Simulation research on high frequency antenna protection design

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Simulation research on high frequency antenna protection design [Copy link]

1. Introduction

Civil aircraft usually use high frequency (HF) communication antennas for long-distance communication between air and ground. Early high frequency communication antennas mainly include cable antennas, tail cap antennas, probe antennas and notch antennas, but they all have insurmountable shortcomings and have been basically eliminated in modern civil aircraft. The slotted parallel-fed antenna has better overcome the shortcomings of previous antennas, but it is necessary to make a slot on the leading edge skin of the aircraft's vertical tail, which will cause electromagnetic leakage into the vertical tail structure, which may cause electromagnetic interference problems inside the vertical tail. For this reason, it is necessary to install a shielding cover for electromagnetic reinforcement. Installing a shielding cover is one of the solutions, so it is necessary to study the impact of the antenna cover installation on the antenna.

2. Basic principles

The peak output power of the civil high-frequency communication system is as high as 400W. Since the antenna is part of the entire vertical tail structure, as shown in Figure 1, its L-shaped insulation area leads to the integrity of the vertical tail structure. The rear of the L-shaped crack is the reinforcement beam of the vertical tail, and there are weight-reducing holes on the beam (as shown in Figure 2). This will cause its high-power electromagnetic radiation to produce strong electromagnetic induction on the vertical tail, which may cause electromagnetic interference to the equipment and cables installed near this area. It is necessary to consider taking corresponding shielding and isolation measures for the radiation energy of the high-frequency antenna without destroying the aircraft's vertical tail structure to reduce the potential impact of the electromagnetic energy of the high-frequency antenna. For this purpose, an antenna shield is designed to isolate the electromagnetic energy coupling path between the antenna and the vertical tail, thereby protecting the safety of the aircraft. The antenna shield is shown in Figure 2.

Figure 1 Schematic diagram of high frequency antenna

Figure 2 Internal structure of L-shaped crack

It can be seen from the figure that the influence of high-frequency antenna on the slit is mainly caused by the coupling of holes and slots on the strengthening beam. At present, there are roughly two methods for the study of hole-slot coupling: the first is the analytical method: for example, Senior obtained an integral equation for the electric (magnetic) current inside the slot filled with lossy materials based on the integral equation of the impedance band according to the Babinet principle. Although the integral equation obtained cannot be solved analytically, if the width of the slot is approximately considered to be very small, it can be compared with the results obtained by the numerical method to derive an accurate empirical formula. The second is the numerical calculation method: such as FDTD method, finite integral method, moment method, finite element method, etc. Numerical methods can transform differential equations into difference equations, and transform the integrals in the integral equations into finite sums to establish a set of algebraic equations. When the boundary conditions are relatively simple, the analytical method can be used to find the exact solution. In actual engineering, problems with complex boundary conditions are often encountered. At this time, the analytical method can no longer solve them well. With the rapid development of large-capacity and high-speed electronic computer technology, numerical calculation methods have been widely used. The simulation speed and calculation accuracy mainly depend on the algorithm. In terms of calculation accuracy, the moment method has the highest accuracy, followed by the finite element method, and the finite difference time domain method has the worst accuracy. Therefore, the CAE software FEKO based on the moment method was selected for research. At the same time, FEKO software provides engineering interfaces such as CATIA, which can accurately model and solve complex targets.

First, the CAE software was verified and the model was established as shown in Figure 3. The simulated cabin size is: 6.5m×1.5m×3.5m, the hole is opened on the surface of 6.5m×1.5m, and the hole diameter is 0.05m. The frequency of the incident wave outside the cabin is 10kHz-200MHz.

Fig.3 Schematic diagram of hole-slot coupling calculation model

Preliminary analysis shows that the model is a resonant cavity opening slit model. When a plane wave irradiates the cavity, TE waves and TM waves will be generated in the cavity. Taking the center point of the body as the sampling point, taking the diagram in the model as an example, the incident wave is a plane wave, the incident direction is parallel to the Z axis, and the electric field polarization direction is along the negative X axis. After the incident wave enters the cavity, it will generate induction fields in various directions. Because its electric field polarization direction is in the X axis direction, the electric field in the cavity is also mainly the largest component in the X axis direction. Ey and Ez also exist, but the values are smaller. Taking EX as an example, it can be seen from the formula that at the center point of the observation body, the TM wave is 0, and there is only a TE wave.

The waveform obtained by CAE simulation is shown in Figure 4:

Figure 4 Ex component waveform at the center of the cabin changing with frequency

From the comparison between the analytical calculation results and the CAE calculation results, the FEKO simulation results are consistent with the formula calculation results, which shows that the FEKO software can be accurately used for hole-slot coupling calculations.

3. Simulation calculation

3.1 Simulation model

Due to the large amount of numerical calculation, considering that the vertical tail part has the greatest impact on the antenna current distribution, it is difficult and unnecessary to solve the entire aircraft. In order to reduce the computational workload, only the entire vertical tail is considered. This paper uses the commercial CAE software FEKO based on the moment method for calculation. The simulation calculation model is shown in Figure 4: The vertical tail structure of a certain type of aircraft is made of aluminum alloy, so the surface is set as a good conductor in the calculation; in order to further reduce the amount of calculation, the dielectric material filled in the "L"-shaped crack of the high-frequency communication antenna is set to free space. The radiated power and feed port voltage of the antenna are both normalized values. Considering the accuracy of the algorithm, the face element variable length is set to λ/6, where λ is the smallest free space wavelength in the antenna operating frequency band. A more precise segmentation is performed on the antenna, and the segmentation unit variable length is λ/20. The calculation band is the high-frequency operating frequency band, that is, 2MHz-30MHz, and the frequency interval is set to 500KHz.

Figure 5 FEKO calculation model

3.2 Calculation results

1. Calculation results of shielding cover protection effectiveness

Firstly, FEKO software was used to calculate the induced current on the internal cables of the vertical tail before and after the installation of the antenna shielding cover, and the electromagnetic protection effect on the internal cables of the vertical tail after the installation of the antenna cover was determined. The calculation results are shown in Figure 6: It can be seen that the shielding cover provides up to 40dB of protection for the cables inside the vertical tail, which shows that the design of the shielding cover has achieved the expected effect.

Figure 6 Induction current in the cable inside the vertical tail before and after the antenna shield is installed

2. Calculation of the influence of radome on antenna impedance

Since the shielding cover is connected to the high-frequency antenna through a low-resistance channel, it will inevitably affect the impedance and radiation of the high-frequency antenna. Therefore, the antenna impedance before and after the shielding cover is installed is further calculated as shown in Figure 6: From the calculation results, the impact of installing the antenna shielding cover on the impedance of the antenna can be ignored. This shows that installing the antenna shielding cover will not affect the tuning and radiation characteristics of the antenna.

Figure 7 Impedance comparison of antenna before and after installing shielding cover

3. Antenna radiation pattern calculation

In order to further verify the influence of the radome on the antenna radiation pattern, CAE calculations were performed on the entire aircraft. The calculated radiation pattern is shown in Figure 8:

Figure 8 Comparison of three-dimensional antenna patterns

Figure 9 is a comparison of antenna radiation patterns before and after the shielding cover is installed at some frequency points. The Farfield_shield curve is the radiation pattern with the shielding cover installed, and the Farfield curve is the radiation pattern curve without the shielding cover installed. It can be seen that the two curves are basically overlapped.

Figure 9 Radiation pattern before and after installing the antenna shield (8MHz)

4. Analysis and Conclusion

From the CAE calculation results of the line impedance and radiation pattern before and after the antenna is installed with the shield, it can provide 40dB attenuation for the cable induced current inside the vertical tail of the aircraft after the shield is installed. At the same time, the antenna impedance and radiation pattern curves change very little before and after, so it is believed that the influence of the shield can be ignored. From the perspective of the entire pattern curve, the radiation in the nose direction is the strongest, and the tail is slightly weaker, which is in line with the design expectations. It can be seen that the high-frequency antenna shield structure design and installation are reasonable, and there is no adverse effect on the radiation pattern of the high-frequency antenna, which meets the design requirements. It can be seen that the modeling method of the calculation model in this article is correct, and the calculation software and algorithm are appropriately selected. At the beginning of the design of the high-frequency antenna shield, the FEKO simulation calculation can show that the design is reasonable and effective, which simplifies the experimental verification in the design stage, speeds up the design process, and reduces the design risk. In actual engineering practice, this method and idea are worth learning from.

References

[1] Liang Fusheng, Wang Guangxue. Aircraft Antenna Engineering Manual. China Civil Aviation Press, 1997
[2] Boileau O C. An Evaluation of High Frequency Antenna for Large Jet Airplane. IRETrans. 1956
[3] Feko user's manual. suite 6.1, July 2011.

Author: Shi Jianfeng, Chen Jie, Wang Leyi