Antenna Design for RF Energy Harvesting

Key Takeaways

• The selection of operational frequency is important, with higher frequencies suitable for long-range power harvesting and lower frequencies for near-field applications.

• Balancing bandwidth is crucial, as wide bandwidth antennas capture incident energy effectively, but are more vulnerable to interference, while narrow bandwidth antennas offer high energy conversion efficiencies, but retrieve a limited amount of energy.

• Efficient integration of individual modules is essential to optimize the overall efficiency of an RF energy harvesting system.

Antenna used in Radio Frequency Identification

Power harvesting, also known as energy harvesting, is a method of gathering energy from the surrounding environment utilizing a variety of different techniques. Among these methods, RF wireless energy harvesting holds significant potential as it can replace batteries or extend their lifespan considerably. Radio frequency energy harvesting represents a promising technology that leverages RF electromagnetic waves found around us, positioning it as a viable energy source for future applications. However, in order to utilize this new technology, intelligent and efficient antenna design for RF energy harvesting is essential.

Antenna Design Tips for RF Energy Harvesting

Antenna Design for RF Energy Harvesting Background

The abundance of wireless signals, including those from mobile base stations, Wi-Fi networks, radio and TV transmitters, and microwave radios and mobile phones, has led to a significant growth in RF energy as an ambient energy source. However, compared to other energy sources, RF energy has a relatively low energy density ranging from 0.2 nW/cm2 to 1 μW/cm2. Harvesting this energy for low-power devices, such as wireless sensor networks (WSNs), can significantly extend their operating lifetime but also presents a challenge as the harvesting system needs to be similar in size to the sensor nodes.

In order to harvest ambient RF energy, highly efficient and high-gain antennas are required to collect the surrounding RF waves, which are then converted into DC voltage through rectifier circuits. The harvested DC energy is subsequently stored in low-loss capacitors or batteries until it is needed by electronic components like IoT nodes, sensors, or microcontrollers. It is evident that the efficiency of antennas, rectification circuits, and power storage components significantly impact the amount of energy harvested and available to the device.

RF Energy Harvesting Types

RF energy can be categorized into ambient and intended RF, depending on its source and purpose. Both types can be harnessed for energy harvesting applications, presenting opportunities for utilizing RF resources to collect ambient energy. Moreover, RF-EH exhibits a lower power density compared to other renewable energy sources. However, the density issues faced by RF-EH are relatively minor, particularly in the sub-microwatt range.

The limited RF density can be effectively utilized in two scenarios: ultrapowered devices that operate continuously (non-duty-cycled), and low-power applications, such as duty-cycled operation and low-powered wireless sensors in delay-limited devices. In these cases, it is essential to harvest a sufficient amount of RF energy before initiating system operation.

RF Energy Harvesting Systems

The efficiency of a wireless power harvesting (WPH) system greatly relies on selecting the appropriate operational frequency that aligns with the application's requirements. 

The operational range primarily depends on the frequency employed. High-frequency transmission is more susceptible to attenuation by atmospheric conditions, while low-frequency signals can penetrate matter more effectively. Thus, for specifically implantable devices in WPH applications, it is advisable to keep the transmitting frequency below the megahertz range.

However, for especially long-range power harvesting or transmission, high frequencies like 2.45 and 5.24 GHz are preferred, while near-field applications necessitate electromagnetic waves in the megahertz range. In dense environments or non-air settings, very low frequencies (~kHz) are more desirable.

In addition to operational frequency and distance, the suitable topology of the voltage multiplier in a WPH system is determined by the required output power and voltage. The design of the antenna must be tailored to match the gain, frequency, and size specifications. Equally important is selecting the right rectifying element for efficient power conversion.

The choice of antenna gain depends on the specific application requirements. In cases where the positions of the source and receiving antennas are known, a high gain rectenna provides an advantage. Conversely, if the positions are relatively uncertain, a low gain antenna allows for collecting signals from various directions simultaneously.

The capacitance and inductance of an antenna are influenced by its frequency and physical size. Larger antennas tend to have lower resonance frequencies, making them suitable for transmitting and receiving low-frequency waves but impractical for small device applications.

An antenna's bandwidth encompasses the frequencies within which it can function optimally. In comparison to a narrow bandwidth antenna, a wide bandwidth antenna is capable of capturing signals from a broader spectrum of frequencies. As a result, a wide bandwidth antenna offers the advantage of effectively capturing incident energy. However, it also poses a higher vulnerability to potential interference caused by unwanted frequencies, such as noise. In other words, wide-band or multi-band frequency antennas can capture more RF energy in space, but they come with tradeoffs of lower overall efficiency and larger aperture requirements. On the other hand, Narrow-band antennas have been proven to offer high energy conversion efficiencies but retrieve a limited amount of energy. 

Good Antenna Design Relies on an Efficient RF Energy Harvesting System

The overall efficiency of a WPH system is determined by the efficiencies of individual modules and their successful integration. To optimize the total efficiency, it is crucial to maximize the efficiency of each module and seamlessly integrate them in a harmonious manner.

The key parts of an RF energy harvesting involve the antenna receiver, impedance matching network, rectifying circuit or voltage multiplier circuit, in addition to a DC output that either directly stores the energy or uses it immediately in an application.

Power leakage during transmission in electrical systems can result in energy insufficiency. To address this issue, the addition of an impedance matching network (IMN) ensures maximum power transfer between the RF source and load. When considering WPH applications, the receiving antenna is regarded as the source, while the rectifier/voltage multiplier is seen as the load. It is important that for optimal power transfer in DC, the resistances of the source and load should be equal. L, Reverse L, T, and Pi networks are all various configurations of common impedance matching networks that can be used for design.

For designers seeking to enhance their antenna design for RF energy harvesting, Cadence AWR software is a valuable tool. With its comprehensive features and simulation capabilities, designers can efficiently analyze and optimize their antenna performance, ensuring maximum energy capture. Take advantage of Cadence AWR software to streamline your antenna design process and unlock the full potential of RF energy harvesting.

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