nd having a maximum in the middle. Longer dipoles follow the same basic principles having further wavelengths. However, a voltage maximum and current mini- mum is found at both ends. The feed point is similarly taken at a point of voltage minimum and current maximum. A three half wavelength antenna is sometimes used, and this could be fed in the middle, or at one of the other current maximum points. With the feed point taken at the point where the voltage is a minimum and the current a maximum it can be understood from Ohm's law that the impedance is low. The actual impedance depends mainly on the proximity of nearby objects, but in free space the value of the impedance is 73 ohms, making it a good match to 75 ohm coax. It is very easy to alter the impedance of a dipole. The proximity of nearby objects has a major effect. The height above ground alters the impedance quite significantly. It is possible to calculate this effect and plot the impedance of a dipole in terms of its height in wavelengths above ground as shown in Figure 4.17. In some antenna designs where the dipole is used as the basic driven element, the feed impedance of the dipole falls to very low values, often to values of 10 ohms or less. This can be very difficult to feed efficiently. Antenna systems 97 Figure 4.17 Approximate impedance of a horizontal dipole at varying heights above ground The impedance can be increased to more suitable values by using what is called a folded dipole. In this type of antenna an additional wire is taken from one end to the other. Different conductor sizes can be used for the original section of the dipole element and the folded section. If they are both the same then the impedance of the dipole is raised by a factor of four. By changing the ratios of the sizes of the conductors it is possible to tailor the impedance. However, in most cases the same sizes are used, giving a basic impedance of 300 ohms for a folded dipole (i.e. 4 X 75 ohms). An additional advantage of using a folded dipole is that it gives an increa