Week 2 - Session 4 2
This Lecture
• The physics of electromagnetic waves• Interaction of EM waves with matter
23-Jan-14
Week 2 - Session 4 4
Intensity of Spherical Waves
The Physics of EM WavesIntensity
Wavefront curvature ˂ λ/16 over a given aperture of dimension D ⇒ planar wave
This condition is met if the distance from the source to the aperture is at least 2D2 / λ.
This is called the far-field or plane wave approximation
23-Jan-14
Week 2 - Session 4 5
Polarization
The Physics of EM Waves
23-Jan-14
𝐸=𝐸0 cos(𝑘𝑧−𝜔𝑡+𝜑 )
radians/m and radians/s
Week 2 - Session 4 623-Jan-14
Interaction of EM Waves with MatterDiffraction
Diffraction is the bending of EM wavesas they propagate through an aperture or around the edge of an object.
Diffraction depends on:• Size of aperture (antenna), a • Wavelength, λ
Week 2 - Session 4 723-Jan-14
Interaction of EM Waves with MatterDiffraction
• Angular shape of wave as it exits the aperture: a sin (x) / x (sinc) function
• Main lobe half-power (-3dB) beamwidth, θ3 of sinc function:
• Diffraction responsible for the formation of:• Antenna pattern• Antenna beam (main lobe)• Sidelobes
Week 2 - Session 4 8
Interaction of EM Waves with Matter
Example 1
A multi-element antenna
Multi-element linear array of radiating elements with in-phase signals and resulting main beam pattern
Example 2
Diffraction
23-Jan-14
Week 2 - Session 4 9
Interaction of EM Waves with Matter
radians𝑭𝒐𝒓 𝒕𝒚𝒑𝒊𝒄𝒂𝒍 𝒓𝒂𝒅𝒂𝒓 𝒅𝒆𝒔𝒊𝒈𝒏𝒑𝒂𝒓𝒂𝒎𝒆𝒕𝒆𝒓𝒔Diffraction
Idealized one-dimensional antenna pattern
23-Jan-14
Week 2 - Session 4 10
Interaction of EM Waves with MatterAtmospheric Attenuation
Rain, fog and clouds further attenuate EM waves
23-Jan-14
Week 2 - Session 4 11
Interaction of EM Waves with MatterAtmospheric Refraction
Difference in wavelength for wavefronts in two materials
Refraction is the bending of EM waves at the interface of two different dielectric materials.
Speed of EM wave is function of ‘optical density’ of the material
Speed decreases in glass, but frequency does not change, therefore, wavelength must decrease
Index of refraction: n = c/v
23-Jan-14
Week 2 - Session 4 12
Interaction of EM Waves with MatterAtmospheric Refraction
Refraction is encountered in radar signals directed upward (or downward) through the atmosphere.
As the index of refraction reduces with altitude, therefore, EM wave will bend back towards the earth.
Adverse effects:
• Target location• Tracking accuracy
Benefits:
• OTH detection• Increase in detection range
Severe ray bending is called ducting.
23-Jan-14
Week 2 - Session 4 13
Interaction of EM Waves with MatterReflection
In radar technology, scattering phenomenology is quantified by the target parameter radar cross section, σ.
RCS is determined by interception, reflection and directivity.
23-Jan-14