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Transcript
The Dipole A dipole can be thought of as a two wire transmission line in which the two conductors are folded out at right angles to each other. Dipole Length, l Transmission line (feed) match Free space This is a useful way to view the dipole, as the theory of transmission lines can be applied to understand how it works. The dipole section provides a ‘match’ between the ‘feed’ line and ‘free space’. For any antenna to launch a signal, the field must be made to ‘fringe’ from the feed line into the space around the antenna. Considering how the field associated with the feed behaves at the dipole will show that as charges move on the surface, the field fringes into the space around the dipole. + - Then looking at the way the field fringes from the dipole, and thinking of it as a section of transmission line, shows that there will be no radiation along the axis of the structure. This is because the tips of the dipole represent an open circuit. Picturing the dipole folded back to be part of the feed line will make this idea clearer. The current and voltage distribution along the dipole are similar to those for a section of transmission line terminated in an open circuit, where power is reflected. The resulting standing wave pattern on the dipole puts a voltage maximum at the tips, where the current is close to zero. The impedance at the feed point will depend upon the distance from the open circuit (ie the dipole half-length). The main difference between the impedance seen at the feed point of the dipole and that which would be seen on a section of line terminated in an open circuit is that power is radiated from the dipole and hence there will be an associated ‘real’ part of the impedance. Open circuit In general, this real part, or resistance, consists of the RADIATION RESISTANCE (Rrad) and the LOSS RESISTANCE (Rloss). The radiation resistance is a ‘fictitious’ resistance that would dissipate the same power as the antenna radiates. When the dipole is half a wavelength long electrically, the input impedance will be purely real and equal to 73 (if there are no losses). The feed point is then at a ‘series resonance’ in the transmission line equivalent circuit, a quarter of a wavelength back from the open circuit termination. The equivalent circuit is: L C RLoss Rrad Prad=I2Rrad In this model, the inductance and capacitance are determined by the dipole length and radius. The longer the dipole, the larger the values of L and C resulting in a lower resonant frequency. The conductor radius controls the Q and hence the ratio of L to C, larger radius giving lower Q. The loss resistance is primarily due to the resistance of the conductor, which will reduce as the radius increases. In practice, the physical length of the dipole must be reduced to approximately 95% of half a wavelength to appear electrically half a wavelength long. The radiation pattern is ‘OMNIDIRECTIONAL’, in other words the gain is constant throughout one of the principal planes. For the dipole, the gain is constant throughout the H-plane, which is perpendicular to the dipole axis. The gain in the E-Plane varies according to a ‘figure of eight’ pattern. E-plane H-plane E-Plane H-Plane G=2.1dB G=2.1dB The radiation pattern for the dipole is of a similar form for lengths from close to zero up to one wavelength, when the three dimensional ‘doughnut’ pattern begins to break into extra lobes. This can be investigated using AERIAL1. The input impedance of the dipole changes dramatically over this range, from a high capacitive reactance, through resonance to an inductive reactance and through resonance again. The Hertzian Dipole A Hertzian dipole is a vanishingly short dipole. In practice, a dipole which is very short (length</50) is a good approximation to this. Referring to the transmission line analogy will show that the input impedance of a very short dipole has a high capacitive reactance and a small radiation resistance. For the Hertzian dipole, these are given by: l Rrad 80 2 2 where l = length and and l ln 2a 1 X a 120 l tan a = radius. directivity = 1.5 (compared to 1.643 for a half wave dipole) effective aperture = 0.122 (compared to 0.132 for half wave) General Dipole far field components In the far field for a dipole of length l, the electric and magnetic field components are given by: H j jt j I oe e 2r E H 2r l l cos cos cos sin