Antenna-pointing mechanisms are used in spacecraft communication systems for directing the spacecraft's antenna toward the desired communication target. An antenna can receive an electromagnetic wave and convert it into an electric signal, or receive an electric signal and radiate it as an electromagnetic wave. This mechanism enables communication with satellites in orbit or ground stations located on the Earth. Antenna pointing mechanisms are designed to be precise, efficient, and reliable while minimizing power consumption and maximizing speed.

Principle of Antenna Pointing Mechanisms

The electric and magnetic fields are interrelated, and they propagate through space as electromagnetic waves. The electric field is produced by static charges, whereas the magnetic field is produced by moving charges. When an alternating current flows through a conductor, it produces an alternating electric field that radiates energy into space as an electromagnetic wave. The antenna design is based on the principle that an alternating current flowing through a conductor produces an electric field that radiates energy into space. The geometry of the antenna determines the radiation pattern, which is the direction in which the energy is radiated. To convert an electric signal into an electromagnetic wave, a closed conductor is used along with the principle of electromagnetic induction. This setup produces a fluctuating magnetic field and an electric field around it. However, the electromagnetic field around the source does not propagate, and it only fluctuates around the source. To transmit signals, the electromagnetic waves need to be separated from the source and they should propagate. 

A dipole antenna is used to separate the electromagnetic waves from the source. A dipole consists of one positive and one negative charge placed at a distance apart. When these charges have oscillated, they produce an electric field. The charged particles undergo continuous acceleration and deceleration, producing a fluctuating electric field. The varying electric field automatically generates a varying magnetic field perpendicular to it, resulting in the propagation of electromagnetic waves. For the production of an oscillating field, A conducting rod with a bend in its center is taken, and a time-varying voltage signal is applied at the center. Due to the effect of the voltage, the electrons will be displaced from the right of the dipole and will be accumulated on the left. This means the other end, which has lost electrons, becomes positively charged. With the variation voltage with time, the positive and negative charges will oscillate to and fro, producing a varying electric field that generates an electromagnetic wave.

Design Considerations of Antenna Pointing Mechanism

The following key specifications are considered when designing an antenna pointing mechanism.

• Power Consumption: Antenna pointing mechanisms should be designed to consume minimum power while maintaining maximum efficiency. The power consumption of the mechanism varies depending on the size and complexity of the system.
• Velocity: The velocity of the antenna pointing mechanism is the rate of rotation or movement of the mechanism. The velocity of the mechanism should be high enough to allow the antenna to be pointed accurately towards the target. The velocity of the mechanism varies depending on the specific system.
• Step Size: The step size of the antenna pointing mechanism is the smallest increment of movement that the mechanism can make. The step size should be small enough to allow accurate pointing of the antenna. The step size varies depending on the specific system.
• Wavelength and Frequency: Wavelength is defined as the speed of light divided by frequency. As frequency increases, wavelength decreases. To have a well-performing antenna, the antenna's physical dimensions should be an appreciable factor of a wavelength, typically a quarter wave or larger. Therefore, the frequency has a direct impact on the device size and antenna performance. 
• Antenna Impedance: Impedance is defined as the ratio of voltage to current at the antenna input terminals. The metrics that define how well the antenna matches the receiver's impedance are Voltage Standing Wave Ratio (VSWR) and Return Loss. VSWR should be less than two, and Return Loss should be less than minus 10dB. This correlates to a 90% transfer efficiency, where we are only losing 10% in our reflections. Maximizing the power transfer from the radio to the antenna needs an impedance-matching network between the antenna and receiver. 
• Antenna Efficiency: Efficiency is the most important antenna parameter for wireless embedded devices. It is defined as the power radiated over the power input to the antenna. The higher the antenna efficiency, the better the antenna. Antenna efficiency is affected by conductor losses, dielectric losses, mismatch losses, and impedance match network losses.
• Directivity: Directivity is the ability of the antenna to focus or concentrate energy in a specific direction. Antennas with high directivity, such as satellite dish antennas, focus energy in a narrow beam, while antennas with low directivity, such as dipole antennas, radiate energy in all directions equally.
• Gain: Gain is the measure of how efficiently an antenna converts input power into radiated power in a particular direction. It is defined as the product of the directivity and the antenna efficiency. It is essential to note that gain is always lower than directivity due to losses that occur during energy transmission. These losses can be attributed to conductor, dielectric, and mismatch losses, as well as losses in the impedance-matching network. Even with losses, antennas can still perform well, as long as they are designed and selected for the specific application and frequency range.

Working of Antenna Pointing Mechanism

The design index of the antenna pointing mechanism mainly depends on the working environment and task requirements. Considering it needs to be folded and fixed on the satellite in the early stage, the antenna pointing mechanism should be designed with three degrees of freedom. Space manipulators are often driven by motors, but the increase in the number of motors will directly increase the quality and power consumption. For an optimized structure, pointing manipulators are used which are mainly composed of the spring hinge mechanism, manipulator arm, and the universal adjustment mechanism is proposed. The spring hinge mechanism has been used widely in space deploy structures with the advantage of low mass, high reliability, and self-locked. Thus, the joint on the satellite base at the other end uses a spring hinge mechanism to fold and unfold the manipulator instead of a motor. 

The universal adjustment mechanism and the spring hinge joint are connected by the manipulator's arm.  The material selection and mechanical analysis are also made. Finally, the pointing angle is described based on the mathematical model of the pointing control mechanism. The antenna pointing mechanism's working principle involves two primary components, the drive system, and the control system. The drive system provides the necessary torque to rotate the antenna, while the control system regulates the direction and rate of rotation. The drive system consists of an electric motor or a hydraulic system that provides the torque to rotate the antenna. The control system consists of sensors that measure the antenna's current position and a computer that calculates the necessary rotation required to point the antenna toward the target. The control system of the antenna pointing mechanism is critical to ensure the accurate pointing of the antenna. The control system consists of sensors and a computer that receives information from the sensors and calculates the required rotation of the antenna. The control system must also compensate for external disturbances, such as wind and vibration, to maintain accurate pointing of the antenna.

The spring hinge mechanism is the key component of the first manipulator joint. It is used for folding and fixing the manipulator on the satellite during the launch phase of the rocket. Before the satellite antenna works, the spring hinge carries the manipulator to the working position and locks it. However, the first joint can’t be retracted and its function becomes to fix the manipulator.

The spring hinge mechanism is mainly composed of the L-shaped tripod, the hollow shaft, the torsion spring, the U-shaped tripod, and the locking bolt.

All parts are connected by a hollow steel shaft and then fixed on the satellite body through the L-shaped hinge. One end of the torsion spring is fixedly connected to the U-shaped frame, and the other end is fixed on the satellite. The torsion spring generates elastic force to drive the manipulator arm to rotate when the U-shaped frame moves relative to the L-shaped frame. After the manipulator is unfolded from the folded state to the working position, this joint will be locked and fixed by the locking mechanism installed on one side of the first joint. Whether the manipulator can work stably after unfolding is largely affected by the locking mechanism on the spring hinge mechanism. The joint locking mechanism is composed of a slide-way clamping groove on the U-shaped frame, a locking bolt, a spring, and a mounting hole on the L-shaped frame.

The antenna pointing mechanism structure consists of a spring hinge mechanism, manipulator arm rod, and universal adjustment mechanism. The spring hinge mechanism is used to replace the ordinary motor unfolding of the manipulator. The manipulator arm rod reduces the thickness of the arm rod and holes are constructed to reduce its weight. Two small motors are used at the end of the manipulator to realize the function of adjusting the antenna pointing. The simulation results show that the spacecraft antenna pointing manipulator can not only ensure light weight and low power consumption but also have good mechanical properties which meet the manipulator design requirements.

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