Impulse Radiating Antenna Modeling using Numerical Electromagnetics Code

Arim Ha and Kangwook Kim

School of Mechanical Engineering, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea

Abstract – In this paper, a mesh impulse radiating antenna (IRA) that can be used in space application is modeled and analyzed with numerical electromagnetics code (NEC). The NEC input file was generated in MATLAB environment. A NEC analysis output was validated through comparing with FEKO simulation. The mesh IRA is shown to be capable of radiating an impulse-like waveform through both simulation results.

Index Terms — impulse radiating antennas (IRAs), numerical electromagnetics code (NEC), ultrawide-band (UWB) antennas.

1. Introduction

Since the Soviet Union launched the world’s first satellite in 1957, many satellites have been performing various space missions such as radio broadcasting communications, Earth observation, navigation, weather observation, etc. [1]. These missions need high gain antennas to overcome the very long communication distance and transmit or receive the signals; therefore, large size antenna is needed. Unfortunately, the large antennas have problems with loading on the launch vehicles. They take up too much space and increase the weight of launch vehicle. It, then, increases the launch cost. Therefore, deployable antennas are widely used in satellites. The most common type of deployable antenna is the mesh antenna [2], [3]. The mesh antennas are advantageous in storage, transportation, and launching. Although the mesh is discontinuous, it has high aperture precision depends on the adequate mesh size.

An impulse radiating antenna (IRA) can be considered for satellite applications. The IRA is a high gain antenna and operates from the very low frequency to high frequency [4], [5]. It is designed to radiate extremely short electromagnetic pulses when fed by a step-like pulse. In this paper, a mesh IRA for deployable antenna was modeled using numerical electromagnetic code (NEC).

2. Impulse Radiating Antenna Design using NEC

The NEC developed by Lawrence Livermore National Laboratory in 1981 is based on the method of moment (MoM) solution [6]. It is appropriate for analysis of thin and perfectly conducting wire structures. To facilitate the construction of wire geometries for NEC, the MATLAB program was employed. The structure of NEC input file has three parts: comment, geometry definition, and program

control. Each part consists of a number of commands called ‘card’. Comment cards do not effect on the computation. After some comments lines, the geometry of IRA is described using a number of GW cards. The geometry of mesh IRA is shown in Fig. 1. According to the optimized IRA design process, the parameters of the typical 2-arm IRA which has F/D ratio of 0.5 were determined [4], [5]. The parameters are presented in Table. I. The meshes were created using Delaunay triangulation method. In the table, each wire length is shorter than λ/10 and wire radius is shorter than λ/20, following the NEC design rule to obtain accurate and reliable results. Note that the area close to the reflector rim has higher mesh density and the wire length is

feed point

1β

0β

2β

matching resistors

(a)

(b) Fig. 1. The geometry of mesh IRA: (a) front view and (b) topview.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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shorter because surface charge density is high in that area and to represent fast varying current. To change the mesh size slowly, λ/15 meshes are generated between the two mesh regions. The feed-arms are connected to the rim of reflector. The feed-arms have uniform mesh density and the wire length is λ/20. Following the geometric command lines, program control commands are continued. LD cards generate 200 Ω resistors between the feed-arms and reflector rim for low frequency matching. EX card generates 1V voltage source at the apex of feed arm as the excitation of the structure. The computation frequency range was set from 50 MHz to 5 GHz by FR card.

3. Simulation Results

The output of the NEC solver is a text file containing the numerical values and the desired target data such as input impedance and electric field in frequency domain. From these results, the radiating waveform in time domain can be calculated.

1

0( ) ( ) [1 ( )] ( )r rinE t E f f V t−= × + Γ ×F F (1)

where 1−F and F are Fourier transform operators, and Γ is the reflection coefficient at the drive point. 0

rE is the NEC-produced, normalized radiated field intensity on boresight, and inV is the input step-like pulse generated as

0 110 90%

1 1( ) erf

2 2

tV t Vt

κ−

= +

(2)

where erf(t) is the error function [7], 10 90%t − is the 10% to 90% rise time of the step-like pulse, and 1κ is the constant defined as

11 2erf (0.8) 1.8124κ −= (3)

The step-like input pulse generation and FFT was

performed using MATLAB. Fig. 2 shows the radiating wave on the boresight when a step-like input was fed into the drive

point. To evaluate NEC result of mesh IRA, the typical solid IRA simulation was also performed using the FEKO solver, which is a commercial MoM software [8]. The FEKO result is shown in Fig. 2 in dash-dot line. The figure shows that two results are well matched. In the figure, as expected, the impulse signal appears at the time 2F/D. The pre-pulse appears before the impulse, and tail signals appear after the impulse signal.

4. Conclusion

In this paper, a mesh IRA was modeled and analyzed using NEC. The time domain response calculated form NEC simulation was well matched to results of a solid IRA. To achieve more accurate results, finer mesh size would be needed, however it costs additional computation time. The mesh IRA may be useful in variety of applications both on Earth and in space.

Acknowledgment

This work was supported by the National Research Foundation of Korea (NRF-2013M1A3A3A02042444).

References

[1] J. N. Pelton, S. Madry, and S. Camacho-Lara, Handbook of Satellite Applications. New York, NY, USA: Springer, 2013.

[2] Space Antenna Handbook, W. A. Imbriale, S. Gao, and L. Boccia, Eds. Chichester, West Sussex, U.K.: Wiley, 2012.

[3] A. W. Love, “Some highlights in reflector antenna development,” Radio Sci, vol. 11, pp. 671–684, Aug. –Sep. 1976.

[4] C. E. Baum, “Radiation of impulse-Like transient field,” Sensor and Simulation Note #321, Weapons Laboratory, Nov. 1989

[5] C. E. Baum, E. G. Farr, and D. V. Giri, “Review of impulse radiating antennas,” in Review of Radio Science, W. R. Stone, Ed. Oxford, U.K.: Oxford Univ. Press, 1999, ch. 16, pp. 403–439.

[6] G. J. Burke and A. J. Poggio, “Numerical electromagnetics code (NEC)—Method of moments,” Rep. UCID18834, Lawrence Livermore Lab., CA, Jan. 1981.

[7] M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. New York, NY, USA: Dover, 1972.

[8] FEKO, http:// www.Feko.info/

TABLE I IRA Design Parameters

parameter value

F/D ratio 0.5

feed arm 0 53.13°

1 46.96° 2 59.96°

mesh density reflector (inner) λ/10

reflector (outer) λ/20

feed arm λ/20

wire radius 0.001 m

Fig. 2. Radiated waveforms on boresight when step-like pulse was fed into the drive point.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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