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Folded Dipole Calculator

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.

β–Ά Start Calculating πŸ“– Theory
d₁ β€” CONTINUOUS dβ‚‚ β€” SPLIT AT FEED ⚑ FEED 300 Ξ© Feedline L = Ξ»/2 = β€” s = β€” f = β€” MHz | Zin = β€” | Ο„ = β€”
Frequency Tuning 144.00 MHz
1 MHz1000 MHz
Top Conductor β€” d₁ (continuous) Bottom Conductor β€” dβ‚‚ (split/fed) Feed Line

βš™ Folded Dipole Engine

Ξ» = β€” m

Bare wire β‰ˆ 0.95. Twin-lead / window line β‰ˆ 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 β†’ 300 Ξ©

Advanced Parameters

End capacitance shortening (0.95–0.98 typical)

More conductors β†’ higher impedance step-up

Balun transforms feedline impedance

m

Affects feedpoint impedance via image theory

Calculated Results

Input Impedance (Zin) β€”Ξ©
Impedance transformation ratio (Ο„) β€”
Element Length (L)β€”
Half-Length (L/2)β€”
Conductor Spacing (s)β€”
Free-space Ξ»β€”
Zin (equal conductors)β€”
Zin (unequal d₁≠dβ‚‚)β€”
SWR vs 300 Ξ©β€”
SWR vs 75 Ξ©β€”
SWR vs 50 Ξ©β€”
Gain (dBi)2.15 dBi
Bandwidth advantageβ€”
Enter values to calculate.

πŸ“– 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 Γ— c Γ— ee) / (2 Γ— f)

Same as standard half-wave dipole with VF and end-effect corrections.

Impedance Transformation

Equal conductors (d₁=dβ‚‚):
Zin = 4 Γ— 73.1 = 292 β‰ˆ 300 Ξ©

Unequal conductors:
Ο„ = (1 + ln(s/a₁)/ln(s/aβ‚‚))Β²
Zin = Ο„ Γ— 73.1 Ξ©

Spacing Rule-of-Thumb

Typical spacing s = Ξ»/100 to Ξ»/30.
For standard TV twin-lead (300 Ξ©), s β‰ˆ 12 mm (Ξ»/100 at ~250 MHz).
Wider spacing β†’ larger Ο„ β†’ higher Zin.

πŸ“‘ Pattern & Impedance Viewer

Live Simulation | β—‰ Pattern & Zin update with spacing / conductor ratio
Display
Spacing (s) 25 mm
dβ‚‚/d₁ ratio 1.0 Γ—
equal10Γ— thicker
E-Plane Pattern 90Β° 270Β° 180Β° 0Β° Antenna axis (null) 2.15 dBi
β„Ή
Design Insights

Enter parameters to see design insights.

Common Applications

πŸ“Ί

FM & TV Reception

The classic 300 Ξ© folded dipole is the standard driven element for FM dipoles and UHF TV antennas, feeding 300 Ξ© twin-lead directly.

πŸ“‘

Yagi Driven Element

Folded dipoles are preferred as Yagi driven elements β€” the ~290 Ξ© impedance naturally matches 300 Ξ© twin-lead or 4:1 balun + 75 Ξ© coax.

πŸ“»

Amateur Radio HF/VHF

Wire folded dipoles on HF bands provide wider bandwidth than simple dipoles and allow 300 Ξ© ladder-line or open-wire feed.

πŸ”

Impedance Matching

Use unequal-diameter conductors to tune the impedance transformation ratio Ο„ and match unusual feed-line impedances without a balun.

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