How does a bifilar helix differ from a monofilar helix?

A monofilar (single-conductor) helix antenna, as described by Kraus, operates in either normal mode (helix circumference << lambda, producing a dipole-like omnidirectional pattern) or axial mode (helix circumference approximately lambda, producing an endfire beam with circular polarization and directivity approximately 10-15 dBi for 5-15 turns). The monofilar helix requires a ground plane at the feed end and produces radiation in the forward direction (away from the ground plane). A bifilar helix uses two conductors, enabling balanced feeding without a ground plane and producing different radiation patterns depending on the winding configuration. Co-wound bifilar (both same-sense, 180-degree feed phase): produces a radiation pattern similar to a folded dipole, with improved impedance matching (higher radiation resistance) and wider bandwidth than a single helix in normal mode. The radiation is broadside (perpendicular to the axis) with linear polarization. Used in VHF/UHF communication antennas. Contra-wound bifilar (opposite-sense winding): produces a backfire pattern (radiation toward the feed end, opposite to monofilar axial mode) with inherent circular polarization determined by the dominant helix winding sense. The backfire mode occurs when the helix circumference is approximately 0.5-0.8 lambda, and bandwidth is approximately 1.5:1.

What is the quadrifilar helix antenna (QHA) and why is it used for GPS?

The quadrifilar helix antenna (QHA) consists of four helical conductors arranged in two orthogonal bifilar pairs on a common cylinder. Each pair is a half-turn or full-turn bifilar helix, and the two pairs are fed with 90-degree phase difference (phase quadrature). This creates right-hand circular polarization (RHCP) over a hemispherical coverage region. Key QHA parameters for GPS (L1 = 1575.42 MHz): helix diameter approximately 30 mm, helix length approximately 70-100 mm (0.25-0.5 lambda turns), conductor width 2-5 mm (etched on flexible PCB wrapped into a cylinder). The QHA produces a cardioid-shaped pattern: maximum gain approximately 3-5 dBic at zenith, gradually decreasing to approximately 0 dBic at 10-degree elevation, and a sharp null below the horizon (back radiation < -15 dBic). This pattern shape is why GPS receivers use QHAs: (1) Hemispherical coverage: receives signals from all visible satellites simultaneously (typically 6-12 satellites above 10-degree elevation), which is essential for position fix. A high-gain directional antenna would need to be pointed at each satellite. (2) Low-elevation gain: the cardioid shape maintains usable gain down to 10-degree elevation, where many satellites are visible. (3) Horizon rejection: the sharp null below the horizon rejects multipath reflections from the ground, which are the dominant error source in GPS positioning (multipath errors of 1-10 meters).

What are the design parameters and trade-offs for bifilar helix antennas?

Bifilar helix design involves several interrelated parameters. Pitch angle (alpha): the angle between the helix conductor and the plane perpendicular to the axis. alpha = arctan(S/(pi*D)), where S is the turn spacing and D is the diameter. For backfire mode: alpha = 12-15 degrees typically. For normal mode QHA: alpha = 20-30 degrees. Higher pitch angles shift the pattern from broadside toward endfire. Circumference (C = pi*D/lambda): determines the operating mode. C < 0.5: normal mode, broadside radiation. C = 0.5-0.8: backfire mode (contra-wound bifilar), backward radiation. C = 0.8-1.2: axial mode (monofilar only), forward endfire radiation. The bifilar backfire mode operates at smaller circumference than monofilar axial mode, enabling more compact designs. Number of turns (N): more turns increase directivity and narrow the beamwidth. For QHA GPS: 0.25-1 turns (wide hemispherical beam). For bifilar backfire communication: 2-5 turns (moderate directivity, 8-12 dBic). For monofilar axial mode: 5-15 turns (high directivity, 10-15 dBi). Conductor width (w): affects input impedance and bandwidth. Wider conductors (larger w/D ratio) lower the impedance and increase bandwidth. Printed QHAs use conductor widths of 2-5 mm on flexible substrates for reproducibility and low cost. Feed network: critical for pattern symmetry and CP purity. QHA requires 0/90/180/270-degree phase feeding.

Antenna / Helical

Bifilar Helix

Q: What is the quadrifilar helix antenna (QHA) and why is it used for GPS?

The quadrifilar helix antenna (QHA) consists of four helical conductors arranged in two orthogonal bifilar pairs on a common cylinder. Each pair is a half-turn or full-turn bifilar helix, and the two pairs are fed with 90-degree phase difference (phase quadrature). This creates right-hand circular polarization (RHCP) over a hemispherical coverage region. Key QHA parameters for GPS (L1 = 1575.42 MHz): helix diameter approximately 30 mm, helix length approximately 70-100 mm (0.25-0.5 lambda turns), conductor width 2-5 mm (etched on flexible PCB wrapped into a cylinder). The QHA produces a cardioid-shaped pattern: maximum gain approximately 3-5 dBic at zenith, gradually decreasing to approximately 0 dBic at 10-degree elevation, and a sharp null below the horizon (back radiation < -15 dBic). This pattern shape is why GPS receivers use QHAs: (1) Hemispherical coverage: receives signals from all visible satellites simultaneously (typically 6-12 satellites above 10-degree elevation), which is essential for position fix. A high-gain directional antenna would need to be pointed at each satellite. (2) Low-elevation gain: the cardioid shape maintains usable gain down to 10-degree elevation, where many satellites are visible. (3) Horizon rejection: the sharp null below the horizon rejects multipath reflections from the ground, which are the dominant error source in GPS positioning (multipath errors of 1-10 meters).

Q: What are the design parameters and trade-offs for bifilar helix antennas?

Bifilar helix design involves several interrelated parameters. Pitch angle (alpha): the angle between the helix conductor and the plane perpendicular to the axis. alpha = arctan(S/(pi*D)), where S is the turn spacing and D is the diameter. For backfire mode: alpha = 12-15 degrees typically. For normal mode QHA: alpha = 20-30 degrees. Higher pitch angles shift the pattern from broadside toward endfire. Circumference (C = pi*D/lambda): determines the operating mode. C < 0.5: normal mode, broadside radiation. C = 0.5-0.8: backfire mode (contra-wound bifilar), backward radiation. C = 0.8-1.2: axial mode (monofilar only), forward endfire radiation. The bifilar backfire mode operates at smaller circumference than monofilar axial mode, enabling more compact designs. Number of turns (N): more turns increase directivity and narrow the beamwidth. For QHA GPS: 0.25-1 turns (wide hemispherical beam). For bifilar backfire communication: 2-5 turns (moderate directivity, 8-12 dBic). For monofilar axial mode: 5-15 turns (high directivity, 10-15 dBi). Conductor width (w): affects input impedance and bandwidth. Wider conductors (larger w/D ratio) lower the impedance and increase bandwidth. Printed QHAs use conductor widths of 2-5 mm on flexible substrates for reproducibility and low cost. Feed network: critical for pattern symmetry and CP purity. QHA requires 0/90/180/270-degree phase feeding.

/BY-fil-ar HEE-liks/

Two-conductor helical antenna producing shaped radiation patterns with inherent circular polarization. Contra-wound configuration yields backfire beam (C = 0.5–0.8λ) without ground plane. Quadrifilar derivative (QHA) produces hemispherical cardioid coverage with <3 dB axial ratio over 120° beamwidth. Standard antenna for GPS/GNSS (L1 = 1575.42 MHz), NOAA APT, and satellite telemetry.

Config: Co-wound / Contra-wound

CP purity: <3 dB AR

QHA gain: 3–5 dBic

Understanding Bifilar Helix Antennas

The bifilar helix antenna extends the monofilar (Kraus) helix by adding a second conductor, enabling balanced feeding, ground-plane-free operation, and unique radiation patterns not achievable with a single helix. The contra-wound variant (one right-hand, one left-hand helix) produces a backfire pattern with inherent circular polarization, while the quadrifilar extension (two orthogonal bifilar pairs) creates the hemispherical cardioid pattern essential for satellite navigation receivers.

The QHA's combination of hemispherical coverage, horizon rejection (<−15 dBic below horizon for multipath suppression), and compact size (30 mm diameter at GPS L1) makes it the dominant antenna for GNSS applications. The Kilgus self-phasing design simplifies feeding by using resonant arms of different lengths to create the required 90° phase progression.

Key Design Parameters

Pitch Angle:
α = arctan(S / (πD))
S = turn spacing, D = diameter
Backfire mode: α = 12–15°
QHA normal mode: α = 20–30°

Normalized Circumference:
C = πD / λ
C < 0.5: normal mode (broadside)
C = 0.5–0.8: backfire mode
C = 0.8–1.2: axial mode (monofilar)

QHA Phase Feeding:
Arms: 0°, 90°, 180°, 270°
→ RHCP hemispherical coverage

Helix Configuration Comparison

Type	Conductors	Pattern	CP	Ground Plane	Application
Monofilar axial	1	Endfire	Inherent	Required	Telemetry, point-to-point
Co-wound bifilar	2 (same sense)	Broadside	Linear	Optional	VHF/UHF comms
Contra-wound bifilar	2 (opposite)	Backfire	Inherent	Not needed	Satellite handheld
Quadrifilar (QHA)	4 (2 pairs)	Hemispherical	Excellent	Not needed	GPS, GNSS, NOAA

QHA GPS Advantages

Feature	QHA Benefit	Alternative Limitation
Hemispherical coverage	All visible satellites (6–12)	Patch: narrower beamwidth
Horizon rejection	<−15 dBic below horizon	Monopole: no multipath rejection
CP purity	AR < 3 dB over 120°	Patch: AR degrades off-axis
Compact	30 mm dia at L1	Patch + GP: larger footprint

Common Questions

Frequently Asked Questions

Bifilar vs. monofilar helix?

Monofilar: single conductor, endfire axial mode (C ≈ λ), requires ground plane, 10–15 dBi. Contra-wound bifilar: backfire mode (C = 0.5–0.8λ), no ground plane, compact. QHA (quadrifilar): hemispherical coverage, <3 dB AR over 120°, dominant for GPS/GNSS.

Why QHA for GPS?

Hemispherical cardioid: all satellites visible simultaneously. Below-horizon null (<−15 dBic) rejects ground multipath (1–10 m error reduction). RHCP matches GPS signal. Compact: 30 mm diameter at L1 (1575 MHz). Kilgus self-phasing simplifies feed network.

Design trade-offs?

Pitch angle: 12–15° backfire, 20–30° QHA. More turns = higher directivity but narrower beam. Bandwidth: 5–15% resonant, 30–50% traveling-wave. GPS L1 needs only 0.3%, so resonant Kilgus sufficient. Meander loading: 30–50% size reduction but 2–3% BW.

Related Terms

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