S-Band Antenna

Background

Reliable radio telecommunication systems are among the most important and most challenging aspects of CubeSat development. Many different design considerations and component selections need to be made to ensure sensor data, telemetry, and commands are effectively communicated between a satellite in orbit and its ground station. Antennas are a critical component of these systems, and for many developers and researchers the time, materials, and equipment costs of developing these systems can significantly impact a project’s schedule and budget. High frequency design equipment is not always available to developers, and commercial antenna alternatives typically cost upwards of $5200CAD [1].

For both the ORCASat and the MARMOTSat missions ultra high frequency (UHF) amateur radio was used for TT&C. In order to utilize amateur radio allocations for a satellite mission, the mission must incorporate an amateur radio element. This was acceptable for the above missions but is limiting to what can be done on future missions. For example a purely scientific mission without a major amateur radio component may be ineligible to utilize the amateur bands. Additionally, there are several limitations of the amateur radio spectrum such as limited bandwidth, thus limited data rate, and restrictions on what information can be sent, for example encryption is restricted.

When combined this creates a need for an inexpensive commercial band antenna for use on CubeSat missions. This need is both internal to UVSD and external as other similar teams face these same considerations. The S-Band is considered the standard commercial frequencies to be utilized for satellite TT&C. The goal of this project is to design, build, and test an effective and affordable S-band CubeSat antenna, which will then be released as an open-source design to aid in the development of future CubeSat projects. This antenna will be sufficient to sustain a communications link while in Low Earth Orbit (LEO) and driven by a standard commercially available S-Band radio.

Design

A comprehensive survey of available S-Band CubeSat antennas was undertaken to see the key industry design trends. From this study it was determined that the best option for an inexpensive S-Band antenna is a rectangular patch with circular polarization. This allows for a sufficient gain and a wide enough beam width to ensure operation given the low pointing accuracy which is common in CubeSats. The reason a circularly polarised antenna is preferred to a linear polarized one is the difficulty in controlling the precise attitude of the spacecraft. The use of a circularly polarized antenna decreases the possible polarization mismatch losses that can arise from the transmitting and receiving antennas not being correctly aligned.

The selected design topology is a rectangular patch antenna. It is selected to be circularly polarized cutting out a slot inside the patch. The antenna will be fed from a microstrip line on one of the sides. This microstrip line will be connected to an impedance matching network which matches the antenna to a 50 Ohm coaxial cable which will connect to the PCB.

Through simulation, it was determined that a 39 by 39 milimetre patch yields a resonate antenna at 2.24 GHz. The patch is placed on a 80 by 80 milimetre PCB which can be mounted to the CubeSat chassis. This patch is feed with an impedance matching L-network to match it to 50 Ohms so it can be fed with a standard coax cable. The use of lumped elements in the impedance matching network allows the exact frequency of the match to be adjusted to suit a particular mission. 0.76mm thick Rogers RO4350B (Er = 3.48) was selected as a suitable cost to performance balance. This is a very inexpensive dielectric which can be procured from vendors such as PCBWay or JLCPCB and does not require specialized microwave manufacturing.

Patch antennas radiate across a wide beam width, usually 60-80 degrees. As such, for a CubeSat mission the design may incorporate multiple antennas on opposite sides of the satellite such that one can cover each hemisphere to ensure communications regardless of satellite orientation.

Testing

Anechoic Chamber Testing

Radiation pattern testing was done in the UVic anechoic chamber to eliminate errant signals and isolate the performance of the patch antenna in a free-space environment. The radiation pattern of the patch antenna was characterized in the horizontal plane and vertical plane by a linearly polarized test antenna.

Horizontal Characteristics

The horizontal polarization heavily resembles the simulated case. With a 85 degree half power beamwidth and a maximum gain of 4.3dB, it performs as required. The gain is slightly lower than simulated, this is due to the antenna being designed for circular polarization and being tested with a linearly polarized antenna. Assuming perfect polarization, this polarization mismatch would result in -3dB.

Vertical Characteristics

The vertical polarization is notably worse than the horizontal characterization. While the 80 degree beamwidth is sufficient, the small -0.4dB maximum gain is not. This also demonstrates the antenna is more elliptically polarized than perfectly circular. The close proximity of the matching network to the antenna potentially caused the poor circular polarization and lower peak gain of the vertical plane.

Thermal, vacuum, and vibration testing has not been undergone at this time due to time constraints and limited access to test equipment.

The Team

Blake Baldwin

Evan Peters

Ethan Clarke

Read the Report

Capstone_Report.pdf

More Information and Acknowledgements

Acknowledgements:

Special Thanks to Dr. Levi Smith for design input and the use of testing facilities.

References:

[1] “S-Band Antenna ISM,” EnduroSat. Accessed: Jun. 22, 2024. [Online]. Available: https://www.endurosat.com/products/s-band-antenna-ism/

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