself, but Maxwell discovered that a CHANGING magnetic field will induce a CHANGING electric field in the surrounding region and vice-versa.
Unlike a STATIC field, a WAVE cannot exist unless it is moving. Once created, an electromagnetic wave will continue on forever unless it is absorbed by matter. The changing fields in this surrounding region will, in turn, induce further fields in a still more distant region, and thus the energy continues to propagate on its journey outwards.
These electromagnetic waves require no material medium to support them. They propagate just as well in a completely empty space, in a vacuum, as in the atmosphere.
A wave arriving at a receiver, at a conductor, will induce a current in that conductor, but receivers are a story we will look at later. Meantime, we have a dipole launching a wave, a wave that is made up of changing electric and magnetic fields travelling at the speed of light on its way outwards and towards a distant receiver.
The diagram on the right bottom is to remind us that a wave is not just a single-point event, it covers an area with undefined edges. We just use the simple single vector representation for convenience.
The Moving Wave
The main drawing is another three-dimensional representation.
The source of our signal is at the bottom left and the signal moves to the top right. The field strengths at many points on the way are shown in our vector form and each changes at the frequency of the source.
Put your eyes on any one of the spots and see it go through a complete cycle. Note that the maxima and the minima of the electric and the magnetic fields at any spot occur at the same time.
Note too the sinusoidal outline set out by the tips of the vectors moving in the direction away from the source. These represent changes in the intensity of the outgoing fields. If the wave could be stopped, the wavelength of the wave could be measured. We can’t do it like that, but we will shortly see how we can.
The source dipole is vertical. The E field of the wave is vertical. This wave is described as being “vertically polarised”. Polarisation is described by the direction of the electric field. We will examine polarisation in some of our experiments.