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Antenna Fundamentals part. 1

Antenna Fundamentals part. 1

ies the antenna length is equal to key fractions of a wavelength. The current on a dipole for frequencies resulting in 1/2 and 1 wavelength is shown in Figure 5a and 5b, respectively. At 1/2 wavelength, the current from the source is maximal. The input impedance of the antenna at this frequency is therefore minimum, equivalent to the resistance of the antenna actual + radiation resistance. At a frequency that has a wavelength the same as the antenna length, the current from the source is zero and therefore, the input impedance is infinite. A plot of the impedance vs. frequency is shown in Figure 5c.

DO ANTENNAS RADIATE IN ALL DIRECTIONS?

The power from an antenna radiates in a pattern that may not be uniform in all directions. To characterize the antenna gain, the ratio of the power radiated in a given direction to the power density if radiation occurred uniformly in all directions distributed over the surface of a sphere is used. For a dipole antenna, most of the power radiates in the direction perpendicular to the axis of the antenna as shown in Figure 3. The directivity of an antenna is the gain in the direction of the maximum power, which is the direction perpendicular to the axis of a dipole. Gain is measured in dBi = 10*log Gain.

The three- or two-dimensional radiation pattern from an antenna is also called a power pattern, power plot, or power distribution. It visually illustrates how an antenna receives or transmits in a certain range of frequencies. It is normally plotted for the far field. An antenna radiation pattern is primarily affected by the geometry of the antenna. It is also affected by the surrounding landscape or by other antennas. Sometimes multiple antennas are used in an antenna array to affect directivity. As shown in Figure 6a, two antennas fed by the same source can be used to cancel the fields in the plane of the antennas if they are spaced by ??? wavelength. The top view of this arrangement is shown in Figure 6b with a sketch of the power pattern.

MIRROR, MIRROR ON THE WALL: WHAT IS THE IMPORTANCE OF REFLECTIONS?

When we look into a mirror, we see the effect of reflections of electromagnetic radiation. Why do waves bounce off conductive surfaces? What is the result of these reflections on radiation? The basis for reflections is the boundary condition of the fields on the surface of a conductor. Boundary conditions for E and H fields are shown in Figure 7. Inside the conductor, charges are free to move when influenced by electric fields and current is induced by time-varying magnetic fields. A charge nearby the conductor causes charges to migrate on the conductor surface. Any tangential component of the E field would cause the charges to move until the tangential component of E is zero. The resulting effect is equivalent to the image, or virtual charge, located below the conductor surface shown in Figure 7c. The image isn’t real, but represents the charge that would cause an equivalent effect to the a

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