Planar Antennas for Small-satellite Communications

A 2.45-GHz microstrip patch antenna was designed for small-satellite communications. Optimization was achieved by trimming the copper patch to minimize the input return loss over the desired 2:1 VSWR bandwidth. Final testing of the antenna included a successful long-range data transmission over which pictures taken by the satellite were sent to the ground station. In addition to the microstrip patch antenna, a 7.3-GHz retrodirective antenna array was fabricated and tested. By combining phase detection with a frequency scanning antenna array, retrodirectivity was achieved at interrogator angles of 0°, -20°, and +15°. INTRODUCTION – MICROSTRIP PATCH ANTENNA The University of Hawaii Small Satellite Team completed its third-generation CubeSat in Spring 2006. This satellite named Ho’okele had a geo-referenced imaging payload consisting of a camera, GPS unit, and a nano-IMU. These components would provide a file containing a picture, GPS data, and nano-IMU data which can be used to determine the exact area of earth being imaged. All of the satellite subsystems for this satellite were finalized and a low-profile antenna was needed to complete the satellite. It was desired to have a low-profile antenna to eliminate the need of a complicated antenna deployment mechanism. In addition there were size and form restrictions due to the physical layout of the satellite exterior. The antenna needed to fit on one end of the satellite structure. The area available for the antenna was 10 cm by 10 cm, minus the area needed for the camera aperture. MICROSTRIP PATCH ANTENNA DESIGN AND FABRICATION Microstrip patch antennas are a well-known type of planar antenna. This antenna was chosen for Ho’okele due to its planar structure, and ease of fabrication. Much research has been published about this antenna and numerous programs exist to aide in design. A MATLAB program developed by Arizona State University [1] was the chosen software for the microstrip patch antenna design. The Microstrip Patch Antenna Designer MATLAB script prompts the user to enter data used to calculate the patch antenna dimensions; patch geometry, substrate thickness and dielectric constant, desired input impedance, feed type, and frequency of operation need to be entered. The code then outputs a file with the required patch dimensions in order to meet the specified requirements. Table 1 shows the parameters specified to the program. The output dimensions of the patch antenna are listed in Table 1. Input parameters of patch antenna. Entered MATLAB & RPDESIGN Input Parameters Geometry rectangular Resonant Frequency 2.45 GHz Dielectric Constant 2.2 Substrate Thickness .1575 cm Input Feed Type coaxial Input Impedance 50 Ω Table 2, and Figure 1 shows how the dimensions relate to patch geometry. The data from the design program indicated that the antenna would be larger than the allocated size on Ho’okele. MATLAB Output Dimensions RPDESIGN Output Dimensions Final Antenna Dimensions Width of Patch (W) 4.84 cm 4.83 cm 4.11 cm Length of Patch (L) 4.05 cm 4.07 cm 3.95 cm Feed Position (Yo) 1.42 cm 1.39 cm 2.23 cm Feed Position (Xo) 2.42 cm 2.415 cm 1.45 cm Table 2. Dimensions of antenna calculated by design programs and final antenna dimensions. To verify the results of the MATLAB code, the same parameters were entered into RPDESIGN, another antenna design program. RPDESIGN also produced the same results, leading to the conclusion that the MATLAB dimensions were correct. The next step in the design process was to fabricate the antenna and optimize the design. A prototype was fabricated using copper tape and 62-mil RT/Duroid substrate with a dielectric constant (εr) of 2.2. The copper-tape patch cut to the specified MATLAB dimensions was affixed to the side of the substrate with the ground plane removed. An SMA connector was attached to the specified feed point location. It was then tested on the network analyzer to ensure sufficient 2:1 VSWR (-10 dB) bandwidth needed by the transceiver was available. The 2:1 VSWR bandwidth is a measure of how much power is being reflected by the antenna. In antenna design, bandwidth is commonly determined by the frequency range where S11 (input return loss) is less than or equal to -10 dB. Optimization was necessary to decrease the footprint of the antenna, so it could fit on Ho’okele. Copper tape was cut away in slices to shrink the size of the antenna. This caused the center frequency and bandwidth to change depending on which side of the antenna was cut. For the most part, cutting pieces of copper tape from the radiating edge decreased the bandwidth while cutting pieces from the non-radiating edge increased the center frequency. Although the other parameter (bandwidth or center frequency) changed somewhat, these relationships were the dominant relationships observed. It was noticed that when the patch became nearly square and the feed point was close to the diagonal, the 2:1 VSWR bandwidth would increase dramatically. This was an important discovery because decreasing the patch antenna size would decrease the bandwidth. To ensure that the antenna would work with the transceiver, there had to be enough bandwidth, and all previous antenna optimizations did not meet the required Figure 1. Layout showing the dimensions of a microstrip patch antenna. Figure 2. Input return loss versus frequency for final microstrip patch antenna. bandwidth. When the patch was nearly square, there was enough bandwidth to assure compatibility with the transceiver. The design was tuned a little further and then final fabrication took place. Figure 2 shows the final bandwidth of the antenna. The copper tape antenna prototype was measured and an exact permanent replica was created through photolithography and etching. MICROSTRIP PATCH ANTENNA ANALYSIS After final fabrication, numerous tests were performed to verify proper operation. The bandwidth and center frequency were verified through network analyzer measurements. A polarization measurement was taken to determine the antenna’s polarization. The crosspolarization ratio was about 7 dB, indicating that the antenna is elliptically polarized rather than linearly polarized like it normally should be. The MATLAB and RPDESIGN specifications should produce a linearly polarized antenna. In this case, the trimming of the antenna caused the polarization to change. According to [1], a nearly square microstrip patch antenna with feed point on the diagonal produces a circularly-polarized antenna. This is because a square’s sides are equal length and this means that all edges start to radiate creating circular polarization. The diagonal feed position determines the input impedance of the antenna for matching purposes. The final antenna exhibited some elliptical polarization, but not circular polarization because the antenna was not exactly square and fed on the diagonal. This explains why the optimized antenna exhibited strange characteristics. The final antenna was then mounted onto Ho’okele and field tests were performed. Long-range image transmission tests from Tantalus to UH and Diamond Head to UH were successful. Comparisons between the supplied ground station antenna the extra copper tape antenna showed the microstrip patch antenna improved the signal strength. INTRODUCTION – RETRODIRECTIVE ANTENNA A retrodirective antenna is a self-steering antenna system. Retrodirective antennas redirect their transmitting main beam to point in the direction of the target interrogator. This means that the outgoing transmission departs in the same direction as the received interrogator signal. No previous knowledge of interrogator position is needed to achieve self-steering. The benefits of retrodirective antennas over conventional antennas include security and efficiency. Retrodirective antennas provide secure crosslinks by using a directional beam which points at the target of interest. Retrodirective antennas are efficient because unlike omnidirectional antennas which waste power by radiating in all directions, power is only radiated in the target direction. This is why retrodirective antennas are so attractive; numerous applications such as small satellites require security and efficiency. The work presented in this paper is a new development from a combination of techFigure 3. Block diagram of the entire retrodirective