QFH Quick Design Tool
Use this calculator to estimate starting dimensions for a quadrifilar helix antenna (QFH), commonly used for circularly polarized satellite reception (e.g., weather satellites around 137 MHz).
What this quadrifilar helix antenna calculator does
A quadrifilar helix antenna is made from four helical conductors spaced 90° apart around a central axis. In many practical QFH builds, two elements are slightly longer than the other two. That geometry helps produce circular polarization and a broad, sky-facing pattern that is excellent for low-earth-orbit weather satellites.
This calculator provides a practical first-pass geometry so you can start cutting wire and building a support frame. It is intentionally simple and workshop-friendly: enter frequency, choose a pitch angle, and get the key dimensions.
Formulas used in the calculator
1) Free-space wavelength
λ = c / f
2) Effective wavelength with conductor shortening
λeff = λ × VF
3) Helix circumference and diameter
C ≈ λeff, D = C / π
4) Axial rise per turn (pitch)
S = C × tan(α)
5) Height and wire length
H = N × S, Larm = N × √(C² + S²)
6) Short arm estimate
Lshort = Llong × short factor
These equations are a starting point. Real-world builds still need tuning with an antenna analyzer and possibly small mechanical adjustments.
How to use the output
1) Build the frame first
Use the calculated diameter and height to create your non-conductive support structure (PVC, fiberglass, or 3D-printed spacers). Keep the frame rigid—small shape changes can shift resonance and pattern.
2) Cut long and short arms
- Cut two long helical elements to the calculated long-arm length.
- Cut two short elements using the short-arm factor result.
- Leave a little extra wire for trimming during tuning.
3) Keep spacing symmetric
Place each arm 90° apart around the circumference. Symmetry is critical for good axial ratio and balanced circular polarization. If one arm drifts out of shape, signal quality can drop quickly.
4) Feed and phase carefully
The calculator includes an estimated quarter-wave phasing line length for your chosen coax velocity factor. This is useful for many feed harness approaches, but exact feed topology varies by design. Always verify with measurements.
Typical starting values
- 137 MHz weather satellite reception: Turns = 0.5, Pitch = 10° to 14°, Short factor = 0.90 to 0.94
- Conductor VF: around 0.93 to 0.98 depending on wire diameter and nearby materials
- Coax VF: ~0.66 (solid PE), ~0.78 to 0.85 (foam dielectric), check your datasheet
Tuning checklist after construction
- Measure return loss or SWR around your target frequency.
- Trim long/short arms in tiny equal increments if resonance is low.
- If resonance is high, increase electrical length slightly or reduce trimming.
- Check feedline choke/current balun to reduce common-mode noise.
- Validate performance on real satellite passes, not just bench measurements.
FAQ
Is this calculator accurate enough for a finished antenna?
It is accurate enough for an excellent first build. Final dimensions are still influenced by conductor diameter, nearby dielectric materials, feed method, and installation environment.
Can I use this for other bands?
Yes. Enter any frequency and re-calculate. The geometry scales with wavelength, so the same method works across many VHF/UHF use cases.
Why include a short-arm factor?
Many practical QFH variants intentionally use two slightly shorter arms to improve circular polarization behavior and pattern shape. The factor lets you quickly test that geometry.