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Calculators / Folded Dipole
Folded Dipole
Calculator
Calculate the impedance transformation ratio, total element length, and optimal conductor spacing for a folded dipole antenna. Used in Yagi driven elements, FM broadcast, and twin-lead applications where a higher feed impedance is needed.
Top Conductor — d₁ (continuous)
Bottom Conductor — d₂ (split/fed)
Feed Line
⚙ Folded Dipole Engine
λ = — m
Bare wire ≈ 0.95. Twin-lead / window line ≈ 0.82–0.91
Top element (continuous conductor)
Bottom element (split at feed point)
Centre-to-centre spacing between conductors
Standard folded dipole with equal conductors → 300 Ω
Advanced Parameters
End capacitance shortening (0.95–0.98 typical)
More conductors → higher impedance step-up
Balun transforms feedline impedance
m
Affects feedpoint impedance via image theory
Calculated Results
Input Impedance (Zin)
—Ω
Impedance transformation ratio (τ)
—
Element Length (L)—
Half-Length (L/2)—
Conductor Spacing (s)—
Free-space λ—
Zin (equal conductors)—
Zin (unequal d₁≠d₂)—
SWR vs 300 Ω—
SWR vs 75 Ω—
SWR vs 50 Ω—
Gain (dBi)2.15 dBi
Bandwidth advantage—
Enter values to calculate.
📖 Theory
A folded dipole is a half-wave dipole with a second conductor connecting the two ends, forming a narrow rectangular loop. It has the same radiation pattern as a standard dipole but a higher feedpoint impedance.
Element Length
L = (k × c × ee) / (2 × f)
Same as standard half-wave dipole with VF and end-effect corrections.
Impedance Transformation
Equal conductors (d₁=d₂):
Zin = 4 × 73.1 = 292 ≈ 300 Ω
Unequal conductors:
τ = (1 + ln(s/a₁)/ln(s/a₂))²
Zin = τ × 73.1 Ω
Spacing Rule-of-Thumb
Typical spacing s = λ/100 to λ/30.
For standard TV twin-lead (300 Ω), s ≈ 12 mm (λ/100 at ~250 MHz).
Wider spacing → larger τ → higher Zin.
📡 Pattern & Impedance Viewer
Live Simulation
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◉ Pattern & Zin update with spacing / conductor ratio
Display
Spacing (s)
25 mm
d₂/d₁ ratio
1.0 ×
equal10× thicker
E-Plane Pattern
ℹ
Design Insights
Enter parameters to see design insights.
Common Applications
FM & TV Reception
The classic 300 Ω folded dipole is the standard driven element for FM dipoles and UHF TV antennas, feeding 300 Ω twin-lead directly.
Yagi Driven Element
Folded dipoles are preferred as Yagi driven elements — the ~290 Ω impedance naturally matches 300 Ω twin-lead or 4:1 balun + 75 Ω coax.
Amateur Radio HF/VHF
Wire folded dipoles on HF bands provide wider bandwidth than simple dipoles and allow 300 Ω ladder-line or open-wire feed.
Impedance Matching
Use unequal-diameter conductors to tune the impedance transformation ratio τ and match unusual feed-line impedances without a balun.
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