In the rapidly evolving field of unmanned aerial vehicle (UAV) technology, the reliability of navigation systems is paramount. As a ‘UAV drone’ operates in increasingly complex electromagnetic environments, it becomes highly susceptible to high-power electromagnetic pulses (EMP) that can disrupt or even damage sensitive front-end circuits. To address this challenge for the BeiDou Navigation Satellite System (BDS) B1I band at 1.561 GHz, I have designed a novel circularly polarized (CP) microstrip patch antenna that integrates pulse suppression functionality directly into its radiating structure.
This design innovatively combines a conventional CP patch antenna with PIN diodes and parasitic strip patches. By leveraging the non-linear switching characteristics of the PIN diode, the antenna can seamlessly transition between a normal navigation state and a protection state without the need for bulky external circuitry. This approach is particularly advantageous for ‘UAV drone’ applications where size, weight, and power consumption are critical constraints. The core principle is that under normal signal levels, the PIN diode is in a high-impedance (cut-off) state, allowing the antenna to resonate at the desired B1I frequency. When a strong EMP is incident, the diode undergoes a reversible breakdown, shifting the antenna’s resonant frequency away from the navigation band and effectively reflecting the harmful energy.
The following sections provide a detailed analysis of the antenna’s design methodology, simulation results, and experimental validation, demonstrating its suitability for robust ‘UAV drone’ navigation in hostile electromagnetic environments.
1. Antenna Design and Operating Principle
1.1 PIN Diode Equivalent Model for Pulse Suppression
The core of the pulse suppression mechanism is the PIN diode. According to the datasheet of the BAP51-02 model used in this design, its behavior can be approximated using simplified equivalent circuits in two distinct states. These models are integrated into the electromagnetic simulation software to accurately predict the antenna’s performance.
In the normal operating state (low power), the PIN diode is reverse-biased and effectively OFF. It presents a high impedance and can be modeled as a series combination of a small capacitance \(C_j\) and a parasitic inductance \(L_p\). For the BAP51-02, these values are \(C_j = 0.17\ \text{pF}\) and \(L_p = 0.5\ \text{nH}\).
In the protection state (high power), the incident electromagnetic pulse induces a voltage across the diode that exceeds its threshold. This causes a reversible (ON) breakdown. The I-layer becomes flooded with charge carriers, drastically reducing the resistance. In this state, the diode is modeled as a very small series resistance \(R_s \approx 2\ \Omega\).
| State | Equivalent Model | Component Values |
|---|---|---|
| Normal (Cut-off) | Series LC (\(R_{off} \approx \infty\)) | \(C_j = 0.17\ \text{pF}\), \(L_p = 0.5\ \text{nH}\) |
| Pulse Suppression (ON) | Series R (\(C_{on} \approx \infty\)) | \(R_s = 2\ \Omega\) |
1.2 The Coaxial-Fed Circularly Polarized Patch Antenna
The base of my antenna is a standard microstrip patch antenna designed for the B1I band (1.561 GHz). It is fabricated on an FR4 substrate with a dielectric constant \(\epsilon_r = 4.4\), a loss tangent \(\tan\delta = 0.02\), and a total thickness of \(H = 3.2\ \text{mm}\). The dimensions of the square substrate and patch are \(L = 74.5\ \text{mm}\) and \(W = 41.5\ \text{mm}\), respectively. Circular polarization (RHCP) is achieved through the perturbation method. Specifically, one corner of the square patch is truncated by a size \(X = 5.4\ \text{mm}\), and two L-shaped stubs are loaded on the opposite diagonal. This combination excites two orthogonal resonant modes that are 90 degrees out of phase. The entire structure is fed by an offset coaxial probe located at coordinates \(r1 = 0.7\ \text{mm}\) and \(r2 = 1.5\ \text{mm}\) from the center.
To further improve the impedance bandwidth and axial ratio (AR), a Defected Ground Structure (DGS) is implemented on the bottom ground plane. This DGS consists of a central rectangular slot and two symmetrically placed L-shaped slots. These slots create a high-impedance surface that modifies the surface current distribution, suppresses higher-order modes, and introduces an additional resonance, effectively widening the impedance bandwidth to 85 MHz and ensuring the 3-dB AR bandwidth covers the B1I signal.
| Parameter | Value (mm) | Parameter | Value (mm) |
|---|---|---|---|
| \(L\) | 74.5 | \(W_1\) | 1.25 |
| \(W\) | 41.5 | \(W_2\) | 2.0 |
| \(X\) | 5.4 | \(d\) | 24.5 |
| \(H\) | 3.2 | \(d_1\) | 0.65 |
| \(r_1\) | 0.7 | \(r_2\) | 1.5 |
| \(m_1\) | 10.0 | \(m_2\) | 1.5 |
| \(n_1\) | 1.8 | \(n_2\) | 7.0 |
| \(n_3\) | 1.0 | \(n_4\) | 1.4 |
| \(n_5\) | 3.5 | \(n_6\) | 5.0 |
1.3 Integration of the Pulse Suppression Element
The pulse suppression mechanism is realized by adding a pair of rectangular metallic strip patches close to the edges of the main radiating patch. Two PIN diodes (BAP51-02) are connected across the gap between the main patch and these parasitic strips. As a ‘UAV drone’ operates normally, the received signal is weak, the PIN diodes remain in their cut-off state, and the parasitic strips are electrically isolated. The antenna behaves identically to the standard CP design, resonating at 1.561 GHz.
When a high-power electromagnetic pulse is incident, the induced voltage across the PIN diodes forces them into the reversible breakdown state. This creates a low-impedance path (ON state), connecting the parasitic strips to the main patch. This effectively increases the electrical length of the radiating structure. The antenna’s resonant frequency shifts downwards to approximately 1.378 GHz. At the original B1I operating frequency (1.561 GHz), the antenna now presents a very high reflection coefficient (\(S_{11}\) close to 0 dB), acting as a stop-band filter for the harmful pulse energy. The reflected energy is scattered by the antenna instead of traveling to the sensitive receiver front-end.
After the EMP subsides, the voltage across the diodes drops below the breakdown threshold. The diodes automatically revert to the cut-off state, disconnecting the parasitic strips. The antenna’s resonance shifts back to 1.561 GHz, restoring normal navigation reception without any external control signal.
2. Simulation and Measurement Results Analysis
I used HFSS (High Frequency Structure Simulator) to design and optimize the antenna. For simulation, the PIN diode was represented by its equivalent circuit model. A prototype was then fabricated and measured in an anechoic chamber to validate the performance. The following sections compare the simulated and measured results for the antenna in both its normal and protection states.
2.1 Impedance Bandwidth (S11) Performance
The \(S_{11}\) parameter is a critical indicator of impedance matching. In the normal state (diode cut-off), the antenna resonates at 1.561 GHz. The simulated \(-10\ \text{dB}\) impedance bandwidth is 85 MHz, and the measured bandwidth is 82 MHz. The measured resonant frequency shows a slight shift to 1.522 GHz, which is attributed to fabrication tolerances of the FR4 substrate and the parasitic effects of the diode’s physical package. Despite this shift, the B1I band is still well within the \(-10\ \text{dB}\) impedance bandwidth. In the protection state (diode ON), the resonance shifts dramatically. The simulated center frequency moves to 1.378 GHz, and at the original frequency of 1.561 GHz, the simulated \(S_{11}\) is about -0.3 dB. The measured results confirm this, showing an \(S_{11}\) of -1.45 dB at 1.561 GHz and -1.89 dB at 1.522 GHz. This high \(S_{11}\) value (near 0 dB) means that over 95% of the incident power is reflected back, effectively protecting the receiver.
| State | Metric | Simulated | Measured |
|---|---|---|---|
| Normal (Diode OFF) | Resonant Frequency | 1.561 GHz | 1.522 GHz |
| -10 dB BW (MHz) | 85 | 82 | |
| Pulse Suppression (Diode ON) | Resonant Frequency | 1.378 GHz | 1.326 GHz |
| \(S_{11}\) @ 1.561 GHz (dB) | -0.3 | -1.45 |
2.2 Axial Ratio and Radiation Pattern
The axial ratio (AR) is a measure of polarization purity. A value below 3 dB is the standard for CP antennas. In the normal state, the antenna exhibits excellent CP performance. The simulated AR at 1.561 GHz is 1.44 dB, while the measured value is 2.02 dB at the same frequency and reaches a minimum of 1.43 dB at 1.555 GHz. The measured 3-dB AR bandwidth covers the B1I band. In the protection state (diode ON), the AR at 1.561 GHz degrades to 3.92 dB (simulated) and is significantly above 3 dB in measurements, confirming that the circular polarization purity is destroyed, which further helps in attenuating the interference.
The radiation patterns were measured at 1.561 GHz. The antenna shows good broadside radiation patterns with a measured peak RHCP gain of 3.85 dBi, which is close to the simulated value of 3.5 dBi. The cross-polarization (LHCP) level is better than -20 dB in both the E-plane and H-plane, indicating a high polarization purity essential for accurate ‘UAV drone’ navigation.
| Metric | Normal State (Sim./Meas.) | Protection State (Diode ON) |
|---|---|---|
| Resonant Freq. (GHz) | 1.561 / 1.522 | 1.378 |
| -10dB BW (MHz) | 85 / 82 | N/A |
| AR @ 1.561 GHz (dB) | 1.44 / 2.02 | 3.92 |
| Peak Gain (dBi) | 3.5 / 3.85 | – |
| Cross-pol. Level (dB) | -20.15 (E-plane) / -19.26 (H-plane) | – |
3. Comparison with Other State-of-the-Art Designs
The following table provides a comparative analysis of my proposed antenna with several recently published circularly polarized navigation antennas. The comparison focuses on key performance metrics and unique features. The electrical size (\(\lambda_0\)) is calculated at the lowest operating frequency.
| Reference | Electrical Size (\(\lambda_0\)) | Impedance BW (%) | AR BW (%) | Peak Gain (dBi) | Pulse Suppression |
|---|---|---|---|---|---|
| [20] | 0.33 × 0.35 × 0.02 | 1.15 | 0.58 | 4.40 | No |
| [21] | 0.34 × 0.34 × 0.01 | 2.54 | 1.02 | 3.31 | No |
| [22] | 0.53 × 0.53 × 0.02 | 2.03 | —— | 5.40 | Yes |
| [23] | 0.37 × 0.37 × 0.01 | 1.79 | 1.40 | —— | No |
| [24] | 0.29 × 0.29 × 0.02 | 2.16 | 2.03 | 3.62 | No |
| This Work | 0.39 × 0.39 × 0.02 | 5.45 | 1.22 | 3.52 | Yes |
As shown in Table 5, my proposed antenna achieves a significantly wider impedance bandwidth compared to other designs. While its peak gain is moderate, it is fully sufficient for standard ‘UAV drone’ navigation applications. The most critical advantage is the integrated pulse suppression capability, a feature not present in the other compact designs. Reference [22] also offers pulse suppression, but its electrical size is significantly larger, making my design more suitable for space-constrained ‘UAV drone’ platforms. The combination of compact size, wide bandwidth, good CP performance, and inherent pulse protection makes this antenna an ideal candidate for robust ‘UAV drone’ navigation systems.
4. Conclusion
I have successfully designed, simulated, and fabricated a compact, pulse-suppressing circularly polarized antenna for the BeiDou B1I navigation band specifically intended for ‘UAV drone’ applications. By integrating two PIN diodes and parasitic strips onto the radiating patch, the antenna achieves a self-adaptive protection mechanism. In normal operation, it functions as a high-performance CP antenna with a measured impedance bandwidth of 82 MHz, a peak gain of 3.85 dBi, and excellent polarization purity. When subjected to a high-power EMP, the diodes switch ON, causing the antenna’s resonance to shift away from the B1I band, thereby acting as a protective shield for the sensitive receiver front-end. The antenna’s low profile, simple structure, and automatic recovery make it an ideal solution for ensuring the reliability of ‘UAV drone’ navigation in complex electromagnetic battlefields or high-interference industrial settings.
