Log Periodic Antenna Calculator
Calculate dimensions, bandwidth ratio, and estimated gain for a log-periodic dipole array antenna.
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Inputs
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Math
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Enter parameters and click Calculate to view results
Formula & Theory
Ln = L0 × τⁿThis formula is used to calculate antenna parameters for log periodic antenna calculator.
Overview
The Log Periodic Antenna Calculator helps RF engineers, antenna designers, amateur radio operators, broadcast engineers, researchers, and students calculate the key dimensions and performance of a Log Periodic Dipole Array (LPDA). By entering the centre frequency, tau ratio, and number of antenna elements, the calculator determines wavelength, longest and shortest element lengths, boom length, bandwidth ratio, and estimated antenna gain. It is widely used for HF, VHF, UHF, television reception, spectrum monitoring, EMC testing, military communication, radio direction finding, and broadband wireless communication systems.
Input Guide
Enter Center Frequency, Tau Ratio, Number of Elements exactly in the units shown by this log periodic antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Center Frequency — use MHz.
- Tau Ratio.
- Number of Elements.
Output Guide
The results describe the calculated log periodic antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The Log Periodic Antenna uses Ln = L0 × τⁿ. Supply Center Frequency (MHz), Tau Ratio, Number of Elements in the displayed units, then use the calculated values as the first engineering target for this directional antennas design or analysis.
Design Notes
A Log Periodic Dipole Array (LPDA) is a broadband directional antenna that maintains relatively constant gain, impedance, and radiation characteristics over a wide frequency range. Unlike narrowband antennas such as the Yagi-Uda, the LPDA uses a logarithmic progression of element lengths and spacing controlled primarily by the tau (τ) ratio and sigma (σ) spacing factor. The tau ratio determines the relationship between adjacent element lengths, directly affecting antenna gain, boom length, and operating bandwidth. Lower tau values generally increase gain but require a longer boom and more elements, while higher tau values produce a more compact antenna with slightly lower gain. Practical antenna performance also depends on boom construction, feed-line balance, element alignment, conductor diameter, balun design, mechanical tolerances, and installation environment.
Build and Tuning Notes
Use the calculated element lengths and boom dimensions as the starting point when designing or constructing a log periodic antenna. Ensure each dipole element is accurately positioned according to the intended logarithmic progression and maintain consistent spacing throughout the array. Use a suitable balanced feed system or broadband balun to achieve good impedance matching and minimise feed-line radiation. After construction, verify return loss (S11), VSWR, impedance, gain, front-to-back ratio, and radiation pattern using a vector network analyser (VNA) and anechoic chamber measurements. Electromagnetic simulation using CST Studio Suite, Ansys HFSS, FEKO, or NEC is recommended to optimise broadband performance before fabrication.
Frequently Asked Questions
What is a Log Periodic Dipole Array (LPDA)?
A Log Periodic Dipole Array is a broadband directional antenna consisting of dipole elements whose lengths and spacing decrease logarithmically. It provides stable impedance, consistent gain, and wide operating bandwidth across multiple frequency bands.
Where are log periodic antennas commonly used?
Log periodic antennas are widely used for television reception, amateur radio, HF, VHF and UHF communication, EMC testing, spectrum monitoring, military communication, radio direction finding, and broadband wireless systems.
What is the tau (τ) ratio?
The tau ratio defines the size relationship between adjacent antenna elements. It controls antenna gain, boom length, and operating bandwidth, making it one of the most important design parameters in an LPDA.
What is the advantage of a log periodic antenna over a Yagi antenna?
A log periodic antenna offers significantly wider bandwidth while maintaining relatively constant impedance and gain across multiple frequencies. A Yagi antenna generally provides higher gain but over a much narrower frequency range.
Why are log periodic antennas considered broadband?
Only a small group of elements is active at any operating frequency, allowing the antenna to maintain efficient radiation over a wide frequency range without significant changes in impedance or radiation pattern.
Why can measured antenna performance differ from calculated values?
Actual performance may vary because of element spacing accuracy, boom construction, balun performance, conductor diameter, feed-line routing, nearby conductive objects, manufacturing tolerances, environmental conditions, and measurement uncertainty. Practical testing and electromagnetic simulation should always be used to validate the final antenna.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.