log periodic antenna calculator

Log Periodic Dipole Array (LPDA) Calculator

Use this calculator to generate first-pass dimensions for a log periodic antenna. Enter your target frequency range and design factors, then click calculate.

Lowest frequency (MHz)

Highest frequency (MHz)

Tau (τ) - element length ratio (0.5 to 0.99)

Sigma (σ) - spacing factor (0.02 to 0.30)

Velocity / end-effect factor (0.90 to 1.00)

Build note

Formulas are for preliminary geometry only. Real-world tuning depends on boom, conductor diameter, feedline, balun, and environment.

What this log periodic antenna calculator does

A log periodic dipole array (LPDA) is a wideband directional antenna made from multiple dipole elements that get progressively shorter toward the front. This tool estimates the key dimensions you need to start a build: element lengths, spacing, boom length, and element count for your chosen frequency range.

It is especially useful for VHF and UHF projects where one antenna must cover a broad bandwidth, such as scanner reception, TV bands, and multi-band communication work.

Input definitions (in plain language)

1) Lowest and highest frequency

These set your design bandwidth. The longest element is based on the lowest frequency; the shortest element is based on the highest.

2) Tau (τ)

Tau is the ratio between adjacent element lengths. A lower τ usually means more elements and often better bandwidth behavior, but with larger physical size and complexity.

3) Sigma (σ)

Sigma controls spacing between neighboring elements. It affects boom length, apex angle, and pattern behavior. Typical practical values are often in the 0.05 to 0.20 range.

4) Velocity / end-effect factor

Real dipoles are not always ideal half-wave wires in free space. This factor lets you shorten lengths slightly to reflect practical conductor and end effects.

Equations used by the calculator

Element length: L = (c × VF) / (2f)
Length progression: L(n+1) = τ × L(n)
Element spacing: S(n) = 2σ × L(n)
Apex angle estimate: α ≈ 2 arctan[(1 − τ) / (4σ)]
Element count is selected to cover your requested top frequency.

Practical build tips

Keep boom alignment straight and consistent to preserve pattern performance.
Use a proper feedline transition/balun to reduce common-mode current.
Account for nearby metal structures, mast interaction, and mounting height.
After construction, verify SWR and pattern with measurement or simulation tools.

Why this is a starting point, not a final answer

LPDA performance depends on more than just geometry. Conductor diameter, boom material, feed implementation, and even weatherproofing can shift resonance and impedance. Treat this page as your first-pass design engine; then refine in NEC software and validate on-air or with test equipment.