Radio Wave Refraction

Like other forms of electromagnetic wave, radio signals can be refracted when the refractive index of the medium through which they are passing changes.

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It is possible for radio waves to be refracted in the same way as light waves. As both light and radio waves are forms of electromagnetic waves, they are both subject to the same basic laws and principles.

The concept of refraction is generally illustrated in terms of light. On a hot day the surface of the ground can be heated and this causes air to rise. Hot air and the colder air have slightly different values of refractive index and this causes the light to bend. With the movement of the rising air the surface of the ground appears to shimmer.

Refraction of radio waves

In just the same way that light waves are refracted, so too radio waves can undergo refraction.

The classic case for refraction occurs at the boundary of two media. At the boundary, some of the electromagnetic waves will be reflected, and some will enter the new medium and be refracted.

Propagation of reflected and refracted waves
Propagation of reflected & refracted waves

This is best illustrated by placing a straight stick through the surface of a still pond where both the reflection and the refracted waves can be seen.

Radio wave refraction follows exactly the same effects as it does for light.

The basic law for radio wave refraction and light wave refraction is known as Snells Law which states:

η 1 sin ( θ 1 ) = η 2 sin ( θ 2 )

Gradual changes in refractive index

Rather than a sudden boundary to two different media, radio waves will often be refracted by areas where the refractive index gradually changes.

This may happen as the radio waves propagate through the atmosphere where small changes in refractive index occur.

Typically it is found that the refractive index of the air is higher close to the earth’s surface, falling slightly with height.

In this case the radio waves are refracted towards the area of higher refractive index. This extends the range over which they can travel.

Refraction of radio waves in ionised regions

Radio waves are also refracted in regions of ionisation such as the ionosphere.

The ionosphere is a region in the upper atmosphere where there is a large concentration of ions and free electrons, primarily as a result of the effect of the Sun’s radiation on the upper reaches of the atmosphere.

The electrons in the ionosphere are excited by the radio waves and are set in motion by them as a result they tend to re-radiate the signal. As the signal is travelling in an area where the density of electrons is increasing, the further it progresses into the region, the signal is refracted away from the area of higher electron density. In the case of signals below about 30 MHz, this refraction is often sufficient to bend them back to earth. In effect it appears that the region has "reflected" the signal.

The tendency for this "reflection" is dependent upon the frequency and the angle of incidence. As the frequency increases, it is found that the amount of refraction decreases until a frequency is reached where the signals pass through the region and on to the next. Eventually a point is reached where the signal passes through the E layer on to the next layer above it.

The state of the ionosphere is constantly changing, so the degrees of refraction that are encountered will vary continually.

More information about HF band ionospheric propagation can be found in out HF ionospheric propagation pages.

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