Design and Implementation of a High Q-Factor Filter Using Square Loop Resonator with Defected Ground Structure for X-Band Radar

In modern telecommunication systems, especially in radar applications, filters play a critical role in isolating desired signals from unwanted frequencies and noise. A key performance indicator for filters is the Quality Factor (Q-Factor), which indicates the sharpness of resonance and the filter’s ability to select specific frequencies accurately.

This study presents the design, fabrication, and measurement of a compact microwave bandpass filter that operates in the X-band frequency range, specifically around 9.51 GHz. The filter is built using a Square Loop Resonator (SLR) integrated with a Defected Ground Structure (DGS) to significantly enhance the Q-Factor. The addition of DGS helps to improve selectivity by modifying the propagation characteristics of the transmission line.

Background and Motivation

The X-band (8 GHz to 12 GHz) is widely used in radar systems, satellite communication, and military applications due to its ability to support high-resolution imaging and precise target detection. Filters operating in this band must have:

• High Q-Factor for accurate frequency selection

• Compact size to meet space-constrained platform requirements

• Low insertion loss to preserve signal strength

Traditional microstrip filters are often limited by size and bandwidth trade-offs. By link introducing DGS into the filter design, we aim to overcome these limitations and deliver a compact yet high-performance filter.

Filter Design Methodology

1. Square Loop Resonator (SLR)

The core element of the filter is the Square Loop Resonator (SLR), chosen for its simplicity and effectiveness in microwave filter design. The SLR configuration allows the filter to resonate at a desired frequency depending on the loop dimensions and substrate properties.

2. Defected Ground Structure (DGS)

To improve the Q-Factor, a DGS pattern is etched on the ground plane directly beneath the transmission line. DGS acts as a band-stop structure, introducing inductive and capacitive effects that suppress spurious harmonics and increase stopband attenuation, while enhancing the sharpness of the passband.

3. Substrate and Dimensions

• Substrate: FR4 Epoxy with εr = 4.3

• PCB Size: 22 mm × 16 mm

• Simulated Center Frequency: ~9.5 GHz

• Bandwidth Target: ~600 MHz

The design was modeled and optimized using electromagnetic simulation software such as CST Studio Suite or HFSS to achieve the desired performance.

Fabrication Process

The filter was fabricated on a double-sided FR4 substrate using standard photolithography techniques. The top layer included the microstrip line and SLR pattern, while the bottom layer featured the etched Defected Ground Structure. Special care was taken during fabrication to ensure accurate alignment between layers to maintain electromagnetic integrity.

Measurement and Results

The fabricated filter was tested using a Vector Network Analyzer (VNA). The performance parameters measured include:

• Center Frequency (fc): 9.51 GHz

• Bandwidth (BW): 610 MHz

• Insertion Loss: Minimal loss in the passband (noted in dB)

• Return Loss (S11): Better than -15 dB in the operating band

• Q-Factor: Enhanced due to DGS implementation

• Size: Compact form factor of 22 mm × 16 mm

The measurement results closely matched the simulated values, confirming the accuracy of the design and effectiveness of DGS in improving Q-Factor.

Discussion

The integration of DGS into the SLR-based filter has shown significant improvement in Q-Factor and bandwidth sharpness. The measured bandwidth of 610 MHz at a center frequency of 9.51 GHz makes the filter suitable for radar systems operating in the X-band.

The advantages observed include:

• Improved selectivity with sharper cutoff characteristics

• Reduced size due to the compact resonator and efficient ground modification

• Cost-effectiveness, as FR4 material and standard PCB manufacturing were used

However, some limitations such as limited power handling and losses associated with FR4 must be considered in high-power applications.

Applications

This filter design can be integrated into:

• Synthetic Aperture Radar (SAR) systems

• Short-range communication systems

• Aerospace and defense electronics

• Portable radar systems

Its compact size and high performance make it ideal for embedded systems where space and power efficiency are critical.

Conclusion

This study successfully demonstrates the design and implementation of a compact X-band filter using a Square Loop Resonator (SLR) enhanced by a Defected Ground Structure (DGS). The filter operates at a center frequency of 9.51 GHz with a bandwidth of 610 MHz and offers a significantly improved Q-Factor. The results confirm that the incorporation of DGS is an effective technique for achieving high-performance microwave filters in radar applications, especially in systems that require precision, compactness, and cost-efficiency.