Free Log-Periodic Antenna Calculator (LPDA)
Design ultra-wideband log-periodic dipole arrays. Calculate geometric scale factors (τ, σ), dipole element lengths, feeder boom spacing, and directivity gain.
📡 Frequency Band & Geometric Ratios
3.21:1 Continuous Operating Bandwidth (140 - 450 MHz)
The Criss-Cross Feeder Phase Inversion: Unlike a Yagi where parasitic elements are unpowered, EVERY element in an LPDA is directly connected to the dual transmission line boom with an alternating 180° phase reversal between adjacent dipoles.
Recommended Tools & Equipment
Tested hardware and components for high reliability
How the Log-Periodic Dipole Array Works
The Log-Periodic Dipole Array (LPDA) is a frequency-independent directional antenna invented by Raymond DuHamel and Dwight Isbell in 1957. The geometric dimensions of the dipole elements and their relative boom spacings follow a strict geometric progression ratio:
Relative Spacing (σ): σ = d_n / (2 × L_n)
Apex Half-Angle: tan(α) = (1 - τ) / (4 × σ)
Active Region: Only 2 to 3 elements near λ/2 resonance radiate at any given frequency!
Why the Active Region Shifts Seamlessly
As operating frequency increases, the resonant active region moves smoothly forward toward the smaller front elements. The larger elements behind the active region act as reflectors, while the smaller unexcited elements ahead act as directors, maintaining a constant 6.5 to 8.5 dBi forward gain and low SWR across multi-octave bandwidths.
Frequently Asked Questions
Why must the feedline cross over between elements?
The 180° phase inversion created by alternating the connection of adjacent dipole halves across the two boom rails is required to produce end-fire radiation in the direction of the smaller elements. Without the transposition, the antenna radiates broadside.
Where does the coaxial feedline connect to the LPDA?
The coaxial feedline connects at the FRONT (the narrow end with the shortest director elements), with the center conductor tied to one boom rail and the shield to the opposite rail. The coax is typically routed through the inside of one hollow boom tube from the back to act as an integrated sleeve balun.
What is the trade-off between higher Tau and boom length?
Higher values of Tau (e.g. 0.92 vs 0.82) provide slightly higher gain and flatter SWR, but require significantly more elements and a substantially longer, heavier boom.