Log Periodic Dipole Array

Log-periodic dipole arrays (LPDAs) are wideband structures having a geometry such that its impedance and radiation characteristics vary periodically with the logarithm of frequency. This antenna design is commonly used as a receiving antenna at VHF and UHF frequencies. Figure 1 shows the FEKO model of an 18 element LPDA designed for operating in the frequency band 200~600MHz. Non-radiating transmission lines are used to connect consecutive elements and the array is fed at the shortest element. The shortest element is roughly half a wavelength long at the highest design frequency, while the longest element is roughly half a wavelength long at the lowest design frequency.

Figure 1: FEKO model of LPDA

The input impedance of the LPDA across the operating frequency is shown in Figure 2a. While the resistive component oscillates near 70Ω, the reactive component remains close to zero. In Figure 2b the input reflection for a 75Ω system is shown.

Figure 2: Impedance characteristics of LPDA
(a) Input impedance (b) Input reflection (75Ω)

Radiation patterns for the LPDA at different frequencies is shown in Figure 3. The angle phi is measured in the xy-plane from the x-axis and the angle theta is measured from the z-axis. As expected the radiation pattern of the antenna is relatively stable across the 3:1 relative bandwidth.

Figure 3: Radiation patterns of LPDA
(a) 200MHz (b) 400MHz
(c) 600MHz