lpda antenna calculator

Enter your target frequency range and LPDA design factors to estimate element lengths, element spacing, boom length, and apex angle.

Lowest Frequency (MHz)

Highest Frequency (MHz)

Tau (τ) — Scaling Factor (0.7 to 0.98 typical)

Sigma (σ) — Spacing Factor (0.05 to 0.25 typical)

Velocity Factor (wire/end effect correction, usually 0.93 to 0.98)

What this LPDA calculator does

A log-periodic dipole array (LPDA) is a directional broadband antenna made from many dipole elements of different lengths. Instead of tuning one element for one frequency, the LPDA uses a repeating geometric pattern so the “active” part of the antenna shifts as frequency changes. That is why LPDAs are popular for TV reception, scanner monitoring, EMC testing, field strength work, and multiband communication setups.

This calculator gives a fast first-pass mechanical layout: number of elements, each element length, boom spacing, and boom length. It is ideal for planning and prototyping before deeper simulation in NEC, 4NEC2, MMANA-GAL, or RF lab measurement.

Input parameters explained

1) Frequency range

Use the lowest and highest operating frequencies in MHz. The lowest frequency controls your longest element. The highest frequency controls your shortest element and total element count.

2) Tau (τ)

Tau is the scaling ratio from one element to the next:

L(n+1) = L(n) × τ

Smaller τ (like 0.8) gives more aggressive scaling, often wider spacing in electrical terms and potentially more elements needed for smooth response. Larger τ (like 0.9+) can produce tighter scaling.

3) Sigma (σ)

Sigma controls axial element spacing along the boom. In this calculator:

S(n) ≈ σ × L(n)

Increasing sigma typically increases boom length and can affect gain and impedance behavior. Typical practical values are around 0.12 to 0.2 for many builds.

4) Velocity factor (VF)

Real dipoles are usually a little shorter than ideal free-space half-wave formulas due to end effects and conductor geometry. The velocity factor lets you apply a practical correction. Values in the 0.93–0.98 range are common depending on tubing, wire diameter, and mounting style.

How the calculator estimates geometry

The tool uses standard geometric relationships for log-periodic arrays:

Longest element from lowest frequency: L₁ = (150 × VF) / f_low
Element progression: L_n = L₁ × τ^(n-1)
Estimated element count: based on bandwidth ratio and tau
Spacing progression from sigma and current element length
Approximate apex angle: α = 2 × arctan((1 − τ) / (4σ))

These formulas are excellent for getting very close to a workable design quickly. Final optimization is still recommended for feedpoint match, gain flatness, and front-to-back ratio.

Practical build tips for better real-world performance

Use rigid booms: Keeping element positions accurate is important. Mechanical sag changes spacing and can disturb pattern consistency.
Maintain element centering: Every dipole should be centered and aligned the same way to preserve symmetry.
Follow the phasing feed arrangement: LPDAs rely on alternating feed polarity between adjacent elements. Keep cross-connections neat and equal.
Mind feedline routing: Keep the coax routed to minimize common-mode pickup. A current choke at the feedpoint is often beneficial.
Prototype and trim: Build slightly long if possible, then trim toward target after measurement.

Example use case

Suppose you want an LPDA for roughly 470–860 MHz (common UHF region). With τ = 0.86, σ = 0.16, and VF = 0.95, this tool will estimate a compact, practical boom with multiple elements stepping from longest to shortest. You can then model that geometry and tune for your exact boom material and feed details.

Final notes

This calculator is meant as a design starting point, not a full electromagnetic solver. Nearby metal, mast coupling, element diameter, and feed structure all influence final performance. For professional results, combine these estimates with simulation and on-air or bench verification.