Evaluation and Field Testing of an Embedded Antenna in a Small UAV Wing Structure Mohammad S. Sharawi1 , Osamah A. Rawashdeh2 and Daniel N. Aloi2 1
Electrical Engineering Department, King Fahd University for Petroleum and Minerals, Dhahran, 31261 Saudi Arabia, Email:
[email protected] 2 Electrical and Computer Engineering Department, Oakland University, Rochester, Michigan 48309, USA, Email: {rawashd2,aloi}@oakland.edu attractive features such as low cost, low profile, and ease of integration. They posses acceptable gain and radiation efficiency [3], [4]. The use of printed embedded antennas within structural components can reduce weight; thus gaining more flight time and build smaller UAVs. It lowers manufacturing costs, and makes the system more robust by eliminating external components that can be damaged by user or during take offs/landings. Finally, they will reduce drag which will provide longer flight times. This paper presents a small size embedded antenna that is a part of the wing structure of a small UAV, and compares the performance of the antenna against that of a standard wire antenna in the 2.45 GHz ISM band in two environments. The assessment is based on a loop back data transmission test using a ZigBee transceiver on the UAV and the ground station. The paper is organized as follows; Section II illustrates the modeling and construction of the printed monopoles as part of the wing structure for a UAV. Section III presents and discusses the testing environment and measurement results. Finally, Section IV concludes the paper.
Abstract— Unmanned Aerial Vehicles (UAV) are extensively being used in exploration, surveillance and military applications to collect data via special sensors and send the data back to a central station via wireless links. Printed antennas embedded in wing structures will eliminate the drag due to friction, and allow for extended load capability due to their extra light weight. In this work, we assess the performance of an embedded small size monopole antenna within the wing structure of a UAV and compare its performance in two environments with a standard wire type monopole inside the fuselage. Their performance is similar with an average difference of 6% in the open field test.
Index Terms— Printed Monopoles, Embedded Antennas, UAVs.
I. I NTRODUCTION Unmanned aerial vehicles (UAV) are widely used in military and exploration missions. They are used routinely to collect and send information back to a ground station that provides real-time information on the covered area. Flying UAVs have several advantages over their land counterparts, primarily their ability to cover a wider area. The data transmission from both kinds of UAVs (flying and land based) is done via wireless links. Single antennas and antenna arrays are being used for sending the data back to the ground station for analysis and decision making [1], [2]. Current vehicle antennas rely on whip/single element antennas which raise aerodynamic and visual signature concerns. These concerns become even more rectified when several antennas are required to cover multiple bands (and/or functions), creating larger structures, with higher cost, complex mechanical issues and possible interference. Printed antennas have been widely used in a variety of industries. For example, they are used in computers for wireless local area network (WLAN) connectivity in the 2.4 GHz (802.11b,g) and the 5 GHz (802.11a) bands. Also, printed patch antennas are used in automobiles as global positioning system (GPS) and satellite digital audio radio system (SDARS) antennas. Printed antennas are also used in RFID systems and in cellular handsets. They have very
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II. M ODELING OF P RINTED E MBEDDED A NTENNAS A sample wing from a hobby airplane (Mini-Telemaster) is shown in Figure 1. A single wing consist of 9 structural wing slots that are attached together via several beams and thin curved sheets that are all made of light wood (Balsa wood). The full length of the two wing sections (left and right) is 114.3 cm (45 in.), the UAV length is 82.6 cm (32.5 in.), its flying weight is 20 oz. and its wing loading is 9 oz. per square ft. Towards the center of each wing slot, there is a hole that is used to pass an extra supporting wing rod. In the Mini-Telemaster model used for this work, the holes are used to pass the feeding cable to the embedded printed antenna instead of the supporting wing rod. The wing slot structure with an embedded printed Lshaped antenna is shown in Figure 2. The printed antenna is fed via a coaxial cable coming from the WLAN frontend within the control unit of the UAV. The dimensions of the wing slot and the shape of the monopole L-shaped
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A wing section from a Mini-Telemaster UAV.
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frequency of the simulation was 2.423 GHz and the measurement was 2.5275 GHz resulting in an error of about 4 %. The 3dB difference on the low end as well as the ripple shown are attributed to the return loss measurement point and the way the feed was attached, respectively. The return loss measurement was conducted at the end of a 2 ft. RG-361 cable while in the simulation model it was done at the input of the antenna element, this is beleived to have contributed to the lower measurement levels. The ripple is attributed to the hand soldering mechanism that was used to attached the feeding cable to the antenna input. The x-z plane cuts for the co-polarized (co-pol) and crosspolarized (cross-pol) gain patterns at 2.45 GHz are shown in Figure 4.b. Figure 4.c shows the co-pol and cross-pol gain patterns in the y-z plane for 2.45 GHz. The center of the polar plot corresponds to -40 dBi. The gain substitution methods was used to calculate the gain of the antenna. A standard gain reference antenna was used. The maximum gain of the L-shaped antennas was 1.4 dBi. A passive small size antenna is not expected to have a high gain value. Increasing the size of the GND plane will slightly increase the amount of gain obtained by about 0.5-1.0 dBi as has been demonstrated in [5] (maximum gain of 2dBi was obtained at 2.5 GHz). The cross polarization for each frequency is shown in the dashed line within each gain pattern plot. Thus, given the 2/3 size reduction of the L-shaped embedded monopole compared to its T-shaped counterpart presented in [5], the proposed antenna reduced the cost of fabrication (size) without a significant sacrifice in gain. It is worth mentioning that the maximum gain of the wire antenna used was 1.5dBi [8]. The embedded antenna was mounted on the wing structure with a 2 ft. RF cable that connected it to the ZigBee transceiver unit as shown in Figure 5. The wireless link was tested by testing the number of received packets from a loop back test as a function of distance. The ground station consisted of a laptop computer with a transceiver module and a serial loop back packet generator, while the UAV transceiver was connected to the embedded antenna (or the wire antenna) with a serial loop back interface as well. The test was conducted in two environments: 1) outdoors in a parking garage, and 2) outdoors in an open field. Although a UAV is used in an open sky environment,
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antenna are shown in Figure 2. The wing slot with the embedded printed antenna was made from FR-4 material with a thickness of 0.8 mm, r = 4.8 and tanδ = 0.017. The microstrip embedded antenna had a width of 1.5 mm to have a 50 Ω input impedance. The antenna was excited using an RG-136U coaxial cable. The length of the monopole main arm M SL was 33 mm, and the bent had a length ARM = 7 mm. The GND plane on the bottom layer (GN DL ) extended to 17.5 mm from the center of the through hole. III. R ESULTS AND D ISCUSSION The embedded antenna was prototyped on a thin FR4 substrate for ease of integration within the wing structure and to provide light weight. It was modeled using FEKOT M , a full wave Method-of-Moments based 3-D solver. The antenna main arm length (M SL ), GND length (GN DL ), and arm length (Arm) for the L-shaped geometry were varied to obtain the best values that satisfied the bandwidth and resonant requirements keeping the size limitation into consideration. Compared to the results shown in [5] and [6], the fabricated miniaturized antenna occupied an area of 20.8×38 mm2 . This shows an area saving of 65 % and 85% to the antennas presented in [6] and [5], respectively. The length is taken at the edge of the antenna element. The prototyped antenna is shown in Figure 3. Figure 4.a shows the simulated and measured return loss of the L-shaped embedded monopole. The resonance
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the two tests conducted were made to compare the performance vs measurable distance in a repeatable fashion. In each test, 1000 round-trip packets were sent between the ground station and the UAV module. The percentage of the received versus lost packets is displayed as a function of distance between the UAV and the ground station. Figure 6 shows the open parking environment while Figure 7 shows the open field environment as presented by Google earth with the data collection points and their distances relative to one another. The location of the UAV was determined using a mobile GPS module. Figure 8 shows the percentage of received packets versus distance for the simple wire monopole that is shipped with ZigBee kits and the designed embedded in-wing antenna. The performance of the wire antenna was very close to its embedded counterpart with a maximum difference of 1.5% in the parking garage. This environment had a direct line of sight between the transmitter and the receiving antennas with a few trees on the route and cars on the sides. In
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Fig. 4. L-shaped Monopole embedded antenna results. (a) Simulated and Measured |S11 |. (b) Measured Co-Pol (solid) and Cross-Pol (dashed) gain patterns at 2.45 GHz (x-z plane). (c) Measured Co-Pol and Cross-Pol gain pattern at 2.45 GHz (y-z plane).
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average from its counterpart. This small variation might be accepted as a trade of for cost and improving the UAV aerodynamics.
the open field, the transmitter was on a hill top (about 10 ft. difference with respect to the lowest point) and the receivers were moved according to the points shown in Figure 7. The orientation of the embedded antenna was facing the transmitter (θ = 0o ). The performance of the two antennas was very comparable with average difference of 6% over the 7 points taken in the 1500 ft. distance. This is the zone where the lowest point with respect to the hill top was reached, and the embedded antenna showed the highest degradation compared to its wire counterpart, which shows some directive performance of the embedded antenna on the UAV. At the end of the field, a better line of sight was obtained (in a real flying environment, a line of sight is almost always maintained, since the sky is assumed to be clear), and better reception was obtained. It is worth noting that given the advantages of embedded antennas mention in the introduction, the slight difference in performance might be tolerable. One antenna on each wing side can be utilized to accomplish better reception and enhance the radio link performance. This work will be extended to implement and investigate the design of embedded printed antenna phased arrays on UAV wing structures. Antenna arrays will enhance the radio link performance and increase the range of the UAV.
ACKNOWLEDGEMENTS The authors would like to thank the Michigan Space Grant Consortium and the National Science Foundation (NSF) major instrumentation program, 2005, for funding this research. Also, they would like to thank Mr. Bilal Sababha and Mr. Hong Yang for their help in performing the on plane tests. R EFERENCES [1] D. K. Jackson, et. al., ”Evolution of an Avionics System for a HighAltitude UAV,” Proceedings of the AIAA Infotech at Aerospace Conference, Sep. 2005. [2] A. Simpson, et. al., ”Big Blue II: Mars Aircraft Prototype with Inflatable-Rigidizable Wings,” 43rd AIAA Aerospace Sciences Meeting and Exhibit, Jan. 2005. [3] C. A. Balanis, Antenna Theory, 3rd Edition, Wiley Interscience, 2005. [4] G. Augustin, et. al., “Compact dual-band antenna for wireless access point,” Electronics Letters, Vol. 42, No. 9, April 2006. [5] Y. L. Kuo and K. L. Wong, “Printed Double-T Monopole Antenna for 2.4/5.2 GHz Dual-Band WLAN Operations,” IEEE Transactions on Antennas and Propagation, Vol. 51, No. 9, pp. 2187-2192, September 2003. [6] H. M. Chen and Y. F. Lin, “Printed Monopole Antenna for 2.4/5.2 GHz Dual-Band Operation,” IEEE International Symposium on Antennas and Propagation, Vol. 3, pp. 60-63, June 2003. [7] EM Software and Systems Ltd. (EMSS), FEKOT M User Manual, South Africa, July 2005. [8] XBee Pro. Data sheet, Digi International Inc., http://www.digi.com.
IV. C ONCLUSIONS The design and fabrication of a printed small size monopole antenna embedded in an small UAV wing slot operating in the 2.4GHz ISM band was performed and demonstrated. Embedded antennas will provide several advantages for UAVs; reduced weight, lower costs and lower drag. The antenna performance is compared against a commercial wire antenna in two environments to access its performance in a wireless communication link. The performance of the embedded antenna was 6% lower on
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