Radio Wave Reflection

Like other forms of electromagnetic wave, radio signals can be reflected by certain surfaces.

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Radio Propagation Tutorial Includes:
Radio propagation basics     Radio signal path loss     Free space propagation & path loss     Link budget     Radio wave reflection     Radio wave refraction     Radio wave diffraction     Multipath propagation     Multipath fading     Rayleigh fading     The atmosphere & radio propagation    

It is possible for radio waves to be reflected 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.

Visual examples of light reflection are everywhere from specific mirrors to flat reflective surfaces like glass, polished metal and the like.

So too, radio waves can experience reflection.

Radio wave reflection

When a radio wave or in fact any electromagnetic wave encounters a change in medium, some or all of it may propagate into the new medium and the remainder is reflected. The part that enters the new medium is called the transmitted wave and the other the reflected wave.

The rules that govern the reflection of radio waves are simple and are the same as those that govern light waves.

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

When a reflection occurs it can be seen that the angle of incidence, θ1 is the same for the incident ray as for the reflected ray.

Additionally there is normally some loss, as a result of absorption, or signal passing into the medium.

Reflective medium

Conducting media provide the optimum surfaces for reflecting radio waves. Metal surfaces, and other conducting areas provide the best reflections. It is noticeable that for HF ionospheric propagation, when signals are returned to earth and are reflected back again by the Earth’s surface, areas of good conductivity provide the best reflections. Desert areas give poor reflected signals, but the sea is much better and the differences are very noticeable despite the variations in the ionosphere and overall propagation path.

Surface Conductivity (Siemens)
Dry ground & desert 0.001
Average ground 0.005
Fresh water 0.01
Wet ground 0.02
Sea water 5

Multiple reflections

In real transmission paths, radio waves are often reflected by a variety of different surfaces. Although ionospheric reflections are actually caused by refraction, they can often be considered as reflections. Also for shorter range signals like mobile phone or other VHF / UHF communications the signals undergo many reflections.

These multiple reflections lead to the signal arriving at the receiver via several paths. Radio wave reflections normally give rise to multi-path effects.

The multiple reflections and multi-path effects give rise to distortion of the signal and fading.

When a signal arrives by two paths, one is longer than the other and will take longer to arrive than the other. This can mean that the signals either add together if they are in phase, or they can tend to cancel each other out. This results in fading of anything moves of changes, or dead spots in certain areas if the reflective surfaces are fixed.

Additionally the delays in some signal paths can give rise to distortion of the modulation. For audio the signal can literally sound distorted dependent upon the type of modulation used – frequency modulation the audio can become very broken when multiple signals are received. For digital signals, it can result in data becoming corrupt as the data from one path may be delayed compared to the other and the receiver not being able to distinguish where data bits start and stop.

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