HF Helical Antenna Calculator

Calculator Inputs

Frequency

MHz

Operating mode

Coil diameter

Diameter unit

Number of turns

Pitch per turn

Pitch unit

Wire diameter

Wire unit

Coil loss resistance

Ground or mount loss

Extra matching loss

Feedline loss

Transmitter power

Builder notes

Example Data Table

Case	Frequency	Mode	Diameter	Turns	Pitch	Typical Use
Portable 40 m loading helix	7.1 MHz	Normal	10 cm	28	2.5 cm	Compact vertical experiment
Compact 20 m test coil	14.2 MHz	Normal	7.5 cm	18	2 cm	Field tuning practice
Large axial demonstration	28.5 MHz	Axial	3.2 m	5	2.4 m	Directional helix study

Formula Used

Wavelength: λ = c / f.

Coil circumference: C = πD.

Wire length per turn: Lt = √(C² + S²).

Total wire length: L = N × Lt.

Axial length: H = N × S.

Pitch angle: α = tan⁻¹(S / C).

Normal-mode radiation resistance: Rr ≈ 40π²(H / λ)², capped near 36.5 Ω.

Axial-mode input resistance: Rin ≈ 140(C / λ).

Axial-mode gain: G ≈ 15N(C / λ)²(S / λ).

Efficiency: η = Rr / (Rr + Rloss). System efficiency also includes feedline loss.

How to Use This Calculator

Enter the target HF frequency first. Choose normal mode for compact helically wound verticals. Choose axial mode only for large directional helix designs. Enter coil diameter, turns, pitch, and wire diameter using the available unit selectors.

Add realistic coil, ground, matching, and feedline losses. These values strongly affect compact HF antennas. Press Calculate. Review wire length, wavelength ratios, estimated resistance, efficiency, and gain. Export the result when you want shop notes.

Planning an HF Helical Antenna

An HF helical antenna is useful when space is limited. It wraps conductor length into a compact coil. The shape can serve two different jobs. In normal mode, the helix behaves like a shortened vertical radiator. In axial mode, it behaves like a directional helix. HF builders usually use normal mode for compact stations.

Why the Dimensions Matter

Frequency sets the wavelength. Wavelength controls every useful antenna dimension. Diameter sets the coil circumference. Pitch sets the spacing between turns. Turns decide total wire length and overall height. These values affect resonance, current, efficiency, and matching. A small change can move the antenna away from the intended band.

Efficiency and Losses

Short HF helices can have low radiation resistance. That makes losses important. Ground loss, coil loss, and matching loss can waste much power as heat. The calculator compares estimated radiation resistance with loss resistance. It then estimates efficiency and radiated power. This helps you decide whether a larger diameter, better wire, or improved ground system is worth the effort.

Normal and Axial Modes

Normal mode is best for compact HF verticals and portable antennas. It uses a circumference much smaller than one wavelength. Axial mode needs a circumference near one wavelength. It also needs suitable turn spacing. That makes axial mode uncommon on lower HF bands, because the coil becomes large. Still, the mode check is useful for experiments.

Using the Results

Start with the target frequency. Enter realistic diameter, pitch, and turns. Review wire length, axial height, pitch angle, and wavelength ratios. Compare the calculated wire length with quarter wave length. Then check efficiency and realized gain. If losses dominate, improve materials or ground before expecting better performance.

Building Notes

Use strong supports for large coils. Keep spacing steady across every turn. Measure the finished antenna with an analyzer. Trim slowly when tuning. Nearby metal, soil, and feedline routing can change results. Treat the numbers as a design estimate, not a final guarantee. Record every test. Good notes make later band changes much easier.

Safety and Power

High RF voltage can appear on compact coils. Keep hands away during tests. Use low power first. Add weather protection after stable tuning is confirmed and logged.

FAQs

What is an HF helical antenna?

It is an antenna that winds conductor into a helix. On HF, it is often used to shorten a vertical radiator when full length is not practical.

Which mode should I choose?

Choose normal mode for compact HF loading coils and shortened verticals. Choose axial mode only when the circumference and spacing approach axial-mode design rules.

Does wire length alone set resonance?

No. Wire length helps, but coil diameter, pitch, end effects, ground, nearby objects, and feed arrangement also shift resonance.

Why is efficiency sometimes low?

Short HF antennas can have very low radiation resistance. Coil, ground, and matching losses can then consume much of the applied power.

Can this replace an antenna analyzer?

No. Use it for planning and comparison. Final resonance, impedance, and matching should be checked with real measurements.

What does pitch mean?

Pitch is the distance from one turn to the next along the coil axis. Larger pitch increases axial length and changes the pitch angle.

Why enter ground loss?

Ground loss is critical for small HF vertical antennas. Poor radials, lossy soil, and weak mounts can reduce radiated power significantly.

Is axial mode practical on HF?

Usually it becomes physically large. It is more common at higher frequencies, but the calculator includes it for advanced experiments.