What this QFH calculator does
This calculator gives a practical first-pass geometry for a Quadrifilar Helix (QFH) antenna. QFH antennas are popular for satellite reception because they can provide circular polarization and broad sky coverage. If you are building for weather satellites (like NOAA APT around 137 MHz), this tool helps you move quickly from frequency to build dimensions.
How the calculations work
The calculator starts with free-space wavelength:
- Wavelength (λ) = c / f
- Effective wavelength = λ × wire velocity factor
It then applies build factors to estimate:
- Short loop total conductor length
- Long loop total conductor length
- Approximate antenna cylinder diameter (from short loop circumference)
- Overall antenna height
- Quarter-wave coax phasing harness length
These values are intentionally practical, not absolute. Final tuning is usually done with measurements, simulation, or on-air performance testing.
Recommended starting values
For 137 MHz weather satellite builds
- Wire velocity factor: 0.95
- Short loop factor: 1.00 × λ
- Long loop factor: 1.06 × λ
- Height factor: 0.25 × λ
- Coax velocity factor: 0.66 (common solid PE coax)
After cutting and assembling, trim in small increments if you are tuning with a VNA. Mechanical symmetry often matters as much as exact wire length.
Build tips that improve real-world results
- Keep both loops physically smooth and symmetrical.
- Use a non-conductive former (PVC, fiberglass, or 3D-printed supports).
- Avoid tight bends at feed points; strain relief helps long-term stability.
- Mount the antenna away from large metal objects.
- Use quality coax and weatherproof outdoor connectors.
- Document your dimensions so adjustments are repeatable.
Example quick check
At 137.1 MHz using the defaults above, you should see dimensions close to: short loop around 2.08 m, long loop around 2.20 m, height around 0.52 m, and a quarter-wave coax section around 0.36 m. Your exact result depends on the factors you set.
Final note
Think of this as a design launcher: it gets you close fast. QFH performance depends on geometry, feed arrangement, mounting environment, and receiver chain quality. Start here, then refine with testing.