Advanced QFH design inputs
Example data table
| Use case | Frequency | Circumference factor | Long factor | Short factor | Typical note |
|---|---|---|---|---|---|
| Weather satellite receive | 137.5 MHz | 0.15 | 0.50 | 0.48 | Good starting point for broad sky coverage. |
| VHF test build | 145 MHz | 0.14 | 0.50 | 0.47 | Keep conductors symmetrical and supported. |
| UHF prototype | 435 MHz | 0.16 | 0.49 | 0.46 | Small errors become more important at UHF. |
Formula used
The effective wavelength is λ = c / f × VF. Here, c is light speed, f is frequency in hertz, and VF is the element velocity factor.
The circular support circumference is C = πD. In automatic mode, C = λ × circumference factor. The radius is D / 2.
For target length mode, each helical path uses L = λ × length factor. The pitch per turn is P = √((L / N)² − C²). The pitch angle is atan(P / C).
For fixed pitch mode, the pitch per turn is P = C × tan(angle). The conductor length is L = N × √(C² + P²).
The 90 degree phasing line estimate is line = (c / f × coax VF) / 4. Skin depth uses the copper high frequency approximation.
How to use this calculator
Enter the operating frequency first. Pick air velocity factor unless the element is insulated or loaded. Choose automatic diameter for a fast start. Use fixed diameter when you already have a former, pipe, or printed frame.
Select target conductor length when you want the tool to solve pitch and height. Select fixed pitch when your mechanical design already sets the helix slope. Compare long and short paths, then review the 90 degree phasing line estimate.
Cut conductors slightly long. Build the antenna with equal spacing. Keep feed wiring short. Measure resonance and pattern quality. Trim slowly, because small changes can shift circular polarization and impedance.
Quadrifilar helix antenna design guide
What this calculator estimates
A quadrifilar helix antenna, often called a QFH antenna, uses four helical conductors. The conductors form two crossed loops. These loops are phased close to ninety degrees. That phasing helps the antenna produce circular polarization. Circular polarization is useful for satellites. It reduces signal fades caused by rotation.
Why wavelength matters
Every major dimension begins with wavelength. Frequency sets wavelength. The velocity factor then shortens the electrical path. This matters when wire insulation, nearby plastic, or support material changes the field. The calculator lets you adjust that factor. It also shows quarter wave and half wave references.
Geometry and pitch
The diameter controls circumference. Circumference controls how much wire is used in each turn. Pitch controls vertical height. More pitch makes a taller helix. Less pitch makes a compact shape. A balanced QFH needs clean symmetry. Both pairs should keep stable spacing from top to bottom.
Long and short helices
Many practical QFH builds use one longer pair and one shorter pair. This creates the phase relationship needed for circular polarization. The exact values depend on construction details. Feed wiring also matters. The calculator gives length factors, pitch, and cut ranges. These are starting values, not final guarantees.
Feeding and matching
The feed point should stay neat and repeatable. A quarter wave coax section can help create a ninety degree delay. Its physical length depends on coax velocity factor. The tool estimates that length. It also gives a rough matching impedance. Use an analyzer for final trimming.
Build and test process
Start with accurate marks. Use a strong support tube. Keep opposite conductors equal. Avoid sharp bends near the feed. After assembly, test return loss. Then test real signal strength. Rotate the antenna and watch fades. Good circular polarization should give smoother reception over a wide sky area.
Record every change in a simple log. Note wire length, pitch, feed method, and measured return loss. Outdoor tests are also important. Satellite signals arrive from many angles. A useful build should give steady audio or data through most of the pass. Small mechanical errors can reduce that smooth coverage. Measure twice before trimming final conductor ends carefully.
FAQs
What is a quadrifilar helix antenna?
It is a four conductor helical antenna. It is designed to produce circular polarization. It is often used for satellite reception because the signal angle changes during a pass.
Can this replace antenna simulation?
No. It gives strong starting dimensions. Simulation and measurement are still needed. Nearby materials, feed routing, and construction tolerances can change the final resonance and pattern.
What frequency should I enter?
Enter the center frequency of the band or channel. For satellite reception, use the main downlink frequency. A small frequency change can shift dimensions.
What does circumference factor mean?
It sets the support circumference as a fraction of wavelength. A value near 0.15 is a practical starting point for many compact QFH prototypes.
Why are long and short helices different?
The two conductor pairs need a phase relationship. Different path lengths help create quadrature behavior. This supports circular polarization when the feed is built correctly.
What is the 90 degree phase line?
It is a quarter wavelength transmission line section. It delays one feed path by about ninety degrees. Its length depends on frequency and coax velocity factor.
Why does conductor diameter matter?
Thicker conductors can improve bandwidth and reduce loss. They also change practical spacing and bending. Very thin wire is harder to keep stable.
How should I trim the antenna?
Cut conductors slightly long first. Measure with an analyzer. Trim in small steps. Keep the four conductors symmetrical after every adjustment.