lpda calculator

What Is an LPDA Calculator?

An LPDA calculator helps you design a Log-Periodic Dipole Array, a wideband directional antenna commonly used for TV reception, ham radio, EMC testing, and broadband monitoring. Unlike narrowband antennas, an LPDA covers a large frequency range by using multiple dipole elements that progressively change in size.

This tool gives you a fast first-pass design by estimating key geometry values from standard LPDA relationships: frequency range, element scaling, spacing, and boom dimensions.

Inputs Explained

1) Frequency range (f_min and f_max)

These define your target operating band. The lowest frequency sets your largest element; the highest frequency sets your smallest active element.

2) Scale factor (τ)

τ controls how quickly element lengths shrink from one dipole to the next. Lower τ usually means more aggressive tapering and potentially broader coverage, but can change gain and boom requirements.

3) Spacing factor (σ)

σ controls spacing between neighboring elements. Larger spacing can increase boom length and change forward gain and impedance behavior.

4) Velocity factor (VF)

VF adjusts electrical wavelength to better match real-world materials and construction effects. A VF near 1.0 is common for initial geometric estimates.

Formulas Used in This LPDA Design Calculator

• Band ratio: B = f_max / f_min
• Estimated element count: N = ceil(1 + log(B) / log(1/τ))
• Dipole length: L = (150 × VF) / f(MHz), in meters
• Element progression: L_n+1 = τL_n
• Spacing progression: S_n = σL_n
• Apex angle estimate: α = 2 arctan((1 − τ) / (4σ))

How to Use the Results

After calculation, use the element table as your fabrication guide. Each row gives a dipole length and the spacing to the next element. Keep element alignment, feedline routing, and boom symmetry consistent to reduce pattern distortion.

Before final construction, run a simulation (for example in NEC-based software) and fine-tune dimensions for your chosen boom material, element diameter, feed method, and mounting environment.

Practical LPDA Build Tips

• Use consistent element diameter and good mechanical rigidity.
• Maintain accurate center spacing; small errors accumulate across the boom.
• Account for balun and feedline effects, especially at upper frequencies.
• Leave extra boom length and trim after verification.
• Validate SWR and front-to-back ratio across the full band.

Limitations and Engineering Notes

This calculator is intended for preliminary sizing. Real LPDA performance depends on conductor thickness, insulation proximity, boom coupling, matching network, and nearby structures. For mission-critical designs, treat these outputs as a starting point and verify with full-wave simulation plus measurement.