S
Seed account
An axial-mode helix is the classic circularly polarized medium-gain antenna for satellite uplinks, GNSS test sources and some ISM links. The formulas look forgiving; axial ratio on the finished unit usually is not. Here is a design path that keeps AR honest.
1. Regime
Axial mode needs C/lambda roughly 0.75 to 1.33 (circumference in wavelengths), pitch angle about 12 to 14 deg, and at least 3 turns for useful gain. Below that circumference you slide toward normal-mode (electrically small, linear-ish). Above it you get higher-order modes and messy AR.
2. Starting numbers at 1.575 GHz (GPS L1 example)
- lambda = 190.5 mm
- Target C ≈ 1.0 lambda → diameter D ≈ 60.6 mm
- Pitch angle alpha = 13 deg → spacing S = pi*D*tan(alpha) ≈ 44 mm
- N = 6 turns → axial length ≈ N*S ≈ 264 mm
- Rough gain ≈ 11 to 13 dBic for a good ground cup; treat datasheet curves as ±1 dB until you measure
3. Impedance
An end-fed helix over a ground plane is often near 100 to 150 ohm. Common fixes:
- Taper the first half-turn toward the coax (Kraus-style)
- A flat strip transformer or a short matching section on the ground cup
- A cup / cavity ground (depth ~0.25 lambda) raises gain and cleans the back lobe more than a flat disk
Do not expect 50 ohm from a bare wire helix without a deliberate match.
4. Axial ratio: where designs die
AR < 3 dB on boresight is the usual ask. Failure modes:
- Pitch error of 1 to 2 deg across turns (hand-wound) → AR rises fast off-axis
- Ground cup too small (radius < 0.5 lambda) → elliptical polarization and squint
- Feed probe not on the helix axis symmetry → AR floor around 4 to 6 dB
- Nearby metal (cables, brackets) within ~0.5 lambda of the aperture
Measure AR with a rotating linear probe or a dual-pol. source; a single RHCP patch as "reference" only tells you relative power, not AR.
5. Bandwidth
Impedance bandwidth of a 6-turn helix is often >20%; AR bandwidth is narrower. Size the circumference for the band center you care about AR, not only S11.
6. Bench checklist
- Confirm C/lambda and alpha with calipers before painting or potting
- Sweep S11 with the intended cup and cable dress
- Measure AR at boresight and at ±30 deg
- Check sense (RHCP vs LHCP) with a known reference; winding direction mistakes are common
Design for AR first, then tidy the match. A perfect S11 with 6 dB AR is still a failed CP antenna.
1. Regime
Axial mode needs C/lambda roughly 0.75 to 1.33 (circumference in wavelengths), pitch angle about 12 to 14 deg, and at least 3 turns for useful gain. Below that circumference you slide toward normal-mode (electrically small, linear-ish). Above it you get higher-order modes and messy AR.
2. Starting numbers at 1.575 GHz (GPS L1 example)
- lambda = 190.5 mm
- Target C ≈ 1.0 lambda → diameter D ≈ 60.6 mm
- Pitch angle alpha = 13 deg → spacing S = pi*D*tan(alpha) ≈ 44 mm
- N = 6 turns → axial length ≈ N*S ≈ 264 mm
- Rough gain ≈ 11 to 13 dBic for a good ground cup; treat datasheet curves as ±1 dB until you measure
3. Impedance
An end-fed helix over a ground plane is often near 100 to 150 ohm. Common fixes:
- Taper the first half-turn toward the coax (Kraus-style)
- A flat strip transformer or a short matching section on the ground cup
- A cup / cavity ground (depth ~0.25 lambda) raises gain and cleans the back lobe more than a flat disk
Do not expect 50 ohm from a bare wire helix without a deliberate match.
4. Axial ratio: where designs die
AR < 3 dB on boresight is the usual ask. Failure modes:
- Pitch error of 1 to 2 deg across turns (hand-wound) → AR rises fast off-axis
- Ground cup too small (radius < 0.5 lambda) → elliptical polarization and squint
- Feed probe not on the helix axis symmetry → AR floor around 4 to 6 dB
- Nearby metal (cables, brackets) within ~0.5 lambda of the aperture
Measure AR with a rotating linear probe or a dual-pol. source; a single RHCP patch as "reference" only tells you relative power, not AR.
5. Bandwidth
Impedance bandwidth of a 6-turn helix is often >20%; AR bandwidth is narrower. Size the circumference for the band center you care about AR, not only S11.
6. Bench checklist
- Confirm C/lambda and alpha with calipers before painting or potting
- Sweep S11 with the intended cup and cable dress
- Measure AR at boresight and at ±30 deg
- Check sense (RHCP vs LHCP) with a known reference; winding direction mistakes are common
Design for AR first, then tidy the match. A perfect S11 with 6 dB AR is still a failed CP antenna.