Calculate helical antenna wire length, turn circumference, or pitch for one turn from any two known values in meters, cm, mm, inches, or feet.

Helical Antenna Calculator
Common axial-mode starting values are C/λ ≈ 1.0 and S/λ ≈ 0.25.
Results
Wavelength
Helix Circumference
Helix Diameter
Turn Spacing
Total Antenna Height
Approx. Wire Length
Pitch Angle
Approx. Input Impedance
Approx. Gain
Approx. HPBW
Approx. FNBW

Related Calculators

Helical Antenna Formula

The helical antenna calculator uses the operating frequency to find wavelength, then scales the helix dimensions from the circumference ratio and turn spacing ratio you enter.

λ = c / f
C = R_Cλ
D = C / π
S = R_Sλ
H = NS
L = N√(C² + S²)
α = tan⁻¹(S / C)
Zᵢₙ ≈ 140R_C
G₍dBi) ≈ 10.8 + 10log₁₀(R_C²NR_S)
B = R_C√(NR_S)
HPBW ≈ 52 / B
FNBW ≈ 115 / B
  • λ = wavelength
  • c = speed of light, 299,792,458 m/s
  • f = frequency in Hz
  • C = helix circumference
  • RC = circumference ratio, C/λ
  • D = helix diameter
  • S = spacing between turns
  • RS = turn spacing ratio, S/λ
  • N = number of turns
  • H = total antenna height
  • L = approximate wire length
  • α = pitch angle
  • Zin = approximate input impedance in ohms
  • GdBi = approximate gain in dBi
  • B = beamwidth factor used for HPBW and FNBW estimates

The frequency unit selector first converts your frequency to Hz. The length output selector only changes the displayed units, not the underlying calculation. The circumference ratio sets the helix circumference relative to wavelength, while the spacing ratio sets the distance between turns relative to wavelength. From those values, the calculator estimates the physical size, wire length, pitch angle, input impedance, gain, half-power beamwidth, and first-null beamwidth.

Typical Axial-Mode Helical Antenna Starting Values

These values are common starting points for an axial-mode helical antenna. Final dimensions may need adjustment for conductor diameter, ground plane size, matching method, and nearby materials.

Parameter Common starting range Calculator input or result
Circumference ratio 0.8 to 1.2 λ C/λ
Turn spacing ratio 0.2 to 0.3 λ S/λ
Pitch angle About 12° to 14° Pitch angle result
Number of turns 3 to 15 turns Number of turns
Input impedance Often near 140 Ω when C/λ = 1 Approx. input impedance

Example Calculations

Example 1: 2.4 GHz helix with 8 turns

Suppose you enter a frequency of 2400 MHz, 8 turns, C/λ = 1.0, and S/λ = 0.25.

  • Wavelength: about 12.491 cm
  • Helix circumference: about 12.491 cm
  • Helix diameter: about 3.976 cm
  • Turn spacing: about 3.123 cm
  • Total antenna height: about 24.983 cm
  • Approximate wire length: about 103.014 cm
  • Pitch angle: about 14.04°
  • Approximate input impedance: 140 Ω

Example 2: 915 MHz helix with 6 turns

Suppose you enter a frequency of 915 MHz, 6 turns, C/λ = 1.0, and S/λ = 0.25.

  • Wavelength: about 32.764 cm
  • Helix circumference: about 32.764 cm
  • Helix diameter: about 10.430 cm
  • Turn spacing: about 8.191 cm
  • Total antenna height: about 49.146 cm
  • Approximate wire length: about 202.636 cm
  • Pitch angle: about 14.04°
  • Approximate input impedance: 140 Ω

FAQ

What does C/λ mean for a helical antenna?

C/λ is the helix circumference divided by wavelength. A value of 1 means the circumference is one wavelength at the operating frequency. For many axial-mode helical antenna designs, C/λ near 1.0 is a common starting point.

What does S/λ mean?

S/λ is the spacing between adjacent turns divided by wavelength. A value of 0.25 means each turn is spaced one-quarter wavelength from the next turn. With C/λ = 1 and S/λ = 0.25, the pitch angle is about 14 degrees.

Why are gain and beamwidth labeled approximate?

The gain, HPBW, and FNBW formulas are simplified estimates for axial-mode helices. Real antenna performance also depends on wire diameter, feed matching, ground plane size, construction accuracy, losses, and objects near the antenna.