Research on the Coupling Effect of HIRF on UAV

The electromagnetic environment of low-altitude airspace has become extremely complex due to the rapid advancement of radio technology. In particular, the electromagnetic energy of HIRF (High Intensity Radiation Field) that is frequently emitted by radar, radio stations, radios, and various ships/airborne impacts negatively aircraft flight safety. In this paper, based on the modeling of a consumer UAV, the HIRF effect is simulated using UWB-HPM, and the electric field variation law at key position and antenna coupling law of the UAV under plane wave irradiation in different directions are analyzed with the help of CST simulation software. The results show that for front-door coupling, the shorter the pulse width of the interference signal is, the stronger the interference for the antenna is, while for back-door coupling, irritation direction and pulse width play a significant role in the coupling effect.


Introduction
HIRF (High Intensity Radiation Field) electromagnetic energy generated by radar, broadcast stations, radio, and various ship/aircraft is common in the low-altitude electromagnetic environment, which threats ranging from 10 kHz-40 GHz.Due to the advantages of high mobility and low loss, consumer UAVs have been increasingly used in many fields in recent years, including agricultural plant protection [1] , power inspection [2] , and traffic regulation [3] .With high internal circuit integration and low operating voltage of UAV, electronics in UAV's datalink are more susceptible to HIRF interference.Some researchers have proven that when HPM carrier frequencies fall into UAV's datalink operating band, electromagnetic interference can easily cause permanent physical damage to the receiver by means of front-door coupling [4] .Some researchers verified through simulation and experimentation that high repetition frequency UWB-EMP can cause interference to GPS receivers [5,6] .Some studies on HIRF have been conducted in recent years.Composite fuel tanks have relatively low shielding effectiveness in a HIRF environment [7] .Both simulation and experimental methods have been used to analyse the differences in the induced electric fields coupled into the aircraft interior in a HIRF environment [8,9] .However, previous studies on HIRF have focused on large civil aircraft and equipment, and there is little relevant study on the impact of HIRF on civilian UAVs.What's more, due to the extensive use of carbon fibre composites for the UAV's shell, its electromagnetic shielding capability is greatly reduced, and thus HIRF can also penetrate the shell through the back-door coupling and cause interference to the internal electronics.
In this paper, a consumer UAV model is established and the effect law of HIRF on the UAV using CST's FDTD solver for both front-door coupling and back-door coupling is analysed and summarized, respectively, including the effect law of signal pulse width on antenna coupling voltage and the effect law of irradiation direction and pulse width on the coupling electric field at a key location within UAV.

High Intensity Radiation Field
Because the Gaussian pulse which is one of the main pulse interference signals, has a rich spectrum, with characteristics of short rise time, wide band distribution, and large pulse intensity, UWB-HPM short-time Gaussian pulse with a field strength of 100 V/m is chosen as simulation excitation source to simulate the HIRF effect.Its mathematical expression is as follows: where λ is the pulse peak,  is the time delay, and  is the pulse width.
Varying the signal pulse width constant  and analyzing its time-domain and frequency-domain characteristics, the results are shown in Figure 1 When   =  , the energy distribution of the low-frequency part is more intensive, while the energy density of the high-frequency part decreases significantly; when   decreases, the energy distribution gradually shifts towards high frequencies and the trend towards decay of the high frequency component becomes slower.Theoretically, it is sufficient to affect electronics operation inside the UAV.

Modeling settings
A real consumer UAV is used as the research object, whose geometric model is established in CAD and imported into the electromagnetic simulation software CST for irradiation simulation, as shown in Figure 2. The metal material of the bottom of the airframe is set to magnesium alloy with a conductivity of 2.27×10 7 S/m, the other parts of the airframe are set to carbon fibre composite material, and the material of operating equipment suspended from the bottom is set to the ideal conductor PEC (Perfect Electric Conductor); the material of the antenna patch is selected as pure copper, and the dielectric substrate is selected as FR-4.The simulation boundary conditions are set to open boundary, while the filling medium is set to air.In order to simulate the possible impact caused by HIRF from different directions during the flight of the UAV, the simulation of a plane wave in three directions is completed, where the upper HIRF interference is mainly from the aircraft above, while the rest is mainly from radio, radar, etc.The direction of irradiation is represented schematically in Figure 3.A 50 Ω discrete port is set at the antenna, which operates in the 2.35-2.45GHz band, covering this UAV's operating frequency of 2.4 GHz.In addition, as the PCB is located close to the metal housing with a heat sink underneath the body, which is called the key position, an electric field monitor in the Z=30 plane is set to investigate the trend of the ambient electric field.

Front-door coupling
The antenna coupling time domain voltage signal is obtained through the pre-set antenna discrete port.According to the antenna gain diagram, the antenna coupling voltage is maximum when the electromagnetic wave is incident from the side.The worst case is analysed.From Figure 4, it can be concluded that under the same electromagnetic irradiation energy, the smaller the signal pulse width is, the larger the amplitude of the coupling voltage is generated at the back end of the antenna.The more violent the waveform oscillation is, the longer the waveform trailing time is.As the rising and falling edges of the narrowband signal are steeper, the electromagnetic energy component of the antenna operating frequency is higher, and the simulation results are consistent with the theory.

Back-door coupling
Due to the UAV's carbon fiber composite shell and its numerous heat sink holes at the bottom, the backdoor coupling effect of HIRF cannot be neglected.To better analyse the coupling capability of the UAV body to HIRF signals of different pulse widths from a qualitative point of view, the relative energy coefficient η is defined using the time-domain shielding effectiveness, with the expression as follows: where  is the incident field strength and  is the maximum field strength in the time domain at a key location.1, it can be concluded that there is a negative correlation between the electric field strength at the key position inside the UAV and the pulse width of the excitation signal, but the difference of η for different pulse width signals is not significant.
Due to the presence of the metal plate on the bottom of UAV, the electric field strength at the key position by electromagnetic waves incident from the bottom (Figure 5(a)) is minimal.The electric field strength at the key position inside the UAV by electromagnetic waves incident from the top (Figure 5(b)) is significantly higher than in the other two cases.The main reason is that most material of the shell is carbon fibre composite, which results in stronger electric fields coupled at the key location inside the UAV for electromagnetic waves incident from the top.All three cases show that as the pulse width increases, the coupled electric field strength changes more dramatically, the stronger the coupling energy and the electric field energy is more concentrated in the low frequency part, which also coincides with the spectral characteristics of the excitation source signal.It is worth noting that, compared to the other two cases, when the electromagnetic wave is incident from the side (Figure 5(c)), the coupled electric field intensity at the key location inside the UAV oscillates significantly with frequency.And as the pulse width increases, it decays more quickly.

Conclusion
In this paper, some simulations were carried out to investigate the influence caused by HIRF that the UAV may suffer during flight, and the influence law of different pulse width signals on the front-door coupling and back-door coupling of the UAV was analysed.It was concluded that due to the poor electromagnetic protection of the UAV shell and the distribution over a wide frequency band of the energy of HIRF, front-door coupling and back-door coupling can occur simultaneously on a UAV.For front-door coupling, the shorter the pulse width of the interference signal, the stronger the interference for the antenna.As for back-door coupling, the irradiation direction of electromagnetic wave and pulse width play a significant role in the coupling effect.

Figure 3 .
Figure 3. Schematic diagram of irritation direction and antenna position

Figure 5 .
Figure 5. Electric field frequency response at the key location