Quiz 15: Radio-Wave Propagation
1. Similarities between radio waves and light waves: 2. Like light waves, radio waves are also one form of electromagnetic radiation. 3. Electromagnetic radiation is considered to be a stream of particles called photons. 4. At radio frequencies the wave model is generally more appropriate, while light is sometimes better modeled by photons. 5. Radio waves reflect from good conductors, such as copper or aluminum and are refracted as they pass from one medium to another, just as light waves. 6. Electromagnetic waves can travel through free space (that is through a vacuum) and via many materials. 7. Any good dielectric will pass radio waves. The material does not have to be transparent to light. The electromagnetic waves do not travel well through lossy conductors, such as seawater, because the electric fields cause currents that dissipate the energy of the wave very quickly. 8. Radio waves, like light, propagate through free space in a straight line with a velocity equal to 3×10 8 m/s. 9. There is no loss of energy in free space, but there is attenuation due to the spreading of the waves. 10. Both radio waves and light waves will undergo the three properties namely reflection, refraction and diffraction. Therefore, due to above all factors radio waves are said to have some similarities like light waves.
The electromagnetic wave travelling in space is the combination of both electric and magnetic fields. If we have a harmonic time varying source, it produces harmonic space varying waves. The E field producing H field shifted by 90 o and this H field in turn produces E field shifted by 90 o. Therefore the electric and magnetic fields both are perpendicular to each other and this E and H fields shifts continuously, which results the EM wave. If electric field travels in x-direction and magnetic field travels in y-direction, then the resulting EM wave travels in z-direction. The ratio of electric field strength to magnetic field strength is defined as characteristic impedance of the medium. For any medium the characteristic impedance is expressed in E and H field constants as follows: For a free space EM wave propagation, . Hence, Substitute . Therefore, characteristic impedance of free space is .
The difference between power and power density: In lumped electric circuits power can be expressed as shown below. Here, power in watts voltage in volts current in amps resistance in ohms If you know any two parameters of V, I, and R then you can determine power of across an element in the circuit. The power density is nothing but power carried in the direction of propagation with indicating power per unit area. It is also called as instantaneous pointing vector. Here, the area is expressed in square meters and the power is expressed in watts. Therefore power density is expressed in watt/meter square. The power density of an electromagnetic wave is expressed as, Here, electric field strength in V/m magnetic field strength in H/m impedance of the medium in ohms Since the power is transmitted in space and the isotropic radiator radiates equally in all directions, the power density in watts per square meters, would be the ratio of total power and the surface area of the sphere. Hence, The power density of a radiating element can be expressed as, ……. (1) Here, power density in watt per square meter total power in watts distance from the antenna in meters. From equation (1), it is clear that the power density is inversely proportional to square of distance from the antenna. Hence power density decreases by increasing square of distance from the radiating element. Thus, the power density decreases by the square of distance from the source.
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