235 folded dipole antenna figure 28 constructing a

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2.3.5 Folded dipole antenna Figure 2.8: Constructing a folded dipole antenna from a two-wire transmission line. Another common antenna is the half-wave folded dipole. You may be able to find one of these on the back of your FM stereo. The folded dipole offers a simple means of increasing the radiation resistance of an ordinary half- wave dipole. Folded dipoles are therefore often incorporated in parasitic arrays which tend otherwise to have very low radiation resistance. Folded dipole antennas can offer a good match with several kinds of inexpensive, twin-lead feedline and can, in fact, be made from such feedline. To understand how this works, imagine constructing one from a piece of two-wire transmission line one-half wavelength long. Figure 2.8 shows the sense of the current during one particular phase of the wave. The end is shorted and therefore corresponds to a current maximum. A current node exists one quarter wavelength closer to the transmitter, and another current maximum exists at the feedpoint. We suppose that deforming the line into the folded dipole antenna at the right leaves the sinusoidal current distribution unchanged. The feedpoint remains a current maximum, the endpoints current nodes. Clearly, the two wires comprising the folded dipole have identical current distributions that are also identical to that of an ordinary half-wave dipole. Since there are two wires carrying current, the total current is doubled compared to an ordinary half-wave dipole. Consequently, for some input current, The electric field produced is doubled. The Poynting flux and total radiated power increase by a factor of 4. Since the feedpoint currents are the same, the radiation resistance must have increased by a factor of 4 (to approximately 300 , which is the intrinsic impedance for one common kind of twin-lead feedline). The radiation pattern is meanwhile unchanged from that of a half-wave dipole. The directivity, effective area, HPBW, and beam solid angle are all unchanged. 40
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2.3.6 Short dipole It is often necessary or desirable to use wire antennas that are very short compared to the wavelength. This situation comes about when space is limited or when the wavelength is very long, as with VLF and ULF communications. A short antenna is not an elemental dipole. The latter is an idealization and requires that the current on the antenna be uniform along its length. In practice, the current vanishes at the ends of a short wire antenna, just as for long wires. Nothing about the calculations in section 1.5 necessitated that the wire antenna be long, and those results are completely applicable to the short dipole. However, we can simplify things considerably with a few well justified assumptions. The first is that the distances measured along the wire antenna are so small (compared to λ ) that the sinusoidal current distribution we assumed earlier can be replaced by its argument (i.e. the first term if its Taylor series expansion): I ( z ) I parenleftbigg 1 | z | L/ 2 parenrightbigg The current distribution is approximately triangular on any sufficiently short wire. Furthermore, we can approximate
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