Dual-Sense Circularly-Polarized PCB Antenna for GPS L1

Project Overview

This project involves the design and development of a compact, dual-sense circularly-polarized PCB antenna for GPS L1 band reception at 1575.42 MHz. The antenna employs a unique two-layer slotted circular patch design inspired by the dual-sense circular polarization concept from Xiulong Bao and M. J. Ammann's work in IEEE Transactions on Antennas and Propagation (2008).

Design Architecture: The antenna features a circular patch element on the top layer with two orthogonal slot lines cut into the patch. On the bottom layer, an additional slot line creates the feed structure. This orthogonal slot configuration excites two degenerate resonant modes with 90° phase difference, generating circular polarization without requiring corner truncation.

Key Innovation: Unlike conventional GPS antennas that support only Right-Hand Circular Polarization (RHCP), this design offers switchable polarization capability - supporting both RHCP and Left-Hand Circular Polarization (LHCP) depending on the feed configuration. This versatility makes it suitable for GPS reception as well as experimental GNSS applications and multipath analysis.

The project has completed electromagnetic simulation and optimization in Ansys HFSS, achieving excellent axial ratio and gain performance. PCB fabrication files have been submitted to JLCPCB and manufacturing is underway. Awaiting delivery for assembly and testing phase.

Design Foundation & My Implementation

Inspiration from Bao & Ammann (2008)

The conceptual foundation for dual-sense circular polarization is inspired by: "Dual-Frequency Dual-Sense Circularly-Polarized Slot Antenna Fed by Microstrip Line" by Xiulong Bao and M. J. Ammann (IEEE Transactions on Antennas and Propagation, Vol. 56, No. 3, March 2008, pp. 645-649).

My Original Design Implementation

While inspired by the dual-sense concept, my implementation is a completely different physical design specifically engineered for GPS L1 band operation:

Unique Design Architecture:

• Two-Layer Circular Patch Design: Unlike the paper's square slot antenna, I designed a circular patch radiator for omnidirectional radiation pattern ideal for GPS satellite tracking
• Orthogonal Slot Configuration: The circular patch features two orthogonal slot lines cut directly into the patch surface. These perpendicular slots perturb the current distribution, exciting two degenerate modes with 90° phase difference required for circular polarization
• Bottom Layer Slot Feed: On the bottom layer (Layer 2), an additional slot line serves as the electromagnetic feed structure. This slot couples energy from the microstrip feed line to the radiating patch above
• No Corner Truncation: Unlike traditional circularly-polarized patch antennas that use corner cuts, this design achieves CP through the orthogonal slot mechanism, maintaining circular symmetry
• Single-Frequency Optimization: Focused exclusively on GPS L1 (1575.42 MHz) for maximum performance at the target frequency, unlike the paper's dual-frequency design
• FR-4 PCB Implementation: Designed for cost-effective standard FR-4 substrate (εr ≈ 4.4, h = 1.6 mm) available at JLCPCB, with all dimensions optimized for this specific material
• Dual-Sense Capability: By controlling which orthogonal slot mode is excited through feed positioning or dual-feed phasing, the antenna can generate either RHCP or LHCP
• 50Ω SMA Integration: Custom feed network designed for direct SMA connector mounting with excellent impedance matching (S11 < -10 dB) across GPS L1 bandwidth

How the Orthogonal Slots Generate Circular Polarization:

Academic Credit: This design acknowledges the dual-sense circular polarization concept introduced by Bao & Ammann, while implementing a completely different antenna topology (circular patch with orthogonal slots vs. square slot with corner truncation) optimized for GPS L1 reception.

Design Specifications

Design Methodology

1. Two-Layer Slotted Circular Patch Architecture

The antenna employs a unique two-layer design with orthogonal slot configuration:

• Top Layer (Patch Side): Circular patch radiator with two orthogonal slot lines cut into the copper. These perpendicular slots (e.g., aligned along 0° and 90° axes) perturb the patch's current distribution, exciting two degenerate resonant modes with 90° spatial phase difference
• Bottom Layer (Feed Side): Contains an additional slot line that serves as the electromagnetic feed structure. This slot couples energy from the 50Ω microstrip feed line to the radiating patch above
• Circular Polarization Generation: The orthogonal slots on the patch create two resonant modes (aligned with each slot) with equal amplitude and 90° phase difference. Their vector superposition produces the rotating E-field characteristic of circular polarization - no corner truncation required
• Low Profile: Two-layer PCB construction keeps the antenna thin and lightweight, ideal for portable GPS receivers and UAV/drone applications
• Omnidirectional Coverage: Circular patch geometry provides uniform azimuthal radiation pattern, ensuring consistent GPS reception regardless of device orientation

2. Electromagnetic Simulation in HFSS

Ansys HFSS (High Frequency Structure Simulator) was used for rigorous full-wave 3D electromagnetic analysis:

Parametric Optimization:

• Circular Patch Diameter: Swept patch radius to achieve resonance at 1575.42 MHz on FR-4 substrate
• Orthogonal Slot Dimensions: Optimized slot length, width, and positioning on the patch to maximize axial ratio bandwidth and circular polarization purity (target AR < 3 dB)
• Bottom Layer Slot Feed: Tuned the feed slot dimensions (length, width, position) to efficiently couple energy from microstrip to the patch while maintaining 50Ω input impedance
• Inter-Layer Coupling: Analyzed electromagnetic coupling between the bottom slot feed and top patch through the substrate to optimize energy transfer and minimize losses
• Substrate Thickness Study: Verified design performance with FR-4 standard thickness (1.6 mm) and evaluated sensitivity to substrate εr variations (4.2-4.6)
• Ground Plane Optimization: Sized the ground plane for optimal radiation efficiency and pattern stability while maintaining compact form factor

Performance Verification:

• Return Loss (S11): Achieved S11 < -10 dB across GPS L1 bandwidth, confirming excellent impedance matching
• Axial Ratio: Verified AR < 3 dB at boresight and across ±40° angular range for satellite visibility
• Gain Performance: Measured realized gain > 2 dBic at 1575.42 MHz
• Radiation Pattern: Confirmed hemisphere coverage with RHCP (or LHCP) polarization
• Front-to-Back Ratio: Evaluated rejection of ground-reflected multipath signals

3. Dual-Sense Polarization Mechanism

The orthogonal slot configuration enables switchable RHCP/LHCP operation:

• Orthogonal Mode Excitation: The two perpendicular slots create two independent resonant modes (Mode 1 aligned with Slot 1, Mode 2 aligned with Slot 2)
• Phase Quadrature: Due to the slot geometry and feed coupling asymmetry, these modes naturally exhibit ~90° phase difference at the resonant frequency
• Equal Amplitude Condition: Slot dimensions are balanced to ensure both modes have equal excitation strength, required for pure circular polarization (AR → 0 dB theoretically)
• Polarization Sense Control:
  • RHCP when Mode 1 leads Mode 2 by 90° (clockwise E-field rotation when viewed from receiver)
  • LHCP when Mode 2 leads Mode 1 by 90° (counter-clockwise E-field rotation)
• Switching Implementation: Polarization sense can be reversed by:
  • Rotating the feed point by 90° relative to slot orientation
  • Using dual-feed configuration with controllable phase shifter
  • Electronic switching with PIN diodes or RF switches (future enhancement)

4. Impedance Matching & Feed Design

Achieving robust 50Ω input impedance for SMA connector integration:

• Microstrip Feed Line: Calculated trace width for 50Ω characteristic impedance on FR-4 (εr = 4.4, h = 1.6 mm) using transmission line equations
• Bottom Slot Feed Coupling: The slot on the bottom layer acts as an aperture coupling mechanism, transforming the microstrip impedance to match the patch input impedance
• Stub Length Optimization: Adjusted the microstrip stub length beyond the slot for impedance fine-tuning and resonance adjustment
• Transition Design: Smooth taper from SMA connector (2.5 mm center pin) to microstrip line minimizes reflection and VSWR degradation
• S11 Verification: HFSS Smith chart analysis confirmed impedance locus near 50Ω point at 1575.42 MHz with return loss better than -15 dB

5. PCB Layout for JLCPCB Manufacturing

Translating HFSS electromagnetic design into manufacturable PCB files:

• Design Rule Compliance: All features meet JLCPCB's standard capability: minimum trace width (6 mil / 0.15 mm), spacing (6 mil), and drill size (0.3 mm for vias)
• Layer Stackup Definition:
  • Layer 1 (Top): Circular patch with orthogonal slots
  • Substrate: FR-4, 1.6 mm thickness, εr = 4.4
  • Layer 2 (Bottom): Feed slot line and microstrip traces
  • Copper weight: 1 oz (35 μm) standard
• Gerber File Generation: Exported HFSS 3D geometry to CAD software (KiCad/Altium), refined traces, and generated Gerber RS-274X files for manufacturing
• SMA Connector Footprint: Edge-mount SMA connector with proper pad sizing and mechanical support, positioned for direct connection to microstrip feed
• Mounting Holes: Four corner mounting holes (3.2 mm diameter) for mechanical attachment to GPS receiver enclosure or UAV airframe
• Silkscreen Markings: Added polarization indicators (RHCP/LHCP), frequency label (GPS L1 1575 MHz), and coordinate reference for test measurements
• Surface Finish: ENIG (Electroless Nickel Immersion Gold) selected for reliable RF performance and easy SMA soldering

GPS L1 Band Characteristics

Understanding the GPS L1 signal requirements for antenna design:

• Center Frequency: 1575.42 MHz (L1 C/A code for civilian GPS)
• Signal Bandwidth: ±10 MHz (1565-1585 MHz) to accommodate Doppler shift and signal spreading
• Polarization: GPS satellites transmit RHCP signals; antenna must receive RHCP for optimal SNR
• Signal Strength: Extremely weak (-130 dBm typical) at ground level; antenna gain critical for receiver sensitivity
• Multipath Environment: Reflections from buildings/ground create LHCP components; good AR rejects multipath
• Elevation Angles: Satellites visible from 0° (horizon) to 90° (zenith); wide-beam antenna needed

Current Progress & Timeline

• ✅ Literature Review: Studied Bao & Ammann paper and circular polarization antenna theory
• ✅ HFSS Modeling: 3D antenna model created with parametric dimensions
• ✅ Simulation Optimization: Slot size, truncation ratio, and feed position optimized for GPS L1
• ✅ Performance Verification: Axial ratio, gain, S11, and radiation patterns meet specifications
• ✅ PCB Design: HFSS geometry converted to manufacturable PCB layout in Altium Designer
• ✅ JLCPCB Fabrication: Gerber files generated and PCBs manufactured successfully
• ✅ PCB Received: Prototype PCBs delivered and SMA connector assembled
• ✅ S11 Measurement: Return loss measured using nanoVNA - excellent match at GPS L1
• 🔄 Signal Reception Testing: VNA transmission testing and signal analysis (In Progress)
• ⏳ Gain Measurement: Awaiting access to anechoic chamber or outdoor gain test setup (Planned)
• ⏳ GPS Receiver Integration: Testing with live GPS signals for positioning validation (Planned)

Tools & Technologies

HFSS Simulation Results

3D Model Views

S-Parameters & Impedance Matching

Axial Ratio Performance

Dual-Sense Capability: RHCP & LHCP

Experimental Testing & Measurements

The antenna PCB has been successfully manufactured by JLCPCB and received for testing. Initial measurements have been performed using a nanoVNA (Vector Network Analyzer) to characterize the antenna's impedance matching and return loss performance.

S11 Measurement Using nanoVNA

Testing Observations

• Impedance Match Validation: The nanoVNA Smith chart shows the antenna impedance is well-matched to 50Ω transmission line near the GPS L1 frequency, confirming successful design translation from HFSS simulation to physical PCB
• Manufacturing Accuracy: JLCPCB's 2-layer FR-4 fabrication met all design specifications with proper trace widths, slot dimensions, and layer alignment - critical for maintaining circular polarization performance
• Assembly Quality: The SMA edge-mount connector was carefully soldered to ensure good ground plane contact and minimal discontinuity in the 50Ω feed line
• Frequency Response: The measured S11 curve shows clear resonant behavior in the GPS L1 band, though slight frequency shift from simulation may be present due to FR-4 dielectric constant variations (εr tolerance)
• Portable Testing Advantage: The nanoVNA's compact form factor enables quick field measurements and iterative testing, making it an invaluable tool for antenna prototyping and validation
• Gain Measurement Challenge: Professional gain measurements require calibrated test equipment not currently available. Alternative approaches being considered include GPS receiver signal strength testing (C/N₀ measurements) or comparative testing with commercial GPS antennas

Key Learnings

• Circular Polarization Theory: Deep understanding of axial ratio, polarization purity, and CP generation through orthogonal mode excitation with 90° phase quadrature
• Two-Layer Antenna Architecture: Mastered the design of multi-layer slotted structures where electromagnetic coupling between layers creates the radiating mechanism
• Orthogonal Slot Perturbation: Learned how perpendicular slots on a circular patch perturb current distribution to excite degenerate modes required for circular polarization - no corner truncation needed
• Aperture Coupling Theory: Understanding of how bottom layer slot feeds couple energy through the substrate to the radiating patch above, including coupling coefficient optimization
• HFSS Advanced Simulation: Proficiency in 3D EM modeling, adaptive mesh refinement, eigenmode analysis, multi-parameter optimization, and field visualization for complex multi-layer RF structures
• GPS L1 Signal Requirements: Deep knowledge of GPS satellite signal characteristics (1575.42 MHz, RHCP polarization, -130 dBm sensitivity), multipath rejection, and satellite visibility angles
• Dual-Sense Antenna Design: Practical implementation of switchable RHCP/LHCP capability for both GPS reception and experimental GNSS/atmospheric research applications
• FR-4 PCB RF Design: Adapting high-performance antenna designs for cost-effective FR-4 substrate (lossy, variable εr) while maintaining excellent performance
• Impedance Matching with Complex Loads: Multi-element impedance transformation using slot coupling and microstrip stubs to achieve 50Ω match for reactive patch impedance
• PCB Manufacturing for RF: Translating EM simulation geometry into manufacturable PCB layouts with JLCPCB design rules (minimum features, layer stackup, surface finish selection)
• Academic Research Adaptation: Translating theoretical dual-sense concepts from IEEE papers into a completely different physical implementation tailored for specific GPS application
• Multi-Objective Optimization: Balancing competing requirements (axial ratio, bandwidth, gain, efficiency, size, cost) through systematic parametric studies and trade-off analysis

Design Challenges & Solutions

Challenge 1: Orthogonal Slot Balancing for Low Axial Ratio

Problem: Initial simulations showed axial ratio > 5 dB at boresight due to unequal excitation of the two orthogonal modes. One slot mode dominated, creating elliptical rather than circular polarization.

Solution: Performed extensive parametric sweeps on:

• Slot lengths (varied from 0.3λ to 0.45λ on patch surface)
• Slot widths (0.5 mm to 2.0 mm range)
• Slot positioning relative to patch center
• Slot orientation angles (ensuring perfect 90° orthogonality)

Result: Found optimal slot dimensions that balance mode amplitudes and achieve 90° phase quadrature. Final design achieves AR < 2 dB at boresight and < 3 dB over ±40° cone angle.

Challenge 2: Bottom Layer Slot Feed Coupling Optimization

Problem: Weak electromagnetic coupling between the bottom feed slot and top circular patch resulted in poor radiation efficiency (~60%) and high insertion loss.

Solution: Optimized the aperture coupling mechanism through:

• Increased bottom slot length to enhance E-field penetration through substrate
• Adjusted slot-to-patch alignment for maximum coupling coefficient
• Fine-tuned microstrip stub length beyond the slot for impedance transformation
• Analyzed H-field coupling using HFSS field monitors

Result: Improved radiation efficiency to > 85% while maintaining S11 < -15 dB across GPS L1 bandwidth.

Challenge 3: Impedance Matching with Two-Layer Slot Structure

Problem: The complex impedance presented by the coupled slot-patch system made 50Ω matching difficult. Smith chart showed high reactive component (capacitive), causing poor S11 performance.

Solution: Multi-parameter optimization approach:

• Adjusted microstrip feed stub length to compensate for capacitive reactance
• Modified bottom slot position relative to microstrip centerline
• Fine-tuned the offset between top patch center and bottom feed slot
• Used FR-4 substrate tolerance analysis (εr = 4.2 to 4.6) to ensure robust matching

Result: Achieved S11 < -20 dB at 1575.42 MHz center frequency with > 50 MHz bandwidth below -10 dB, providing excellent margin for manufacturing variations.

Challenge 4: Circular Symmetry vs. Polarization Purity Trade-off

Problem: Maintaining perfect circular patch symmetry while introducing asymmetric orthogonal slots created unwanted higher-order modes that degraded axial ratio at off-axis angles.

Solution: Carefully balanced slot perturbation strength:

• Slots long enough to generate sufficient mode coupling for CP
• Slots not too long to avoid exciting TM₂₁ and higher modes
• Symmetric slot placement about patch center to preserve azimuthal pattern uniformity
• Analyzed modal content using HFSS eigenmode solver

Result: Clean single-mode operation at GPS L1 with AR < 3 dB maintained over wide angular range, ensuring reliable GPS satellite tracking from horizon to zenith.

References & Resources

• Primary Reference: Bao, X., & Ammann, M. J. (2008). "Dual-Frequency Dual-Sense Circularly-Polarized Slot Antenna Fed by Microstrip Line." IEEE Transactions on Antennas and Propagation, 56(3), 645-649. https://doi.org/10.1109/TAP.2008.916894
• Balanis, C. A. (2015). Antenna Theory: Analysis and Design (4th ed.). Wiley.
• Pozar, D. M. (2012). Microwave Engineering (4th ed.). Wiley.
• Ansys HFSS Documentation - Antenna Design and Analysis
• GPS Interface Specification IS-GPS-200 - GPS L1 Signal Characteristics
• JLCPCB PCB Capabilities - PCB Manufacturing Specifications

Future Work & Testing Plans

• PCB Fabrication: Order prototype PCBs from JLCPCB (2-layer FR-4, 1.6mm, ENIG finish for SMA soldering)
• VNA Measurement: Measure S11 return loss and compare with HFSS predictions using Vector Network Analyzer
• Anechoic Chamber Testing: Professional radiation pattern and axial ratio measurements (if facility access available)
• GPS Receiver Integration: Connect antenna to commercial GPS module (e.g., u-blox NEO-M8N) for real-world testing
• Position Accuracy Comparison: Compare positioning accuracy with commercial GPS antennas as benchmark
• Multipath Rejection Testing: Evaluate performance in urban environment with building reflections
• LHCP Mode Validation: Test polarization switching mechanism and measure LHCP performance
• Potential Publication: Document design process and measurements for IEEE conference or journal paper