Modern battlefield environments are increasingly saturated with electromagnetic radiation, posing a significant threat to small fixed-wing drones. These unmanned aerial vehicles, prized for their cost-effectiveness, endurance, and stability, are vulnerable to disruption from communication jammers, navigation spoofers, and high-power microwave weapons. Electromagnetic interference can enter a drone through two primary pathways: front-door coupling, which occurs via antennas and is often mitigated by filters and limiters, and back-door coupling, which exploits apertures, seams, and cables within the structure. For compact, lightweight fixed-wing drones, back-door coupling is particularly insidious due to the dense integration of electronic systems and the inevitable compromises in shielding integrity. Among these pathways, field-to-wire coupling, where electromagnetic fields induce currents on internal wiring, is a critical mechanism. In this study, we combine CST simulation with controlled exposure experiments to elucidate the continuous-wave electromagnetic radiation field-to-wire coupling mechanism in small fixed-wing drones.
1. Introduction to Back-Door Coupling Paths
Small fixed-wing drones consist of several key subsystems: the airframe, power system, flight control system, avionics, data link, navigation system, payload, and ground control station. Unlike multirotor drones, the slender fuselage and tail section of fixed-wing drones house long, parallel control cables for the servos that drive the elevators and rudder. These cables, along with wires for power, data link antennas, and GPS, span the length of the drone and are prime candidates for electromagnetic energy collection. We identified three main back-door coupling paths for electromagnetic energy into the drone:
- Cable conduction via servo signal wires to the flight controller.
- Radiated field coupling through structural apertures or material penetration into the tail section.
- Interference with attitude and position sensors, causing false signals.
Based on the physical layout and function of the drone, we hypothesize that the servo signal wires represent the most efficient field-to-wire coupling path, as they are effectively long, unshielded transmission lines connected directly to sensitive control electronics.
2. Field-to-Wire Coupling Simulation
2.1 Cable Parameter Model
We constructed a three-dimensional model of the small fixed-wing drone using SolidWorks and imported it into CST Microwave Studio. The fuselage base, motor, and camera mount were defined as magnesium-aluminum alloy, while other structural components were treated as dielectric material. Internal cables were modeled based on actual physical measurements, which are listed in Table 1. Five primary cable types were considered: two single-conductor servo signal wires for the rudder and elevator (C1), a coaxial cable for the data link (C2), three-phase power wires for the brushless motor (C3), a coaxial cable for the GPS antenna (C4), and a single power supply wire (C5).
| Cable | Length (mm) | Conductor Radius r1 (mm) | Insulator Outer Radius r2 (mm) | Outer Conductor Radius r3 (mm) | Jacket Outer Radius r4 (mm) |
|---|---|---|---|---|---|
| C1 (Rudder) | 620.4 | 0.4 | 0.6 | – | – |
| C1 (Elevator) | 815.6 | 0.4 | 0.6 | – | – |
| C2 (Data Link) | 216.7 | 0.5 | 0.84 | 1.0 | 1.5 |
| C3 (Power) | 230.9 | 0.5 | 1.2 | – | 1.8 |
| C4 (GPS) | 226.0 | 0.5 | 0.84 | 1.0 | 1.5 |
| C5 (Battery) | 64.7 | 1.0 | 1.5 | – | – |
2.2 Simulation Setup
The incident field was defined as a uniform plane wave with parameters for elevation angle ($\theta$), azimuth angle ($\phi$), and polarization angle ($\alpha$). The electric field can be described by:
$$ E(x,y,z) = E(t)(e_x \hat{a}_x + e_y \hat{a}_y + e_z \hat{a}_z)e^{-j(k_x x + k_y y + k_z z)} $$
where the components and propagation constants are given by:
$$
\begin{aligned}
e_x &= -\cos\phi\cos\theta \sin\alpha – \sin\phi\cos\alpha \\
e_y &= -\sin\phi\cos\theta \sin\alpha + \cos\phi\cos\alpha \\
e_z &= \sin\theta \sin\alpha
\end{aligned}
$$
$$
\begin{aligned}
k_x &= -k \sin\theta\cos\phi \\
k_y &= -k \sin\theta\sin\phi \\
k_z &= -k \cos\theta
\end{aligned}
$$
Here, $E(t)$ is the field amplitude, and $k$ is the wavenumber in free space. The simulation covered four representative scenarios: horizontal polarization ($\alpha=0^\circ$) and vertical polarization ($\alpha=90^\circ$), each incident from the nose ($\phi=180^\circ$) and from the left side of the fuselage ($\phi=270^\circ$). A Gaussian pulse equivalent to a multi-frequency continuous-wave signal was used as the excitation, with an incident field strength of 100 V/m. All cable terminals were terminated with 50 $\Omega$ loads, and the simulation boundary was set to “Open (add space)”.
2.3 Simulation Results
Figure 6 presents the coupling voltage at the terminals of key cables as a function of frequency for the four irradiation scenarios. Across all scenarios, the rudder signal wire (C1-Rudder) consistently exhibited the highest coupling voltage, followed by the elevator signal wire (C1-Elevator). The shielded coaxial cables for the data link and GPS (C2, C4) showed very low coupling (under 0.46 V), confirming their effective shielding. The motor power wires (C3) showed peak coupling voltages of up to 1.13 V, which is below typical operating voltages.
The most significant coupling was observed for the servo signal wires (C1). The rudder cable’s maximum induced voltage reached 7.74 V under horizontal polarization from the fuselage side, a 68% increase compared to the 4.61 V observed under vertical polarization from the same direction. This confirms that horizontal polarization, where the electric field is parallel to the long axis of the fuselage and signal wires, is far more efficient for field-to-wire coupling. The coupling response was highly frequency-selective, with resonant peaks clearly visible. The main resonance frequencies for the vertical tail (rudder) signal wire were identified around 181 MHz, 261 MHz, and 336 MHz.
The difference in the peak coupling voltage between the two servo wires under the same conditions highlights the sensitivity of the coupling process to the exact wire layout and its orientation relative to the applied field. The simulation unequivocally identifies the servo signal wires as the most sensitive back-door coupling path within the small fixed-wing drone.
3. Continuous-Wave Irradiation Test
3.1 Experimental Setup
To validate our simulation findings, we conducted a continuous-wave irradiation experiment in a microwave anechoic chamber. The drone was placed on a foam stand 1.2 m above the ground, with its motor off but avionics and flight control systems powered. A navigation signal simulator was used to provide a stable satellite signal. A transmission antenna, placed 2 m from the drone, radiated a continuous-wave signal amplified by a power amplifier. A field probe monitored the local field strength, and a high-speed camera recorded the motion of the tail surfaces. We scanned the frequency range from 30 MHz to 500 MHz over four polarization/incidence angle combinations, matching the simulation scenarios.
3.2 Test Phenomena and Thresholds
Two primary back-door coupling effects were observed during the scans: uncommanded jittering of the tail surfaces and fluctuations in sensor readings (e.g., pitch and roll angles). The tail jitter, specifically, was a distinct and repeatable effect. Table 2 summarizes the field strength thresholds required to induce observable tail jitter at the most sensitive frequency points for each scenario. The experimental results show that the sensitive frequency bands were consistent regardless of polarization or incidence angle. The threshold curves exhibited a distinct “V” shape, with minimum thresholds at specific center frequencies: approximately 165 MHz, 241 MHz, and 337 MHz.
| Irradiation Condition | Sensitive Center Freq. (MHz) | Min. Threshold (V/m) |
|---|---|---|
| Horizontal, Left Side | 165, 241, 337 | 52.6 |
| Vertical, Left Side | 165, 241, 337 | 68.0 (approx.) |
| Horizontal, Nose-on | 165, 241, 337 | 80.2 |
| Vertical, Nose-on | 165, 241, 337 | >120 |
3.3 Threshold Analysis
The experimental thresholds corroborate the simulation results. Horizontal polarization from the side of the fuselage, which aligns the electric field with the servo wires, is the most effective coupling condition, yielding the lowest jitter threshold of 52.6 V/m. This is a strong experimental validation of the field-to-wire coupling path. The jitter was non-periodic and high-frequency (up to tens of Hz), which is inconsistent with the drone’s 10 Hz attitude control cycle. This observation rules out a sensor-based interference mechanism, pointing directly to a disruption in the servo control loop itself.
We propose that the uncommanded jitter is caused by the conversion of a common-mode current induced on the servo signal wires into a differential-mode voltage at the servo controller’s input. This differential-mode voltage, $\Delta V_{dm}$, can be expressed as:
$$ \Delta V_{dm} = I_{cm} \cdot Z_{unbalance} $$
where $I_{cm}$ is the induced common-mode current and $Z_{unbalance}$ is the impedance imbalance. This noise voltage is superimposed on the Pulse Width Modulation (PWM) control signal sent from the flight controller to the servo. By modulating the effective pulse width, the noise causes the servo’s comparator to misinterpret the target angle, leading to the observed erratic motion.
4. Mechanism Analysis and Discussion
The strong agreement between the simulated resonant peaks and the experimentally observed sensitive frequencies is the cornerstone of our mechanism analysis. Table 3 shows a comparison of the measured and theoretical resonance frequencies, calculated using the half-wavelength resonance formula:
$$ f_w = \frac{n v}{2L} = \frac{n c}{2L \sqrt{\varepsilon_r}} $$
where $L$ is the cable length (0.6204 m for the rudder), $c$ is the speed of light, $\varepsilon_r$ is the relative permittivity of the insulation (2.2), and $n$ is an integer. The theoretical values of 163 MHz, 248 MHz, and 326 MHz (for n=1, 2, 3) are remarkably close to the simulated and experimental sensitive frequencies, confirming that the field-to-wire coupling is dominated by cable resonance. The slight deviations are attributed to the wire’s non-ideal routing and the absence of an ideal ground plane in the simulation.
| Type | First Resonance (MHz) | Second Resonance (MHz) | Third Resonance (MHz) |
|---|---|---|---|
| Theoretical (cable) | 163 | 248 | 326 |
| Simulated | 172 | 261 | 336 |
| Experimental | 165 | 241 | 337 |
The conversion of the common-mode current into a differential-mode voltage is the core of the interference mechanism. In an ideal, perfectly balanced circuit, a common-mode signal would be rejected. However, the three-wire servo cable (signal, power, ground) is not perfectly balanced, and the servo’s internal PCB input presents asymmetrical impedances. This impedance imbalance, $Z_{unbalance}$, acts as a transformer, converting the large common-mode current arriving at the resonance frequency into a differential voltage, $\Delta V_{dm}$, that is injected into the PWM signal path. As the incident field strength increases, $I_{cm}$ increases, leading to a larger $\Delta V_{dm}$ and more severe tail jitter, which is exactly the continuous relationship we observed in our tests.
5. Conclusion
In this study, we have systematically investigated the continuous-wave electromagnetic radiation field-to-wire coupling mechanism in small fixed-wing drones. The key findings are as follows:
- Primary Coupling Path: The servo signal wires, specifically those controlling the rudder and elevator, constitute the most sensitive field-to-wire coupling path. This is due to their physical length and layout within the drone’s slender tail section.
- Resonance-Driven Sensitivity: The coupling is highly frequency-selective and dominated by the half-wavelength resonance of these signal wires. We identified three primary sensitive frequencies (165 MHz, 241 MHz, and 337 MHz for the rudder cable), which match theoretical calculations and experimental thresholds.
- Interference Mechanism: The mechanism for tail jitter is the conversion of the induced common-mode resonance current into a differential-mode voltage via an impedance imbalance in the servo circuit. This spurious voltage modulates the PWM control signal, causing erratic servo motion.
- Optimal Threat Condition: Horizontal polarization with the source positioned on the left side of the fuselage provides the most efficient field-to-wire coupling, resulting in the lowest disruption threshold (52.6 V/m for the first sensitive frequency).
Our findings provide a critical theoretical and experimental foundation for understanding the vulnerability of small fixed-wing drones to electromagnetic threats. Future work will focus on comparing the performance of different cable configurations (parallel vs. shielded vs. twisted wire) to develop effective mitigation strategies for these critical platforms.