To achieve good directional pattern characteristics and at the same time provide good matching to practical coaxial lines with 50- or 75-ohm characteristic impedances, the length of a single wire element is usually chosen to be λ/4 ≤ l < λ. The most widely used dipole is that whose overall length is l= λ/2, and which has an input impedance of Zin =73 + j42.5 and directivity of D0 = 1.643. In practice, there are other very common transmission lines whose characteristic impedance is much higher than 50 or 75 ohms. For example, a “twin-lead” transmission line (usually two parallel wires separated by about 5/16 in. and embedded in a low-loss plastic material used for support and spacing) is widely used for TV applications and has a characteristic impedance of about 300 ohms. In order to provide good matching characteristics, variations of the single dipole element must be used. One simple geometry that can achieve this is a folded wire which forms a very thin (s << λ) rectangular loop as shown in Figure 1. This antenna, when the spacing between the two larger sides is very small (usually s <0.05λ), is known as a folded dipole and it serves as a step-up impedance transformer (approximately by a factor of 4 when l = λ/2) of the single-element impedance. Thus when l = λ/2 and the antenna is resonant, impedances on the order of about 300 ohms can be achieved, and it would be ideal for connections to “twin-lead” transmission lines A folded dipole operates basically as a balanced system, and it can be analyzed by assuming that its current is decomposed into two distinct modes: a transmission-line mode Figure (b) and an antenna mode Figure (c). This type of an analytic model can be used to predict accurately the input impedance provided the longer parallel wires are close together electrically (s<< λ).

A two-element folded dipole is widely used, along with “twin-lead” line, as feed element of TV antennas such as Yagi-Uda antennas. Although the impedance of an isolated folded dipole may be around 300 ohms, its value will be somewhat different when it is used as an element in an array or with a reflector. The folded dipole has better bandwidth characteristics than a single dipole of the same size. Its geometrical arrangement tends to behave as a short parallel stub line which attempts to cancel the off resonance reactance of a single dipole. The folded dipole can be thought to have a bandwidth which is the same as that of a single dipole but with an equivalent radius (a < ae< s/2).Symmetrical and asymmetrical planar folded dipoles can also be designed and constructed using strips which can be fabricated using printed-circuit technology. The input impedance can be varied over a wide range of values by adjusting the width of the strips. In addition, the impedance can be adjusted to match the characteristic impedance of printed-circuit transmission lines with four-to-one impedance ratios.