qfh antenna calculator

What is a QFH antenna?

A QFH (Quadrifilar Helix) antenna is a circularly polarized antenna commonly used for receiving low-earth-orbit weather satellites. It is especially popular for NOAA APT and Meteor reception because it has a broad sky view and handles right-hand circular polarization well.

How this qfh antenna calculator works

This calculator starts with wavelength and scales the main geometry from practical build ratios. It gives you dimensions that are suitable as a first build, then you can trim and tune for your specific materials and environment.

Core formulas

• Free-space wavelength: λ = c / f
• Adjusted wavelength: λ_adj = λ × wire velocity factor
• Long loop: λ_adj × long multiplier
• Short loop: λ_adj × short multiplier
• Height: λ_adj × height factor
• Diameter: λ_adj × diameter factor
• Phasing coax (quarter-wave): (λ × coax VF) / 4

How to use these results in a real build

1) Pick your target frequency

Choose the center of the band you care about. For NOAA weather satellites, many builders start around 137.1 to 137.9 MHz.

2) Cut slightly long

Always cut conductors a little longer than calculated values. You can trim down during tuning, but adding length later is harder.

3) Keep symmetry

QFH performance depends on balanced geometry. Small asymmetries can increase pattern distortion and reduce overhead performance.

4) Tune and verify

If possible, check return loss/SWR with a VNA. Then test on live satellite passes and compare signal quality over multiple elevations.

Practical build tips

• Use a non-conductive former (PVC or fiberglass) for mechanical stability.
• Label each wire path before final assembly to avoid crossing errors.
• Weatherproof feed-point and coax connections carefully.
• Mount in open sky, away from roofs, gutters, and nearby metal structures.
• Use a common-mode choke on the feedline to reduce pattern distortion.

Important note about calculators

No calculator can guarantee perfect final dimensions because insulation type, conductor diameter, feed arrangement, and nearby objects all shift resonance. Treat this tool as a strong starting point, not a final tune. A quick measurement-and-trim cycle usually produces the best result.