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An Extendable Solar Array Integrated Yagi-Uda UHF

Antenna for CubeSat Platforms

Waleed Alomar*, Jonas Degnan, Steven Mancewicz, Matthew Sidley, James Cutler and Brian GilchristDepartment of Electrical and Computer Engineering

University of Michigan

Ann Arbor, MI USA

E-mail: walomar@umich.edu

Abstract The popularity of nano-satellites in academic,

commercial and government organizations has risen dramatically

in recent years, due largely to their low design and deployment

costs. Despite their many successes, fundamental limits on their

capabilities can be attributed in part to their limited

communications packages. Using 70 cm amateur bands allows for

lower power communications systems, however due to the larger

wavelengths, managing antenna dimensions presents a design

challenge. To improve mission capability, the current dipoleconfigurations, that offer gains of approximately 5 dB, needed

significant improvement. This paper presents a high-gain

solution by incorporating a 6-element linear Yagi-Uda antenna

into a deployable solar array prototype. Measured array gain

was 11.5 dBi with a 53 MHz bandwidth, E- and H-plane half-

power beamwidths of 46 and 58 degrees respectively, and an S11

of -19 dB at 435 MHz.

KeywordsNano-satellite, CubeSat, Yagi-Uda, UHF antenna

I. INTRODUCTIONThe role of miniaturized satellites in space research has

been invaluable as their reduced complexity and expensefacilitated their adoption into many university and business

development programs. Standards of modularization anddeployment mechanisms have also been developed, such asCubeSat specifications and the Poly-PicoSatellite OrbitalDeployer (P-POD). Using these guidelines, considerable effortshave been made to augment CubeSat architectures to improvesatellite performance and mission capabilities. In 2010, theStudent Space Systems Fabrication Laboratory at theUniversity of Michigan developed a prototype for a deployablesolar array system for CubeSats, the eXtendable Solar ArraySystem (XSAS) [1].

The XSAS system is capable of providing an average of 23W of continuous power, occupies a 1.5U (10x10x15 cm)volume and when fully deployed, extends approximately 1.2 m.The expanded area provided a suitable boom to mount a

directional antenna to fully capture the enhanced capabilitiesprovided by XSAS.

Typical CubeSat systems are constrained by theircommunications systems in three significant ways: deploymentaltitudes, transceiver data rates, and ground receiver diversity.Higher gain antenna systems would allow for higher altitudedeployments resulting in less atmospheric drag, andconsequently improved mission durations, without losing the

ability to close the link. Similarly, higher gain CubeSatantennas at typical launch altitudes (approximately 600 km)will be able to maintain uplinks using lower antenna gainground targets, e.g. animal migratory tracking systems.

Several antenna types were assessed based on gain, bandwidth, polarization, ease of deployment and integration,and mass. The required wavelengths made all but helical and

Yagi-Uda antennas practical. The Yagi-Uda was selected dueto the increased complexity necessary to integrate a helicaldesign. Optimization simulations were conducted in AnsoftHFSS to achieve optimal inter-element spacing. Measurementresults revealed a gain of 11.5 dBi, a 53 MHz bandwidth, half-power beamwidths of 46 (E-Plane) and 58 (H-Plane), and anS11 of -19 dB at 435 MHz.

II. ANTENNA DESIGNA. Antenna Design

Designing a high gain antenna at 435 MHz requires larger

antenna dimensions given the 69 cm wavelength (). With

antenna elements roughly /2, each must be stowed for launch

as there can be no protrusion beyond the 1U frame while aCubeSat remains in a P-POD. Once the satellite reached its

orbit, the P-POD will deploy the CubeSat and the elements

will be released and the antenna will be oriented as shown in

Fig. 1.

The unique geometry of the XSAS platform constrained

the element spacing, length and director quantity. Fig. 1 shows

XSAS fully extended at approximately 1.2 m in length with

each solar panel tilted by about 30 degrees. These dimensions

allowed for six elements: one reflector, one driven element,

and four directors. The solar array structure plays an important

role in the antenna design by having a dielectric constant of

4.5 and metallic mechanical connections. Ultimately, the

radiation pattern of the antenna will be affected by XSAS.

In this antenna design, the goal was to optimize the gain,

which is mostly dependent on director spacing, director length,

and the total length of the antenna [2]. Since the CubeSat

length is fixed, length cannot be optimized to improve gain.

Simulations revealed that reflector spacing and length had

little effect on forward gain.

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Figure 1. Photo of fabricated Yagi-Uda array integrated with XSAS

prototype.

The radiator element length also has little effect on forward

gain and was only controlled to match input impedance. Thelength and spacing of the directors had a significant effect on

forward gain, and were most critical for maximizing gain.

Optimization of the gain was determined through a series of

simulations. Director spacing was tested first and a series of

iterations were done to find the highest gain. After finding the

optimal director spacing for high gain, the individual director

lengths were optimized. All optimizations sought to minimizeS11 values throughout the operating frequency range. Table I

contains the optimized design parameter values.

III. ANTENNA SIMULATIONS AND MEASUREMENTSA. Antenna Simulations

An HFSS model of XSAS and a Cubsat has been createdfor the optimization purpose. The model was simplified toreduce computation times by removing inter-panel hinges andarmature fasteners. Early simulation revealed minimal impactto radiation patterns from these components. The overlaidradiation pattern and model are shown in Fig. 2. The finalsimulation results of the S11 and gain of the antenna are in Fig.3 (a) and Fig. 4 (a) respectively.

B. Antenna MeasurementsThe antenna was integrated with XSAS and mounted on a

Styrofoam platform using tape and wooden pegs to hold it inplace for the measurement. The S11 and radiation pattern havebeen measured during this test is shown. Fig. 3 (b) illustratesthat the measured S11 at the center frequency was -19 dB with-10 dB bandwidth of 40 MHz. Fig. 4 (b) and Fig. 4 (c) showthe simulated and measured radiation patterns in the E and H

planes with half power beam widths of 46 and 58respectively.

TABLE I. GAIN OPTIMIZATION LENGTHS FORSIX-ELEMENTINTEGRATED YAGI-UDA ARRAY

ElementSpacing Reflector Driver

Director1

Director2

Director3

Director4

0.250 0.472 0.456 0.438 0.444 0.432 0.404

Figure 2. HFSS radiation pattern simulation of Yagi-Uda array integrated inthe XSAS prototype. Peak simulated gain of 11.8 dB.

(a)

(b)

Figure 3. The simulated (a) and measured (b) S11 data.

The directivity D can be approximated [3] to be

(1)

Based on the simulation, the antenna efficiency can be

approximated to be 0.95, resulting in the gain of 11.5 dBi forthe antenna. A summary of the final antenna measurements areshown in Table II.

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(a)

(b)

(c)

Figure 4. The simulated (a) gain plot and simulated and measured E- (b) and

H-plane (c) radiation patterns.

TABLE II. EXPERIMENT DATA FORSIX-ELEMENT INTEGRATED YAGI-UDA ARRAY

Frequency 435 MHz

S11 Transmitter -18.3 dB

S11 Receiver -21 dB

Source Power 7.8 dBm

Max Received Power -32.17 dBm

Receiver Gain 2.15 dBi

Separation Distance 11.5 m

HPBW (E-Plane) 46

HPBW (H-Plane) 58

IV. FUTURE WORKWhile the antenna measurements proved the potential for

the antennas incorporation into future XSAS development,

the parallel prototyping of both XSAS and this antenna system

hindered efforts to develop proper mounting and deployment

mechanisms. Availability, cost and legacy drove the antennaelement material selection and in a stored configuration, the

cross-curved steel strip elements increased XSAS depth by

approximately 17 mm, necessitating the removal of panels to

maintain CubeSat specifications. HFSS simulations using

lower profile 4 mm diameter cylindrical elements showed no

significant impact on antenna performance. Development of

an antenna deployment system that incorporates the XSAS

cutter system is also needed. Examination of atmospheric drag

effects must also be conducted to determine if a more suitable

element design is necessary.

V. CONCLUSIONThe integrated Yagi-Uda antenna provided a considerable

improvement in antenna performance for CubeSat platforms. Next generation CubeSats incorporated with XSAS and thissystem will experience vastly improved capabilities, allowingfor increased experiment and mission complexity.

ACKNOWLEDGMENT

We would like to thank Professor James Cutler for hissupport and sponsorship of this project as well as the XSASteam at the University of Michigan for their assistance and forallowing our team access to their prototype for testingpurposes.

REFERENCES

[1] Senatore, P., Klesh, A., Zurbuchen, T., McKague, D., & Cutler, J.(2010). Concept, Design, and Prototyping of XSAS: A High PowerExtendable Solar Arrar for CubeSat Applications. 40th AerospaceMechanism Symposium. Ann Arbor, MI: University of Michigan.

[2] Balanis, C. (2005). Antenna Theory: Analysis and Design. Hoboken,New Jersey: John Wiley & Sons Inc.

[3] Kraus, J. D. (1988). Antennas. New York: McGraw-Hill Publishing.

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