Design and Development of a Dual Patch Microstrip Antenna for Aircraft Collision Avoidance System

Debajit De 1, P. K. Sahu 2

1 de.debajit118@gmail.com, 2 pksahu@nitrkl.ac.in 1, 2 Department of Electrical Engineering, National Institute of Technology, Rourkela, India.

Abstract - ACAS (Aircraft Collision Avoidance System) / TCAS (Traffic Collision Avoidance System) is an airborne system designed to increase cockpit awareness of nearby aircraft and to service as a last defense against mid-air collisions between the aircrafts. In the existing system, four monopole stub elements are used as TCAS directional antenna and one blade type element is used as TCAS omni-directional antenna. The transmission and reception frequencies of the TCAS antenna are 1.03 GHz and 1.09 GHz respectively. The existing TCAS antenna has some drawbacks such as low gain, large beamwidth, frequency and beam tuning/scanning issues etc. Antenna issues like unwanted signals reception may create difficulties in identifying the possible threats. In this paper, the focus is on the design and development of a Novel Microstrip Antenna which can be used for ACAS / TCAS application. The proposed model is a Dual Patch Microstrip Antenna, designed in CST MWS Studio 2015 and FEKO Suite 7.0.1 to meet the current needs of aircraft collision avoidance system and to overcome the possible limitations associated with the existing techniques. The performance and other antenna characteristics have been explored from the simulation results followed by the antenna fabrication and measurement. A good Reflection Coefficient and VSWR with proper 50 Ω Impedance matching, high Gain and perfect directional Radiation Pattern at the operating frequencies make this proposed antenna a good candidate for ACAS application. The proposed antenna would be expected to meet the requirements of the advanced avionics standards in terms of design simplicity, lightweight and high performance. Keywords – ACAS, TCAS, Airborne System, Microstrip Antenna, Reflection Coefficient, VSWR, Beamwidth, Gain.

I. INTRODUCTION

The Aircraft Collision Avoidance System (ACAS) / Traffic Collision Avoidance System (TCAS) provide a solution of reducing the risk of mid-air collisions between aircraft. ACAS is a family of airborne systems that function independently of ground-based air traffic control (ATC) to provide collision avoidance protection. The ACAS concept makes use of the radar beacon transponders carried by aircraft for ground ATC purposes and provides no protection against aircraft that do not have an operating transponder [1]. After many years of extensive analysis, development, and flight evaluation by the Federal Aviation Administration (FAA), Civil Aviation Authorities (CAAs), and the aviation industries, Traffic Collision Avoidance System (TCAS) was developed to reduce the risk of mid-air collisions between aircraft [2]. The main feature of TCAS, which was first proposed by Dr. John S. Morell in 1955, is that the function is according to time criteria and not the distance. From several successive replies,

TCAS calculates a time to reach the CPA (Closest Point of Approach) with the intruder, by dividing the range by the closure rate [3]. ACAS/TCAS employs radio signals for surveillance of nearby aircraft, and in dangerous encounters, it warns the aircraft pilot by means of cockpit displays and auditory alarms. To detect the presence of nearby aircraft, TCAS transmits interrogations at a steady rate, nominally once per second, and employs a receiver for detecting the replies to these interrogations from the transponders on nearby aircraft as shown in Fig. 1.

Figure 1. Air-to-Air Surveillance obtained through Interrogation and Reply

Fig. 2 shows the basic block diagram of overall ACAS system. ACAS consists of the Mode S/TCAS Control Panel, the Mode S Transponder, the ACAS Computer, Antennas, Traffic and Resolution Advisory Displays, and an Aural Annunciator. Control information from the Mode S/TCAS Control Panel is provided to the TCAS Computer via the Mode S Transponder [2].

Figure 2. ACAS System Block Diagram

ACAS uses a directional antenna, mounted on top of the aircraft. An Omni-directional transmitting and receiving antenna is mounted at the bottom of the aircraft to provide the

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range and altitude data to ACAS from traffic that is below the aircraft. In addition to the two ACAS antennas, two antennas are also required for the Mode S transponder. These antennas enable the Mode S transponder to receive interrogations at 1.03 GHz and to reply for the received interrogations at 1.09 GHz [1]. In this work, Microstrip Patch Antenna has been used due to its extensively uses in commercial and communication systems that include Military Applications, GPS technology, Bluetooth, WLAN, Satellite Communication etc. The important properties like lightweight, low power consumption, low cost, high reliability, and mobility capability which make it more reliable than other antennas. Simple patch antennas are well-suited to this application because of its narrow bandwidth characteristics. Frequency Tuning, Radiation Beam Shaping, Tuning and Scanning are possible in Microstrip Antenna. These antennas are very much mechanically robust and that is a significant advantage in this application [4].

II. EXISTING ACAS ANTENNA & IT’S FEATURES

The antennas used by ACAS include a directional antenna (ANT-81A) that is mounted on the top of the aircraft and either an Omni-directional or a directional antenna mounted on the bottom of the aircraft.

Fig. 3 Directional Antenna Fig. 4 Omni-Directional Antenna Fig. 3 shows the Directional Antenna, which contains an array of four passive, steerable, radiating elements mounted at 00, 900, 1800, 2700 in relationship with the vertical axis of the antenna. This whole assembly is mounted directly to the fuselage of the aircraft. During Mode S and C interrogation message transmission, the directional antenna transmits 1030 MHz pulses on the four radiating elements. During TCAS receptions, each of the four directional antenna elements detects 1090 MHz RF signals that are present. The phasing of this received signals are determined by the direction from which the RF energy is received [5]. Fig. 4 shows the Omni-Directional Antenna, which is mounted directly to the underside of the aircraft. It consists of L-band blade type, dipole element. It connects to the ACAS Processor via a single coaxial cable. The other three bottom antenna connectors to the processor are left open. Cable resistance is 50 ohms or less. The Omni antenna must be certified to C66b, C74, C112 and C119 ( ) [5].

Table I shows the features of existing ACAS antenna.

TABLE I SPECIFICATIONS OF EXIXTING ACAS ANTENNA

Specifications Detailed Values

Transmitting Frequency 1.03 GHz

Receiving Frequency 1.09 GHz

V.S.W.R 1 to 1.5

Impedance 50 Ohm

Power 10 Watt, 1000 W/Peak

Directional Mode Polarization : Linear, Gain : 3.6 dB,

Side Lobe : -8 dB Sector Beamwidth : More than 900

Omni-Directional Mode Gain : 1 dB

Fig. 5 shows the radiation pattern of a single monopole and 4 monopole stubs respectively. The 4 monopoles are oriented in such a way that the overall radiation pattern can cover all the surveillance area around the aircraft.

Figure 5. Radiation Pattern of 1 and 4 Monopole Elements

III. PROPOSED DESIGN AND ANALYSIS

First, a Dual Patch Single Feed Slotted Microstrip Antenna has been proposed. The proposed antenna which is designed in FEKO Suite 7.0.1 and in CST MWS Studio 2015 has been shown in Fig. 6 (a) & in 6 (b) respectively. After that, we have designed the same antenna with the dual feeding technique and simulated it using both the tools.

Figure 6 (a). Designed Dual Patch Antenna in FEKO and (b) in CST Tool

The Dual Patch Single Feed Microstrip Antenna has been fabricated and it is shown in Fig. 7.

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Figure 7. Fabricated Dual Patch Single Feed Antenna

FR4 Epoxy of height 4 mm. has been taken as the substrate material whose dielectric constant (εr) is 4.4. The dimensions of these 2 proposed antennas have been calculated using the formulae available in the literature [6]. Here, patch 1 and patch 2 has been treated as primary radiator and secondary radiator respectively. A triangular shaped metallic cap made of thin copper foil has been designed above the patch 2. This triangular cap is basically blocking the back radiation, and as a result, the antenna can radiate in a particular horizontal direction that is in the azimuth plane, which is also the desired radiation pattern for this application. One shorting pin / inductive post made of copper has been put inside the substrate under the patch 2 and two small slots have been cut in the patch 2. This shorting pin and those 2 slots have been used for frequency tuning of that antenna. Shorting pin provides inductance and slots having length less than λ0/4 (Length of Quarter Wave Transformer) provide capacitance to the overall patch antenna.

IV. RESULTS AND DISCUSSIONS

The Single Feed and Dual Feed Dual Patch Slotted Microstrip Antennas have been simulated in FEKO Suite 7.0.1 and in CST MWS 2015. The Dual Patch Single Feed Slotted Antenna has also been fabricated and tested in the laboratory with the help of V.N.A (Vector Network Analyzer). A. Simulation and Measurement Results of the Dual Patch

Single Feed Slotted Microstrip Antenna

Fig. 8 shows the FEKO and CST simulation results of the Return Loss (S11). From this plot, it has been shown that, the antenna has the dual resonating frequencies. At 1.03 GHz, S11 is -16.8 dB and at 1.09 GHz, S11 is -27.8 dB.

Figure 8. Simulation Results of S11 in CST and in FEKO Tool

Fig. 9 shows the measurement result of S11. In the measurement plot, the resonating frequencies are slightly deviated from the simulation result. The error percentage of deviation is around 2.1%. In this plot, at 1.045 GHz, S11 is -21.5 dB and at 1.12 GHz, S11 is -37.5 dB.

Figure 9. Measurement Results of S11

Fig. 10 shows the FEKO and CST simulation result of the V.S.W.R. From this plot, it is clear that VSWR is 1.4 at 1.03 GHz and VSWR is 1.3 at 1.09 GHz. Fig. 11 shows the measurement result of the V.S.W.R. In V.N.A, at 1.045 GHz, VSWR is 1.18 and at 1.12 GHz, VSWR is 1.1.

Figure 10. Simulation Results of VSWR in CST and in FEKO Tool

Figure 11. Measurement Results of VSWR

Fig. 12 shows the FEKO and CST simulation result of the Impedance. In this plot, it is observed that, impedance at 1.03 GHz is around 53 Ω which is satisfactory and at 1.09 GHz, it is 56.97 Ω. Fig. 13 shows the snapshot of the measurement result of impedance in V.N.A. In V.N.A, at 1.045 GHz, impedance is 52.02 Ω and at 1.12 GHz, it is 48.88 Ω.

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Figure 12. Simulation Results of Impedance in CST and in FEKO Tool

Figure 13. Measurement Results of Impedance

Fig. 14 and 15 show the CST and FEKO Simulation results of Beam Pattern respectively. From these 3D plots, it is obtained that, the maximum Gain of the antenna for 1.03 GHz is around 4 dB at θ = 140 with respect to positive Z axis in the ZY Plane (i.e. ϕ = 900). It is also obtained that, for 1.09 GHz, the maximum Gain of the antenna is around 5 dB at θ = 900 in CST and at θ = 600 in FEKO tool.

Figure 14. CST Simulation Results of Beam Patten at 1.03 GHz & 1.09 GHz

Figure 15. FEKO Simulation Results of Beam Patten at 1.03 GHz & 1.09 GHz

Fig. 16 shows the CST Simulation results of the E Field Distribution in the antenna at 1.03 GHz and at 1.09 GHz.

Figure 16. E Field Distribution at 1.03 GHz and at 1.09 GHz

Similarly, Fig. 17 shows the FEKO Simulation results of the Surface Current Distribution in the antenna at 1.03 GHz and at 1.09 GHz.

Figure 17. Surface Current Distribution at 1.03 GHz and at 1.09 GHz

A CAD Model of Boeing-787 has been designed. The antenna has been simulated with the aircraft model in the transmitting mode i.e.; at 1.03 GHz. The purpose of this study is to observe the effect of the aircraft body on the antenna beam pattern. From Fig. 18, it is shown that, at 1.03 GHz the antenna is radiating in the broadside direction. The beam pattern has not been distorted. The gain is around 5.4 dB. This simulation has been carried out in FEKO tool.

Figure 18. Simulation Results of the Antenna on the aircraft at 1.03 GHz

B. Simulation Results of the Dual Patch Dual Feed Slotted Microstrip Antenna

. Fig. 19 & 20 shows the CST and FEKO simulation results of the Return Loss: S11 & S22 respectively. From this plot, it has been shown that, in CST, at 1.03 GHz S11 is -17.6 dB and at 1.09 GHz, S22 is -15.7 dB. Again, in FEKO, at 1.03 GHz S11 is -42.3 dB and at 1.09 GHz, S22 is -24.8 dB.

48.88 Ω

52.02 Ω

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Figure 19. Simulation Results of S11 & S22 in CST Tool

Figure 20. Simulation Results of S11 & S22 in FEKO Tool

Fig. 21 & 22 shows the CST and FEKO simulation results of the VSWR respectively. From this plot, it has been shown that, in CST VSWR at port 1 is 1.28 and at port 2, VSWR is 1.35. Again, in FEKO at port 1 VSWR is 1.1 and at port 2, VSWR is 1.18.

Figure 21. Simulation Results of VSWR at Port 1 & at Port 2 in CST Tool

Figure 22. Simulation Results of VSWR at Port 1 & at Port 2 in FEKO Tool

Fig. 23 & 24 shows the CST and FEKO simulation results of the Impedance respectively. From the plot, it has been shown that, in CST Impedance at port 1 is 53.2 Ω and at port

2, Impedance is 48.7 Ω. Again, in FEKO at port 1 Impedance is 51.1 Ω and at port 2, Impedance is 55.4 Ω.

Figure 23. Simulation Results of VSWR at Port 1 & at Port 2 in CST Tool

Figure 24. Simulation Results of VSWR at Port 1 & at Port 2 in CST Tool

Fig. 25 and 26 show the CST and FEKO Simulation results of Beam Pattern respectively. From these 3D plots, it is obtained that, for 1.03 GHz, the maximum Gain of the antenna is around 6 dB at θ = 900 with respect to positive Z axis in the ZY Plane (i.e. ϕ = 900). It is also obtained that, for 1.09 GHz, the maximum Gain is around 5.85 dB at θ = 900, which is exactly the azimuth plane of the antenna.

Figure 25. CST Simulation Results of Beam Patten at 1.03 GHz & 1.09 GHz

Figure 26. CST Simulation Results of Beam Patten at 1.03 GHz & 1.09 GHz

Fig. 27 shows the CST Simulation results of the E Field Distribution in the antenna at 1.03 GHz and at 1.09 GHz.

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Figure 27. E Field Distribution at 1.03 GHz and at 1.09 GHz

Similarly, Fig. 28 shows the FEKO Simulation results of the Surface Current Distribution in the antenna at 1.03 GHz and at 1.09 GHz.

Figure 28. Surface Current Distribution at 1.03 GHz and at 1.09 GHz

This antenna has been again simulated with the previously mentioned aircraft model in the transmitting mode i.e.; at 1.03 GHz. From Fig. 29, it is shown that, at 1.03 GHz the antenna is radiating almost in the azimuth plane, which is the desired beam patter for this application. Here, the radiation beam has also not been distorted. In this case, the gain is around 5 dB. As before, this simulation has been carried out in FEKO tool.

Figure 29. Simulation Results of the Antenna on the aircraft at 1.03 GHz

C. Summary of some other Simulation Results During the simulation of these two proposed antennas, the Beamwidth, Directivity, Radiation Efficiency, Radiating Power and Receiving Power have also been obtained. Table II and Table III provides those results of the Dual Patch Single Feed and Dual Feed Slotted Microstrip Antenna respectively.

TABLE II OTHER SIMULATION RESULTS OF THE SINGLE FEED ANTENNA

TABLE III

OTHER SIMULATION RESULTS OF THE DUAL FEED ANTENNA

V. CONCLUSIONS

This paper presents a designing technique for Microstrip Antenna which can be used for ACAS/TCAS application. It is shown that, these proposed antennas provide a better performance like high gain, good directivity and narrow beamwidth compared to that of existing monopole ACAS antenna. This new proposed antenna is also very much light in weight and cost effective. The dual patch microstrip antenna has been designed using the dielectric most suited for the intended frequency of operation. The length and width of the patch has been calculated comply with the space bracket available on the fuselage of the aircraft. The above designed antenna exhibits the parameters similar to the original antenna. This Antenna can be embedded in the body of the aircraft and hence will not offer any resistance to the airflow. Apart from the design considerations it also saves on the maintenance front as it is not directly exposed in the line of the airflow.

REFERENCES

[1] Steve Henely; Rockwell Collins, “The Avionics Hand Book”, 18-TCAS II.

[2] W. H. Harman, “TCAS: A System for Preventing Midair Collisions”, in The Lincoln Laboratory Journal. Volume 2, November 3, 1989.

[3] U.S. Department of Transportation (Federal Aviation Administration), “Introduction to TCAS II”, Version 7.1, February 28, 2011.

[4] D. Orban and G. J. K. Moernaut, “The Basics of Patch Antennas”, Orban Microwave Products.

[5] TCAS S72-1735-25, AIRNC 735, Sensor Systems Inc., Aircraft Antennas since 1961.

[6] Balanies, C.A., “Antenna Theory: Analysis & Design”, 2nd Edition, John Wiley & Sons, Inc., 1997.

Resonating Frequencies

(In GHz)

Beamwidth (In Degree)

Directivity (In dBi)

Radiation Efficiency

(In %)

Power (In Watt) [Input Power 25 W]

Radiating Receiving In CST MWS 2015 Tool

1.03 (Tx fr) 45 9.8 44 8.1 N/A 1.09 (Rx fr) 37 10.4 N/A N/A 13.8

In FEKO Suite 7.0.1 Tool 1.03 (Tx fr) 52 9.1 45 8.05 N/A 1.09 (Rx fr) 50 9.6 N/A N/A 14.3

Resonating Frequencies

(In GHz)

Beamwidth (In Degree)

Directivity (In dBi)

Radiation Efficiency

(In %)

Power (In Watt) [Input Power 25 W]

Radiating Receiving In CST MWS 2015 Tool

1.03 (Tx fr) 75 7.58 49.6 7.85 N/A 1.09 (Rx fr) 58 9.73 N/A N/A 15.1

In FEKO Suite 7.0.1 Tool 1.03 (Tx fr) 80 7.1 47.2 8.25 N/A 1.09 (Rx fr) 60 9.2 N/A N/A 14.7

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