Goal: Design, simulate, and measure a custom Meandered Inverted-F Antenna (MIFA) for the 2.4 GHz ISM band (Wi-Fi/Bluetooth).
Project Scope:
- Literature Review: Studied reference designs and application notes from credible silicon manufacturers (TI, NXP).
- RF Simulation: Modeled and optimized the antenna geometry using Ansys HFSS.
- Hardware Prototyping: Exported the optimized geometry to KiCad, designed a 2-layer test PCB, and manufactured the prototype.
- VNA Verification & Matching: Measured the physical antenna with a LiteVNA to verify simulation results, and designed a Pi matching network to correct impedance offsets.
The baseline parametric model was created in Ansys HFSS based on Texas Instruments' AN043. Following best practices from PCBSync’s guide on 2.4 GHz PCB trace antennas, the initial dimensions were iteratively adjusted. To streamline the simulation space, the feed line and connector were omitted from the HFSS model, a standard approximation technique outlined in NXP Semiconductors' AN11994.
The design relies heavily on the ground counterweight for the monopole. Because altering the ground plane drastically shifts the resonant frequency, Ansys Optimetrics sweeps were utilized to calculate multiple variables automatically, locking in the geometry that provided the best response on a minimum 30x30 mm ground plane.
Key PCB Simulation Parameters:
- Substrate Material: FR4
- Dielectric Constant (Er): 4.4
- Substrate Thickness: 1.6 mm
- Total PCB Dimensions: 50.3 x 30.3 mm
- Top/Bottom GND Plane: 40 x 30.3 mm
S11 Reflection Coefficient
The target acceptable S11 reflection parameter was < -10dB in the target 2.4–2.5 GHz Wi-Fi band. The optimized HFSS geometry successfully met this criteria.
Smith Chart & Impedance
While the S11 parameter was acceptable, the Smith chart indicated that ideal 50Ω matching was not achieved. At 2.45 GHz, the normalized impedance was 1.3 + 0.46j, requiring a matching network for optimal power transfer.
Directional Gain
The peak directional gain simulated at -0.97 dB (approximately 80% isotropic power). While a perfectly tuned directional antenna should exhibit higher gain, this result suggests strong capacitive coupling to the ground plane resulting in signal power loss.
The HFSS dimensions were mapped directly into KiCad to create a custom footprint. The antenna feed was routed to a U.FL connector using a grounded coplanar waveguide (CPW). Calculating the feed line as a CPW rather than a standard microstrip allowed for a physically narrower 50Ω trace on the 1.6 mm 2-layer board.
To calibrate the LiteVNA for the U.FL connector, a custom Short-Open-Load calibration was performed directly on the unpopulated Pi filter pads.
Initial VNA measurements diverged entirely from the HFSS simulations, presenting an open circuit with fully capacitive loading. Physical inspection of the PCB revealed the root cause: a manufacturing defect left the CPW feed line and the antenna trace unconnected.
Left: Intended Gerber trace. Right: Actual manufactured PCB showing the broken feed.
Due to timeline constraints, fabricating a corrected board was not feasible. Instead, the troubleshooting process successfully validated the VNA setup, and the HFSS simulation data was deemed verified for the purpose of designing the compensation network.
Using the simulated antenna impedance of 65 + 23j Ω, a Pi filter compensation network was designed using onlinesmithchart.com. By plotting the antenna's load impedance and navigating to the 50Ω center point, the necessary passive components were calculated.
As seen from the transceiver to the antenna, the matching network consists of:
- Shunt Capacitor: 1.6 pF
- Series Inductor: 3.9 nH
- Shunt Capacitor: 1.3 pF
This network successfully transforms the simulated antenna impedance to a near-perfect match of 51 + 2.7j Ω. (Note: If this antenna were paired with a transceiver requiring a different characteristic impedance, the Pi filter values would need to be recalculated accordingly).