233 full wave dipole another seemingly obvious choice

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2.3.3 Full-wave dipole Another seemingly obvious choice is the full-wave dipole ( L = λ ). Following the procedures above, one can show that the full-wave dipole has a narrower beam with a HPBW of 48 . However, note that the standing wave pattern assumed for the current distribution has a null at the feedpoint for a full-wave dipole. This implies a very high driving point impedance, usually making the antenna impractical. Wires with increasing lengths have increasingly narrow main beams, but these come at the expense of grating lobes. The utility of long wire dipoles is therefore limited. 2.3.4 Quarter-wave monopole ~ I ~ I I (+q) (+q) (-q) Figure 2.7: Monopole antenna above a perfectly conducting ground plane. We can think of the current in the monopole being carried by positive charge carriers moving in the direction of the current. The corresponding image charges are negative and move in the opposite direction, and the image current is consequently in the same direction as the original current. A common antenna configuration is illustrated in Figure 2.7. Here, a vertical monopole wire antenna is excited from a feedpoint at ground level. The monopole antenna could be a large tower, as is used for AM radio transmission. Charge and current induced in the ground plane will affect the radiation, but it is possible to account for these with the method of images. There will obviously be no electromagnetic fields below the ground plane, in the lower half space. The uniqueness of solution of Maxwell’s equations allows us to compute the fields in the upper half space by replacing the ground plane with whatever sources we like in the lower half space. So long as the boundary conditions are maintained, so is the solution in the upper half space. In this case, the boundary condition is that the fields vanish at the ground plane. We can preserve this by discarding the ground plane, replacing it with an image monopole antenna below ground as shown. The sense (direction) of the current in the image antenna is the same as in the actual one. Consequently, in the upper half space, a monopole antenna above a perfectly conducting ground generates the same electromagnetic fields as a dipole antenna in free space. 39
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With the realization that the radiation from the monopole antenna is identical to that of a dipole above the ground plane and nonexistent below it, we can characterize its behavior without performing any new calculations. By com- parison to the dipole antenna with identical feedpoint current P is unchanged (max. at broadside) P total is halved (no radiation below ground plane) R rad is halved (from Ohm’s law) D = 4 πP /P total is doubled. HPBW and beam solid angle are halved. In practice, the Earth’s surface behaves more like a lossy dielectric than a perfect conductor. Wires are sometimes run radially above or on the ground to better simulate an ideal ground plane. In the absence of such wires, the results presented above must be regarded as approximations. Their accuracy depends on frequency, soil type, and conditions.
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