The Folded Dipole Antenna

A folded dipole is a dipole antenna, with the ends folded back around and connected to each other, forming a loop as shown in Figure 1. Figure 1. Folded dipole of length L. Typically, the width d of the folded dipole is much smaller than the length L. Because the dipole is a closed loop, one would expect the input impedance to depend on the input impedance of a short-circuited transmission line of length L (although unfortunately it depends on a transmission line of length L/2, which doesn't quite make intuitive sense to me). Also, because the dipole is folded back on itself, the currents can reinforce each other instead of cancelling each other out, so the input impedance will also depend on the impedance of a dipole antenna of length L. Letting Zd represent the impedance of a dipole antenna and Zt represent the transmission line impedance given by: The input impedance ZA of the folded dipole is given by: The folded dipole is resonant and radiates well at odd integer multiples of a half-wavelength (0.5, 1.5, ...). The input impedance is higher than that for a regular dipole. The antenna impedance for a half-wavelength folded dipole antenna can be found from the above equation for ZA; the result is ZA=4*Zd. At resonance, the impedance of a half-wave dipole antenna is approximately 70 Ohms, so that the input impedance for a half-wave folded dipole is roughly 280 Ohms. Because the characteristic impedance of twin-lead transmission lines are roughly 300 Ohms, this dipole is often used when connecting to this type of line, for optimal power transfer. The radiation pattern of half-wavelength folded dipoles have the same form as that of half-wavelength dipoles.