The Short Dipole Antenna

The short dipole antenna is the simplest of all antennas. It is simply an open-circuited wire, fed at its center as shown in Figure 1. Figure 1. Short dipole antenna of length L. The words "short" or "small" in antenna engineering always imply "relative to a wavelength". So the absolute size of the above dipole does not matter, only the size of the wire relative to the wavelength of the frequency of operation. Typically, a dipole is short if its length is less than a tenth of a wavelength: If the antenna is oriented along the z-axis with the center of the dipole at z=0, then the current distribution on a thin, short dipole is given by: The current distribution is plotted in Figure 2. Note that this is the amplitude of the current distribution; it is oscillating in time sinusoidally at frequency f. Figure 2. Current distribution along a short dipole. The fields radiated from this antenna in the far field are given by: The above equations can be broken down and understood somewhat intuitively. First, note that in the far-field, only the and fields are nonzero. Further, these fields are orthogonal and in-phase. Further, the fields are perpendicular to the direction of propagation, which is always in the direction (away from the antenna). Also, the ratio of the E-field to the H-field is given by (the characteristic impedance of free space). This indicates that in the far-field region the fields are propagating like a plane-wave. Second, the fields die off as 1/r, which indicates the power falls of as Third, the fields are proportional to L, indicated a longer dipole will radiate more power. This is true as long as increasing the length does not cause the short dipole assumption to become invalid. Also, the fields are proportional to the current amplitude , which should make sense (more current, more power). The exponential term: describes the phase-variation of the wave versus distance. Note also that the fields are oscillating in time at a frequency f in addition to the above spatial variation. Finally, the spatial variation of the fields as a function of direction from the antenna are given by . For a vertical antenna oriented along the z-axis, the radiation will be maximum in the x-y plane. Theoretically, there is no radiation along the z-axis far from the antenna. In the next section further properties of the short dipole will be discussed.