Helical Antenna (Satellite/EME)
Wind a conductor into a helix with a circumference close to one wavelength and the right pitch angle, and it stops behaving like a small loaded loop and starts radiating in "axial mode" — a tight beam straight off the open end, circularly polarized, with gain that climbs as you add more turns. John Kraus documented this behavior in the 1940s, and it's remained a go-to design wherever hams need real circularly-polarized gain in a mechanically simple, single-conductor structure: satellite ground stations and EME (moonbounce) work especially.
Axial mode vs. normal mode
A small helix, with circumference much smaller than a wavelength, radiates broadside like a short loaded dipole — that's "normal mode," the same principle behind a loaded mobile whip's coil. Once the helix's circumference grows to roughly one wavelength, with a pitch angle around 12-14 degrees, the antenna switches to "axial mode": current travels around and along the helix in a way that reinforces radiation straight off the open end, circularly polarized, with real gain that increases as you add turns.
Gain increases with number of turns n and turn spacing S
The ground plane reflector's job
A flat (or shallow cupped) conducting reflector behind the feed end, roughly 0.8-1.1 wavelengths in diameter, directs the helix's radiation forward and establishes a reasonably clean, real feedpoint impedance (commonly cited around 140 ohms for the classic axial-mode helix) — without it, the antenna's pattern and match both suffer.
Why gain scales with turn count — and where that stops helping
The commonly used Kraus approximation for axial-mode gain (in dBi) scales with the circumference squared, the number of turns, and the turn spacing, all in wavelengths — which is why EME stations chasing every possible dB often run very long helices (20-30+ turns, sometimes several helices combined), while satellite ground stations typically use a shorter helix (6-13 turns) that trades some peak gain for a wider, more forgiving beamwidth that's easier to point by hand or with a simple mount during a pass.
- Satellite work: fewer turns, wider beamwidth, easier to track a moving pass.
- EME work: many more turns (often stacked in arrays), maximum gain to overcome the enormous path loss to the moon and back.
Honest accuracy note on the gain figure
The Kraus formula used in the calculator below is a well-known, commonly cited approximation for typical axial-mode proportions (C near 1 wavelength, pitch angle in the usual range) — treat its output as a solid design estimate, not a guaranteed measured number. Real-world gain depends on build precision, reflector size, and matching quality; NEC modeling or on-air comparison against a reference antenna is the way to confirm actual performance.
Installation options
- Fixed mast with az/el rotator: the standard setup for satellite tracking across a full pass.
- Fixed elevation, azimuth rotator only: workable for higher passes where a full elevation range isn't needed, simplifying the mount.
- Dedicated EME array mount: long, high-gain helices (or stacked pairs) for moonbounce typically need a sturdy, precisely trackable az/el mount given the antenna's length and narrow beamwidth.
| Parameter | 435 MHz, 10 turns | Notes |
|---|---|---|
| Helix circumference (C) | ~27.2 in (0.69 m) | ~1 wavelength — the axial-mode condition |
| Helix diameter | ~8.7 in (22.0 cm) | Circumference / pi |
| Turn spacing (S) | ~6.3 in (16.0 cm) | ~0.23 wavelength; sets a ~12-14° pitch angle |
| Axial length (10 turns) | ~62.5 in (1.59 m) | Turn spacing x number of turns |
| Ground plane reflector diameter | ~24.4 in (62 cm) | ~0.9 wavelength; commonly cited range 0.8-1.1 wavelength |
| Approximate feedpoint impedance | ~140 ohms | Commonly cited for the classic axial-mode helix; match to 50 ohms as needed |
Helical Antenna Dimension & Gain Calculator
Materials for Helical Antenna
Building the Helical Antenna
The reflector and feedpoint come first, since the helix winds off of them — getting the pitch consistent along the whole winding is the part that takes the most care.
Build the ground plane reflector
Build or mount a flat (or shallow-cupped) conducting reflector to the calculated diameter, with a feedpoint bracket at its center.
Build the non-conductive support structure
Assemble fiberglass rods or a light frame running the full axial length, spaced to hold the helix winding at the correct diameter along its length.
Wind the helix at a consistent pitch
Wind the conductor around the support structure at the calculated turn spacing, keeping the pitch angle consistent for every turn from the reflector to the open end.
Connect the feed end to the reflector's feedpoint
Connect the helix's first turn to the feedpoint bracket at the reflector, installing the matching section for the antenna's naturally high feed impedance.
Mount the assembly to your rotator or fixed mast
Attach the reflector and helix assembly to an az/el rotator for satellite tracking, or a fixed mast if your build targets a narrower, higher-elevation use case.
Connect coax and sweep SWR
Connect your feedline and sweep the feedpoint for a clean resonant match near your design frequency.
Verify gain and polarization sense on the air
Check performance against a known reference station or satellite pass, and confirm the circular polarization sense matches your intended target before relying on the antenna for critical contacts.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Gain seems lower than the calculated estimate | Uneven turn spacing, undersized reflector, or a poor feedpoint match | Check turn-to-turn spacing consistency and reflector diameter against the design | Correct spacing and reflector size; re-check the matching section |
| SWR is hard to bring down at the feedpoint | Matching section not suited to the helix's actual feed impedance | Measure feedpoint impedance directly with an analyzer | Adjust or rebuild the matching section for the measured impedance |
| Pattern seems broader or weaker than expected | Circumference or pitch angle has drifted from the axial-mode condition | Re-measure circumference and pitch angle against the design values | Correct winding geometry to restore the axial-mode proportions |
How many turns do I actually need?
For satellite work, 6-13 turns is a common practical range, trading some gain for an easier-to-point beamwidth. For serious EME work, many more turns (often 20-30+, sometimes in stacked arrays) are typical, since every dB matters against moonbounce path loss.
Is the gain formula in the calculator exact?
It's the well-known Kraus approximation for typical axial-mode proportions — a solid design estimate, not a guaranteed measured figure. Confirm real performance with NEC modeling or an on-air comparison.
Do I need the ground plane reflector?
Yes — it's not optional. Without it, both the forward pattern and the feedpoint impedance depart significantly from the design this antenna depends on.
What's the difference between normal mode and axial mode?
A small helix (circumference much less than a wavelength) radiates broadside like a loaded dipole — normal mode, the same principle as a loaded mobile whip's coil. This guide's axial-mode helix, with circumference near one wavelength, radiates a circularly polarized beam off the open end instead.
Can I build this for other bands?
Yes, the calculator scales all dimensions to your chosen design frequency — 70cm and 23cm are the most common amateur bands for this design.
Who documented this design?
John Kraus documented axial-mode helical antenna behavior in the 1940s, and the design proportions used here trace directly back to that original work.