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. Figures B-2 and B-3 illustrate the continuing progression and propagation of the EM wave from the dipole. The astute reviewer recognizes that the propagation is circumferentially symmetrical about the z-axis. The initial premise for the generation and propagation of electromagnetic waves is the sinusoidal current on the diode element; more specifically, the acceleration and deceleration of charge within the conductor. Charges moving with a constant velocity do not produce electromagnetic waves. This principle is embodied within Maxwell's Equations which comprehensively describe and characterize the subject of electromagnetics – with the exception of relativistic effects. An interesting property of electromagnetic waves is that, unlike sound, a medium is not required for propagation, and that the principal loss mechanism in free space is the spreading of the wave-front. The spreading loss may be visualized by the observation of a flashlight illuminating a surface while the surface is moved away from the light. The same amount of light, i.e., power, occupies a larger area and the power density is thereby reduced. There are other environmental EM wave loss mechanisms such as oxygen and water molecules which interact with an EM waves at particular frequencies. Other loss mechanisms are rain, ice and other particulate matter which might be encountered within the atmosphere. The fact that no medium is required for EM wave propagation is particularly heartening when one considers the life-enabling EM radiation available from the sun. Although the non-rigorous illustration of dipole radiation illustrates the H-field and E-field coupling as sequentially related, the field vectors for the plane-wave in free space are in-phase. The power density of an EM wave at a point in space is the product of the normal components of the wave-front as represented by the Poynting vector: In this case, the geometry of the dipole example has been utilized such that the E-field polarization is z-directed while the H-field polarization is x-directed. The indicated cross-product is y-directed and the units are Watts/meter 2 , clearly a power density. A similar expression for the power density along the x-axis may be written: t H E ¶ ¶ - = ´ Ñ ! ! µ ( ) x z y H E a H E P × = ´ = ! ! ! ! ( ) y z x H E a H E P × = ´ = ! ! ! ! Figure B-2: Simple Dipole with Sinusoidal Excitation (the initial H-field becomes the source of an E-field) Figure B-3: Simple Dipole with Sinusoidal Excitation (the time varying E-field is the source for the H-field) Microstrip Antenna Design 27 www.cadence.com/go/awr

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