This article presents a design, development, and testing process of a deployable right-hand circular polarization (RHCP) helical antenna for a small satellite in the UHF band. In the stowed configuration the antenna is 5cm and deploys to an operational length of 74cm. Deployment of the antenna is controlled by a DC motor and once released, extends to its full shape under spring power, a 3-turn jack-in-the-box design. The antenna is tuned and matched to correct frequency bandwidth and features an optical deployment confirmation circuit. The antenna produces an axial mode, with an average beamwidth of 60° and this is achieved by winding the helix to the right pitch and spacing. At range testing the antenna achieved an average gain of 7dBi through the frequency and an axial ratio of less than 3dB

Castro, Alejandro

Hill, Brendan

Hiser, Ryan

Mozaffar, Nazia

Obukhov, Dmitriy

Sadowski, David

English

DigitalCommons@USU

Castro 1 37th Annual Small Satellite Conference SSC23-IX-02 Design and Build Process of a Small Satellite Deployable UHF Antenna  Alejandro Castro, David Sadowski, Dmitriy Obukhov, Ryan Hiser, Nazia Mozaffar, Brendan Hill NIWC Pacific 53560 Hull Street, San Diego CA 92152; 619-254-4617 alejandro.t.castro.civ@us.navy.mil  ABSTRACT This article presents a design, development, and testing process of a deployable right-hand circular polarization (RHCP) helical antenna for a small satellite in the UHF band. In the stowed configuration the antenna is 5cm and deploys to an operational length of 74cm. Deployment of the antenna is controlled by a DC motor and once released, extends to its full shape under spring power, a 3-turn jack-in-the-box design. The antenna is tuned and matched to correct frequency bandwidth and features an optical deployment confirmation circuit. The antenna produces an axial mode, with an average beamwidth of 60° and this is achieved by winding the helix to the right pitch and spacing. At range testing the antenna achieved an average gain of 7dBi through the frequency and an axial ratio of less than 3dB.INTRODUCTION Space-based observations using spacecraft change our understanding about the universe. Small platforms such as CubeSats are designed and developed to perform various experiments in space in a more rapid and affordable manner. Crosslink communications done by spacecraft operations use two or more satellites to relay data from one station to the next: earth-to-space, space- to-space, etc. CubeSat communication systems take advantage of many satellites linked together.1 These constellations can comprise of dozens to thousands of satellites. The continuing growth of the small satellite space is reflected by design improvements that many universities and start-up companies achieve in developing high performance hardware that meet mission criteria for a system.  For example, last year’s Small Satellite Conference featured an origami-inspired folded antenna complete with a pop-up deployment mechanism.2 Many antennas were developed to accompanied these mission requirements specifically frequency and bandwidth requirements. However, the main obstacle for antenna designers has been the size of the spacecraft.3 CubeSats are small spacecraft that have dimensional limitations to support various antenna parameters. Antennas which can exhibit specific polarization, adequate bandwidth, and reasonable gain require a relatively large “real-estate”, which may not be available on a CubeSat. ANTENNA DESIGN REQUIREMENTS Antenna Specification Naval Information Warfare Center (NIWC) Pacific is participating in the DoD Space Test Program STPSat-7 experiment, scheduled to launch in 2024. NIWC’s payload is a relay-mode transceiver designed to detect and correct measured Doppler frequency offset in both phase-shift keyed and frequency-shift keyed signals. NIWC selected the UHF band for its experience, and this is required the design and construction of an appropriate antenna as shown in Table 1. Table 1: Antenna Desired Parameters Parameter Value Band of Operation UHF Percent Bandwidth 28% Gain +3 dBi in main lobe Beamwidth of Main Lobe +/- 30 degrees Polarization RHCP Magnitude of S11 -10 dB or lower in desired band Diameter Limit 15 inches Maximum Stowed Height 3 inches Deployable Helical Antenna Solution Helical antennas offer real solution for the desired parameters. This antenna can provide more than 50% bandwidth and reasonable gain.4 Although this antenna can be unreasonably long for a given gain. Deploying antennas that exhibit a gain ≥ 3dBi, circularly polarized, wide bandwidth on the UHF-band and attach to CubeSat platform increases the potential and capabilities of the communication mission. Several antenna types have been proposed for this application such as dipole and patch antennas.5,6 However, these Castro 2 37th Annual Small Satellite Conference types of antennas exhibit too narrow bandwidth, incorrect polarization, or too low relative gain. In UHF-Band for nanosatellite in Low Earth Orbit (LEO), a helical antenna is a good candidate for the payload system. Helical antenna can naturally produce a Circular Polarization (CP). Circular polarization is important to a communication link. Communication links with CP have no issues of Faraday rotation effect and require no polarization alignment between satellites and ground terminals. Mechanically structured helical antennas can be easily compressed, stowed, and deployed. In order to achieve a RHCP, a helical design was selected.  The antenna was intended to be placed on the nadir face of the satellite, so it was desired to have its main lobe in the same direction.  The ratio of the antenna’s circumference to the wavelength was chosen to be between 3/4 and 4/3, and the spacing between its turns was chosen to be a quarter-wavelength.  These choices were intended to achieve an axial mode pattern.4  A wide beam for the main lobe was desired in order to close the communication link for as much of the satellite’s overflight as possible.  The balance between beam width and gain chosen by this design permits a relay-mode operation. A frequency translating relay lowers the signal-to-noise ratio of the signal it receives because part of what it transmits to the ground station receiver is frequency-translated noise it sees on its own receiver.  ANTENNA FABRICATION Successful deployment of a stowed helical antenna using a jack-in-the-box mechanism requires the presence of a nonconductive material surrounding the radiating element (‘the bag’).  This material counteracts the spring force of the radiating element, allowing the antenna to remain rigid and with the correct geometry in its deployed state. A proof-of-concept item was constructed with cotton cloth with a hand-bent wire sewn into it.  Testing showed that even with very loose manufacturing tolerances, the target antenna parameters could be met with the design structure seen in Figure 1. A flight-rated material for bag was carefully selected based on availability, manufacturability, UV resistance, atomic oxygen corrosion resistance, and outgassing characteristics. Considered materials included polyimide, aramid, UHMWPE, and PTFE coated fiberglass. Polyimide and UHMWPE exhibited too much atomic oxygen corrosion.7 Prototypes bags made of aramid and PTFE coated fiberglass were evaluated, with PTFE coated fiberglass showing the best performance for UV resistance, workability, deployability, and low density. Aramid exhibited degradation in UV during manufacturing and requires excess processing to prevent fraying. As a woven material, aramid was easier to compress and stow than a laminated material, the PTFE coated fiberglass. To compensate for this, cutouts were added to the PTFE coated fiberglass bag design. Flight bags are fabricated in house and sewn with PTFE coated fiberglass thread. The radiating element is fabricated in-house out of tempered spring steel and is gold plated for solderability and corrosion resistance. As formed, the helical pitch of the radiating element is greater than the functional, on-orbit configuration. When sewn into the bag, the radiating element is compressed by 60%, preloading the structure and adding rigidity.  Extra attention is given to the feed point on the radiating element. As the antenna is deployed and stowed, the helix angle of the radiating element changes. This requires the connection between the radiating element and the antenna circuit board to pivot. To allow this, a straight section of wire is added to the feed end of the radiating element as shown in Figure 2. This straight section acts as a torsion spring allowing the helix angle of the radiating element to change while the solder connection to the circuit board remains fixed.  Stabilizing flanges and potting epoxy are added to the feed point to further reinforce the solder connection and provide reaction force. The length of the straight section of wire was calculated based on material properties, twist rate, and spring force generated by the radiating element.  Figure 1: Antenna Components Seen in Deployed Configuration Castro 3 37th Annual Small Satellite Conference  Figure 2: Antenna Feed Point Detail, left-side “T” – matching network and right-side copper plate  Figure 3: Antenna in Stowed Configuration Deployment Mechanism In the stowed configuration in Figure 3, the helical element and bag is compressed to a height of ~30mm. The spring force of the compressed radiating element is contained by a rigid, fiberglass plate. The fiberglass plate is secured to the antenna base by a rotating, threaded shaft that is powered by a small DC motor. On orbit, the antenna is deployed by powering the motor causing the rotating shaft to unthread from the plate. Once the shaft fully unthreads, the spring force of the compressed radiating element (~20N) causes the antenna to deploy. Due to the radiating element being preloaded when sewn into the bag, the antenna remains deployed and rigid in its functional configuration. Tabs in the fiberglass plate key into the antenna base, preventing shear loads being transferred to the threaded shaft and motor. These tabs also provide reaction force to the torque of the rotating, threaded shaft and provide mounting locations for remove before flight (RBF) devices. Testing of the deployment mechanism in gravity requires a gravitational compensation system. While the radiating element acts as a spring and is preloaded for rigidity, it is not enough to hold the antenna in a rigid, deployed state in gravity. A simple pulley system with a counterweight is used to compensate for the effects of gravity.  The deployed version of the prototype is shown in Figure 4.  Figure 4: Fully Assembled Antenna Prototype Printed Circuit Board Elements The functions of the antenna printed circuit board are to provide a feed point and impedance match, power the motor, and report to the main payload on the location and Castro 4 37th Annual Small Satellite Conference temperature of the antenna during deployment.  Deployment begins with application of 28V from the satellite main bus power.  This powers several DC-DC converters which lower the voltage to 12 V for the motor and to 5 V for the temperature and proximity sensor circuits.  The temperature and proximity sensors connect to the main payload via a 40-inch cable of differential I2C lines.  On either side of this cable, a circuit convers the parallel lines to single-ended lines and provides buffering. The locations of the proximity sensors were chosen to concentrate their infrared beams on the spokes of the rigid fiberglass plate.  When the power is first applied to the antenna board, the proximity sensors sense the spokes located only 4 cm away.  Figure 5 shows the sensor locations and their field of view when the antenna is fully deployed.  Under this condition, the sensors are likely to report 50 cm instead of the full distance of 72 cm because of the intersection of the third turn of the helix element and the presence of the containment bag.  The team is experimenting with manipulating the calibration of the proximity sensor in order to deliberately restrict the field of view.  This is necessary to reliably measure the full distance between the printed circuit board and the top of the antenna hat after deployment. A separate power port is provided to permit the motor to operate in reverse.  Power is applied via a 9-pin Omnetics Micro-D connector.  This port is diode-blocked from the main DC-DC converters.  The connector is on the top surface of the board, which is the same side that the antenna radiating element is attached to.  It is accessible via the outside of the spacecraft, and can be seen in Figure 6.        Figure 5: Proximity Sensor Locations and Field of View After Antenna Deployment  Figure 6: Antenna Printed Circuit Board (Top View) Most components are populated on the bottom surface of the board, the side of the board facing the interior of the spacecraft (see Figure 7).    Figure 7: Antenna Printed Circuit Board (Bottom View) Impedance Match and Return Loss Measurement As shown in Figure 2, a T-network matching network composed of two capacitors and an inductor is used to bring the impedance match to close to a perfect load as seen at the SMA connector. The inductor helps compensate with the overlength of the antenna and the two capacitors dictate the lower and upper bounds of the frequency bandwidth. The right side of the photo shows a long copper plate tape that helps with the bandwidth of the matching due to the frequency dependency of most lumped components. Figure 8 shows the theoretical magnitude of S11 vs. frequency, and Figure 9 shows the measured network analyzer data plotted in MATLAB. Both simulated and measured S11 verify the matching bandwidth. The antenna was simulated without the long aluminum tape or the extra length of wire at the feed point.  Castro 5 37th Annual Small Satellite Conference Figure 10 shows the axial ratio of the measured antenna is ≤ 3 dB between from midband to the upper end of the desired frequency range, but is ≤ 5 dB between 15 MHz below midband through the rest of the range. This axial ratio could have been better due to the nature of helical antenna. However, a compromise is needed for antenna performance versus antenna deployment. Nevertheless, this did not affect the gain performance of the antenna as shown in Figure 11. The gain of the antenna averages between 7 dBic to 8 dBic between 30 MHz below midband 60 MHz above midband. It is also observed that even with relatively high axial ratio the separation between the RHCP and LHCP is still reasonable.  Figure 8: Simulated |S11| vs. Frequency of Antenna with Impedance Match Circuit  Figure 9: Actual |S11| vs. Frequency of Antenna with Impedance Matching Circuit Seen on Vector Network Analyzer   Figure 10: Measured Axial Ratio vs. Frequency    Figure 11: Measured Gain vs. Frequency   Gain Pattern Testing The antenna pattern was taken using an outdoor as shown in Figure 12. The antenna is position on top of a “mock” satellite with the same dimension and is off center. The off-center position is due to the location of the antenna on the actual spacecraft. The antenna is slightly off center due to sharing space with another two S-Band antennas. The antenna is also lifted up away from the turn table to emulate the small ground plane effect on the measurement. The antenna device under test (DUT) was placed on a platform that can be rotated under motor control. A calibrated test antenna is positioned at a known distance.  Horizontal and vertical cuts of the antenna gain pattern are achieved by keeping the calibrated test antenna stationary and positioning the DUT on its back or its side.   Castro 6 37th Annual Small Satellite Conference  Figure 12: Outdoor Measurement Multiple gain pattern tests were conducted on the design in order to validate the performance at each stage of development.  The first tests used a rigid helix with a large ground plane in order to demonstrate that the beamwidth and bandwidth could be achieved.  An unpopulated circuit board was used for the next series of tests in order to make sure the gain pattern still met specifications with the reduced ground plane.  An antenna mounted on the populated circuit board and with the Kevlar containment bag was used in the next series of tests, and these results are the most recently available.  These tests used 3-D printed plastic in substitution for non-conductive Polyetheretherketone (PEEK) fabricated parts. Figure 13 and Figure 14 show the measured results of the antenna at the Lower Band and the Higher Band, respectively. The antenna measurement demonstrates that the beamwidth is shy by a couple degrees to meet the requirement at the lower band but is achievable in the higher band. It is also measured that the antenna’s main beam is slightly tilted and this is due to the asymmetry position of the antenna on the satellite.  As of the writing of this article, the final series of tests, which include the PTFE bag, populated circuit board, PEEK fabricated antenna elements, motor, motor mount, and gear box have been collected and the results are being analyzed.    Figure 13: Gain vs. Elevation, 0-Degree Cut, Lower Band  Figure 14: Gain vs. Elevation, 0-Degree Cut, Higher Band CONCLUSION The design process described in this article involved a combination of mechanical and radio frequency engineering requirements to achieve a UHF antenna design realization suitable for spaceflight.  Important material considerations and antenna features motivated decisions throughout the design process.  Repeated testing at each stage of development validates the antenna design and allows characterization of the effects of the mechanical parts of the deployment system on the gain pattern and impedance match.  The resulting antenna meets the requirements set out by the design team.   Acknowledgments The authors wish to acknowledge NIWC Pacific’s Naval Innovative Science and Engineering (NISE) program for their continued support of the project that led to this paper and to the papers submitted in previous Small Satellite Conference proceedings. References  1. T. F. C. Leao, V. Mooney-Chopin, C. W. Trueman and S. Gleason, "Design and Implementation of a Diplexer and a Dual-Band VHF/UHF Antenna for Nanosatellites," IEEE Antennas and Wireless Propagation Letters, vol. 12, pp. 1098-1101, 2013. 2. Tomura, Takashi et. al., “Two-layer Pop-up Origami Deployable Membrane Reflectarray Antenna Stowed in 1U CubeSat,” Small Satellite Conference, Logan UT, August 2022. Castro 7 37th Annual Small Satellite Conference 3. A. Narbudowiz, R. Borowjec, and S. Chalermwisutkul, “No-Need-To-Deploy UHF Antenna for CubeSat: Design Based on Characteristic Modes,” in IEEE Antennas and Wireless propagation Letters, vol. 20, no. 4, pp 508-512, April 2021. 4. Balanis, Constantine, Antenna Theory, John Wiley & Sons, second edition, New York, 1997. 5. C. Min, N. Howland and N.R. Potts, "Compact patch antennas on composite printed circuit boards in space", 2016 Loughborough Antennas & Propagation Conference (LAPC), pp. 1-4, 2016. 6. S. Gunaseelan and M. Murugan, "High gain patch antenna for cubesat", IEEE WiSPNET Conference, 2016. 7. De Groh, Kim K. and Bruce A. Banks, “Atomic Oxygen Erosion Data from the MISSE 2-8 Missions,” NASA/TM 2019-219982, May 2019.  

Design and Build Process of a Small Satellite Deployable UHF Antenna

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Design and Build Process of a Small Satellite Deployable UHF Antenna

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