Designing a Compact Multiband Antenna

Intelligent Transportation Systems (ITS) and modern IoT networks need antennas that are small, cheap to manufacture, and capable of working across multiple frequency bands, specifically the 2.4 GHz ISM band for Wi‑Fi/Bluetooth, and adjacent vehicular communication channels. This project covers the design, simulation, and physical fabrication of a planar microstrip patch antenna. To achieve multiband resonance without increasing the overall footprint, the design uses a metamaterial-inspired fractal patch and a Defected Ground Structure (DGS) etched into the ground plane.

Substrate Selection & Design Rationale

For rapid prototyping, I chose a standard FR4 substrate:

• Relative Permittivity ($\varepsilon_r$): 4.4
• Thickness ($h$): 1.6 mm
• Loss Tangent ($\tan \delta$): 0.02 (lossy for RF, but cost-effective for testing)

The antenna layout has a patch on the top copper layer, fed by a $50,\Omega$ microstrip transmission line. The bottom layer carries the DGS. By cutting slots (defects) into the ground plane under the feed line and patch, we alter the distribution of surface current. This changes the effective inductance and capacitance of the transmission line, which improves impedance matching, broadens bandwidth, and generates additional resonant frequencies.

CST Microwave Studio Simulation

I modeled and optimized the geometry in CST Microwave Studio. The simulation targeted a return loss ($S_$) below $-10\text$ (VSWR $< 2$) at the desired resonant bands. The optimization loop adjusted three main variables:

1. Fractal Slot Iterations: Etching concentric shapes into the main patch to lengthen current paths, lowering the fundamental resonant frequency.
2. DGS Slot Dimensions: Adjusting the width and length of the ground plane cuts to match feed impedance near 2.4 GHz.
3. Feed Position: Fine-tuning the microstrip feedline entry point to reduce reflection losses.

The simulation predicted clear resonant points at 2.4 GHz and secondary bands in the 3.5 GHz and 5.2 GHz ranges.

Fabrication and Bench Testing

The prototype was fabricated using a standard chemical PCB etching process. I hand-soldered the SMA connector to the feedline, keeping solder bumps minimal, as bumps introduce parasitic inductance and degrade impedance matching at high frequencies.

To validate the simulation, I connected the fabricated antenna to a Vector Network Analyzer (VNA). The test setup measured:

• $S_$ Parameter: From 1 GHz to 6 GHz to capture all resonant peaks.
• VSWR: To check impedance matching quality.
• Radiation Pattern Approximation: Near-field scans to detect beam distortion.

The measured $S_$ curve aligned closely with the CST simulation. The main resonant peak shifted slightly to 2.38 GHz. This is standard for FR4 prototypes and is caused by minor variations in the dielectric constant of cheap substrates and wet-etching tolerances.

Performance Analysis & Future Enhancements

The physical measurements confirmed several things:

• Impedance Bandwidth: The DGS widened the $-10\text$ bandwidth around the 2.4 GHz center, keeping the VSWR below 1.5.
• Multiband Behavior: Resonant peaks appeared near 2.4 GHz and 5.1 GHz, making the antenna suitable for dual-band IoT setups.
• Substrate Loss: As expected, the FR4 substrate introduced moderate dielectric losses, reducing overall radiation efficiency compared to low-loss substrates like Rogers RO4003.

For future work, moving to a PTFE-based substrate would improve efficiency and gain. FR4 remains a solid choice for validating the geometry and confirming the DGS concept works before committing to a more expensive material.