An Approach to Design and Embed a Printed

An Approach to Design and Embed a Printed

Dipole Antenna inside UAV Empennage

Diptiman Biswas, Sagar Sen, Vamshidhar S, Ramachandra Vulapalli

Aeronautical Development Establishment

DRDO, Ministry of Defence, Govt. of India

New Thippasandra, Bengaluru - 560075

Abstract – This paper describes about the design and development of a Printed Dipole Antenna in UHF band and a procedure to embed the antenna inside the vertical fin of the empennage of a UAV (unmanned aerial vehicle) system. The effect on the impedance and radiation characteristics of the antenna inside the FRP fin has been studied and optimised through comprehensive analysis.

Introduction

The system requirement necessitates an onboard antenna featured with omni directional radiation patternin the yaw plane of the UAV for providing essential wireless datalink communicationwith the GCS (ground control station).However, the fuselage and wings of a complex shaped UAV contributes to distort the omni pattern of the antenna when it is mounted directly on the surface. This may restrictthe operation of continuous and uninterrupted data link when the UAV is expected to fly and maneuver at various aspect angles with respect to the GCS.

The first level of solution approach to this problem is to elevate the onboard antenna appropriately from the surface of the UAV fuselage for obtaining maximum radiation clearance required for line-of-sight communication between the two. Further, there is a possibility of unwanted spurious radiation generated due to various other onboard subsystemswhich may likely to interfere and degrade the performance of a sensitive RF data link receiver. In the present case, we experienced similar problem and elevating antenna alone could not ensure effective communication for receiving command information.This was a crucial issue and to overcome the limitation, we considered to relocate the onboard receiving antenna such that the influence of interference be minimum. A comprehensive study suggested that the antenna near empennage could be a prominent location to satisfy both radiation and interference issues. Embedding an omni antenna inside the vertical fin would provide additional advantage of not being projected outside of the UAV airframe and accordingly no associated aerodynamic drag to be accounted for.

We explored the feasibility of embedding a printed antenna at the top location of the vertical fin of the UAV and carried out required study and analysis for providing solution to a practical system.

Design of Printed Dipole Antenna (PDA)

A Printed Dipole Antenna is a planar and compact radiating device which is used for the wireless communication of vertically polarised radio signal with omni directional radiation pattern. For airborne platform such as a UAV system, the PDA need to be appropriately oriented with necessary encapsulation within aerodynamically shaped radome and with proper reinforcement so as to it can withstand required level of aerodynamic drag, vibration etc.

Fig.1. Schematic of multi-layer Printed Dipole Antenna

The dipole is printed on both sides of the substrate with two radiating arms designed to be very compact so that it can be embedded easily inside the vertical empennage. The schematic of the optimised Printed Dipole is shown in Fig.1. The radiating arms each of length, L are oriented in opposite direction and are open ended. However, the folded and printed ‘balun’ at both sides are of identical geometry and are overlapped. The other end is for feeding through 50  coaxial connector.

Fig.2. Simulated azimuth plane pattern of the PDA

Fig.3. Simulated elevation plane pattern of the PDA

Fig.4. Simulated 3D pattern of the PDA

The length of the PDA has been reduced for making it compact and optimised to be 20% smaller than its usual length of /2 at the operational center frequency. The design has been simulated using Feko 6.1. Simulated radiation patterns of the antenna at its azimuth and elevation planes are given in the fig. 2 & 3 respectively. Fig.4 shows the volumetric radiation pattern of the antenna along with the printed dipole antenna as reference.

Bandwidth Enhancement

There is a need to increase the operational bandwidth of the PDA for meeting the required specification. The width of the radiating arms contributes significantly for the bandwidth of the antenna. A study has been carried out by varying the width, W of the radiating arms in sub-multiple fraction of its length, L. VSWR plots with varying widths have been shown in Fig.5. The dependance of bandwidth as a function of antenna width is shown in Fig.6. Bandwidth increases with the width, W of the dipole arm. The PDA with narrow radiating arms produces a bandwidth of merely 10% of its center frequency. The bandwidth can be increased more than 30% of its center frequencywhen the width of the dipole arm becomes 25% of its length, L.

Fig.5. VSWR Plots of PDAs with varying arm width, W

Fig.6. Bandwidth of PDA Vs. width, W of radiator

Antenna Development

The design of printed dipole antenna has been optimised and fabricated by photo etching of the art work on both sides of copper cladded substrate with dk=2.5 and thickness=1.6mm as shown in Fig.7.

Fig.7. Fabricated PDA indicating printed art-work

at both sides of the substrate

Fig.8. Azimuth plane pattern of the fabricated PDA

Fig.9. Elevation plane pattern of the fabricated PDA

A 50TNC (F) type connector is used appropriately to energise the antenna. The experimental radiation patterns in the azimuth and elevation planes measured in anechoic chamber are given in Fig. 8 & 9 respectively. The experimental VSWR plot of the antenna is shown in Fig.10. When compared with the simulated bandwidth of the PDA, it was found that theoperational bandwidth of the antenna is exactly same that of simulated result i.e. 32% of the center frequency.

Fig.10. Measured VSWR plot of the fabricated PDA

Embedding Methodology

The embedding of printed dipole antenna inside empennage is similar to designing a radome for the antenna to ensure minimum deviation in the original characteristics of the antenna. The presence of dielectric material in the vicinity of the PDA influences the behaviour of the antennawhich may degrade its over all performance. The material properties of radome mainly its dielectric constant (dk) and its thickness are of important concern and need to be optimisedforminimizing unwanted distortion in the radiation pattern and also a noticeable shift in theoperational frequency band of the antenna.To predict performanceof the radome i.e. empennage material in the present case,we need to carry outelectromagnetic study of the effect of theembedding material on antenna.A comprehensive simulation study has been carried out using Feko considering realistic material properties of the empennage of the UAV to quantify their effects and arrive at an optimum configuration.

Fig.11. VSWR plots ofPDA with different embedding material

The study of materials with varying dielectric constants has been compared in Fig.11. It is observed that embedding material with higher dk, the VSWR response of the antenna gets affected adversely. Even with low dk values, there is a marginal amount of shift in the resonating frequency of operation.Further, considering the structural need for providing necessary strength,a fin with low dielectric foamof one inch thickwith the outer covering skinof 2 mm made of FRP found to be appropriate for embedding the PDA. This combiation of empennage material has been simulated along with the PDA sandwiched in between and found to be suitable electrically. As shown in Fig.11,anominal shift in the operational frequency band resulted due to embedding of the antenna with the optimised configuration, however, within acceptable operational limit as the overall operationalbandwidth of the antenna is sufficiently high.The radiation pattern of the antenna as embedded inside the vertical empennage of the UAV has been simulated and its sectional view is shown in Fig.12.

Fig.12. Sectional 3D view of the simulated pattern of the PDA as embedded

V.Conclusion

A compact Printed Dipole Antenna has been designed developed and embedded inside the vertical empennage of a indigenous UAV system. The configuration has improved antenna performance as it is elevated from the fuselage and also minimised the influence of interference as the antenna is placed far from the onboard interfering system.

Acknowledgment

The authors express their sincere thanks to Sri P. Srikumar, Outstanding Scientist and Director, ADE andSri S. Sampath Kumar, Sc ‘G’ and Group Director for their encouragement and constant support to carry out the work and also for permitting this paper for publication and presentation at the conference.

References

[1]Diptiman Biswas, Sagar Sen, Nataraj B, Ramachandra V, “An Analytical Approach for Designing Compact Printed Dipole Antenna in S Band”, Proceedings of 4thIEEE-AEMC-13; Bhubaneshwar, Dec, 2013.

[2]Diptiman Biswas, Krishna Prasad D.S, Sagar Sen, Ramachandra V, “Design of a Novel Printed Dipole Antenna and its Placement Analysis on Indigenous UAV Platform”, Proceedings of 6th Conf. ATMS-13; Kolkata, Feb, 2012.

[3]Mohammad S. Sharawi, Daniel N. Aloi, Osamah A. Rawashdeh, “Design and Implementation of Embedded Printed Antenna Arrays in Small UAV Wing Structures”, IEEETransactions on Antenna and Propagation,Vol.58,No.8,August2010.

BIODATA OF AUTHORS

Diptiman Biswas:Graduate in Electronics Engg from NIT, Jamshedpur in 1993. M. Tech in Microwave Engg. from IIT, BHU, Varanasi in 1995. Joined ADE as Scientist in 1996. At present as Head of Antenna Lab, his prime domain of research work includes configuration design of antennas for various indigenous UAV systems. Using innovative & conceptual approach of design, he has produced a number of critical antennas. He has authored a number of technical papers and presented at various conferences.

Sagar Sen: B.E.in Electronics Comn. Engineering (2013) from C M R Institute of Technology, Bangalore under Visvesvaraya Technological University, Belgaum. Presently working as Graduate Trainee in ADE, DRDO, Bangalore in the area of design and simulation of antennae for various airborne applications.

Vamshidhar S: M.Sc(Physics) in 2006 from University of Hyderabad and M.Tech (Solid State Technology) in 2008 from IIT, Kharagpur.Joined ADE as a Scientist in the year 2008. He is working in the specialised areas of RF and Digital Communications for various UAV systems.

Ramachandra Vulapalli: Post Graduate from Kakatiya University (1980) and Ph.D. from BHU, Varanasi (1988). Joined as a Scientist at ADE in the year 1984. Presently Scientist ‘G’ and Head of Flight Test Telecommand & Tracking Division. He has invented and productionised a number critical products such as Luneberg Lens, DMDI Scoring System forvarious indigenous UAV systems. He has designed and developed number of import substitutes at ADE. Most of his designs / products have been productionized and the items are being used by Indian Armed Forces. He has more than 150 technical publications and internal reports to his credits.He is a member of IEEE.