Tools
Log Periodic Feed Line Calculator
Work out the feed line impedance and element spacing for a log periodic antenna, for round or square tube. Enter the tube size and centre spacing to get the impedance, or enter a target impedance to get the spacing that achieves it.
Leave blank for solid conductor assumption. Applied only when a frequency is also given.
Characteristic Impedance
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Face-to-Face Spacing
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Technical notes & accuracy
- Round conductors.
- Uses the rigorous formula Zc = (376.73 / π√1.00054) × acosh(D/d), based directly on Maxwell’s equations, giving accuracy within 0.1% under ideal conditions.
- Square conductors.
- Implements Owen Duffy’s empirical formula, derived from ATLC electromagnetic field simulation. Valid for 30–300 Ω with under 1% error when D/d ≥ 1.2.
- Aluminium properties.
- When a frequency is specified, the calculation accounts for skin-depth effects in aluminium conductors across the 1.9–72 MHz range typical of log periodic antennas. A wall thickness thin compared with the skin depth means current flows on both the inner and outer surfaces, and the tube no longer behaves like a solid rod of its nominal diameter.
- Verification.
- All formulas were systematically verified against authoritative sources, including NIST physical constants and peer-reviewed electromagnetic research.
- Face-to-face spacing.
- Reported from the nominal tube diameter, not the frequency-adjusted effective diameter — it is the gap you actually measure and cut to on the boom.