Free Axial-Mode Helical Antenna Calculator
Calculate helix coil diameter, turn spacing, pitch angle, directivity gain (dBi), half-power beamwidth (HPBW), and ground plane reflector dimensions using classic Kraus formulas.
📡 Frequency & Design Parameters
Half-Power Beamwidth (HPBW): 41.6° circular cone
• Natural Helix Impedance: ~140 Ω purely resistive.
• 50Ω Coaxial Matching: Solder a triangular copper/brass vane matching strip between the first quarter-turn and ground plane, or use a quarter-wave $approx 84,Omega$ microstrip transformer.
• Total Wire Length: 877 mm (34.5 in) of 12-14 AWG solid copper wire or copper tubing.
Recommended Tools & Equipment
Tested hardware and components for high reliability
Principles of the Axial-Mode Helical Antenna
Invented by Dr. John D. Kraus in 1946, the axial-mode (or end-fire) helical antenna is one of the most widely used high-gain circularly polarized antennas in aerospace, space telemetry, FPV drone video, and amateur satellite communications.
Classic Kraus Formulations
Axial mode operation occurs when the helix circumference $C$ is approximately equal to the free-space wavelength $lambda$ ($0.8 lambda le C le 1.2 lambda$):
Helix Diameter: D = C / π = (C_ratio × λ) / π
Turn Spacing: S = C × tan(α)
Total Axial Length: L = N × S
Kraus Gain Formula: Gain ≈ 10 × log10[ 15 × (C/λ)^2 × (N × S / λ) ] (dBi)
Half-Power Beamwidth: HPBW ≈ 52° / [ (C/λ) × √(N × S / λ) ]
Circular Polarization & Faraday Rotation Rejection
Signals passing through Earth's ionosphere undergo unpredictable Faraday polarization rotation. Linear antennas (vertical or horizontal dipoles) can experience cross-polarization nulls exceeding 20 dB. Circular polarization (RHCP or LHCP) eliminates this fade entirely, as the electric field vector rotates continuously through space.
Furthermore, in FPV drone racing and multipath environments, right-hand circularly polarized waves invert upon bouncing off concrete, trees, or metal obstacles, converting into LHCP. The RHCP receiver antenna suppresses the reflected LHCP ghost signals by 15 to 20 dB, eliminating video breakup and flutter.
Frequently Asked Questions
How do I determine if my antenna is RHCP or LHCP?
Look down the axis of the antenna from the back reflector toward the open front. If the wire coils away from you in a clockwise spiral, it is Right Hand Circularly Polarized (RHCP). If it coils in a counter-clockwise spiral, it is Left Hand Circularly Polarized (LHCP).
Why is the natural impedance ~140 ohms instead of 50 ohms?
The axial-mode helix acts as a travelling-wave antenna with a characteristic terminal impedance of Rin ≈ 140 * (C / λ) ohms. To match standard 50 ohm coaxial cable with low SWR (< 1.2:1), designers solder a flat brass or copper wedge/vane between the first 1/4 turn and the ground plate, which acts as a tapered transmission line transformer.
Can I wind the helix around a PVC pipe?
Be careful! Standard hardware store PVC has a dielectric constant of εr ≈ 3.0 and high dielectric loss (loss tangent) at microwave frequencies (1 GHz to 5.8 GHz). The presence of PVC inside the core slows the wave velocity, detuning the resonant frequency downward by 5% to 15%. For microwave antennas, use thin fiberglass rods, acrylic, low-loss Delrin, 3D printed PETG/PLA skeleton spacers, or keep it air-cored.