LogPeriodicArrays

Log-periodic arrays are a family of broadband antenna configurations characterized by elements whose lengths and spacings vary in a geometric progression. The most common realization is the log-periodic dipole array (LPDA), in which a series of dipole elements are mounted along a common boom with their lengths Ln and center-to-center spacings Sn related by a constant scaling factor τ: Ln+1 = τ Ln and Sn+1 = τ Sn. This geometric scaling produces approximate self-similarity, so when the frequency changes by a factor near 1/τ the array exhibits a similar current distribution and radiation pattern.

In operation, the “active” element at a given frequency is the element whose electrical length is closest to a half-wavelength; as frequency increases, the effective active region moves toward the end of the array. The elements are typically fed by a tapered transmission line or a multi-section transformer to maintain progressive phase and impedance with minimal reflections.

Performance and characteristics: LPDA types offer wide bandwidth with relatively constant input impedance and directional gain over a broad frequency range. The forward beam is narrow and steers slightly with frequency; polarization is usually linear.

Variants and uses: Beyond LPDA, log-periodic concepts apply to arrays of slots or folded elements. They are used for VHF-UHF communications, shortwave direction finding, radar, and amateur radio because of their broadband characteristics and moderate complexity.

Design considerations: The choice of τ (commonly around 0.85–0.95) and the number of elements determine impedance match, gain, and physical size. Manufacturing tolerances and feed network losses can affect performance.