The rapid progress in personal wireless communication devices has made the development of the Electrically Small Antennas (ESAs) the center of research interests. A large variety of miniature antennas has been developed with the emergence of mobile handheld devices. The success of these devices largely relies on the progress and innovation in dielectric materials, the optimization of size, gain, and bandwidth.
Integrated circuit antennas (Chip antennas), Planar Inverted F Antennas (PIFA), and printed circuit board (PCB) antennas (e.g. Meander antennas, inverted L antennas, printed monopole antennas and printed dipole antennas) are popular antennas available in today's market, which are widely used in different wireless hand held devices. However, in order for these antennas to effectively radiate or receive energy when used as transmitting or receiving antennas, they need a ground plane of an appropriate size. Chip antennas from various companies, such as Johanson Technology, Mitsubishi, Matrix Electrica, S.L, Antenna Factor, Raisun, etc., all require a specific PCB size. Usually, at least one edge of these PCBs should have a minimum of a quarter wavelength at its operating frequency.
One of the major design highlights of these commercial antennas is focused on the space/volume dual-usage realized by sharing the ground plane of the antenna and the circuits. Since the current is most significant on the edge of the ground plane, the center portion of the ground plane that serves as the return path of the circuit signals will have less of an effect from the antenna radiation. Some of these antennas are adopted for hand-held applications, such as cell phones and PDAs. Others are used in blue-tooth devices, such as wireless mouse and keyboards. The approximate quarter wavelength ground plane size required by the antenna in these applications is still within the range of the package for the end-user products. Therefore these antennas are widely accepted in wireless devices.
However, in some Wireless Sensor Network (WSN) systems, such as the Smart Dust systems, different application constraints are employed. SmartDust is a Wireless Sensor Network system intended to be used in sensing signals for civil or military purposes. The key challenges of the SmartDust prototyping are power, size, cost and sensing. SmartDusts can detect any target signal, such as sound, vibration, light, the environment temperature, humidity for industry factories, warehouses, plantings, poultry or animal husbandry, or can monitor patients conditions, etc. Some applications require thousands of SmartDust sensors distributed over a large area. They are usually disposable simply because it is not practical to collect SmartDusts and reuse them. Therefore, wireless sensor nodes in the WSN systems with low power consumption and low cost are very important. In military and other applications, it is preferred to hide the SmartDusts, e.g., the size of these sensors should not be noticeable. Ideally, these sensors should be as small as sand or dust. Obviously, antennas requiring a large ground plane are not compatible with SmartDusts and cannot be applied in these areas.
In addition to the many common requirements in ESAs for conventional handheld devices, such as low cost, light weight, compactness, gain and bandwidth performance, antennas in ultra low volume Wireless Sensor Network (WSN) applications, such as in SmartDust systems, have stricter dimensional limitations and demand for omnidirectional radiation for the following reasons:
First, in each WSN transceiver node, all components, such as sensor, antenna, battery, transceiver integrated circuit (IC), as well as the reference ground plane (normally a printed circuit board) for IC and antenna are to be stacked or integrated in a package with a total volume of only a few mm3 to one cm3, where only a fraction of this volume is left for an antenna. The millimeter or centimeter scale dimensions are often much less than a quarter wavelength at the operating frequency (i.e., 0.1λ or less). For example, in conventional ESA designs, a ground plane with a minimum quarter wavelength dimension is often necessary for proper performance. In the ISM bands (916/828/433 MHz), this ground plane size is between 8 to 16 cm. Though this is a reasonable size to be fit within a cell phone or a PDA's housing, it is too large to be integrated into SmartDust sensor nodes in WSN communication package, whose node size is on the order of a few cm3 or smaller. A package with a low height and a large ground plane area is not suitable for WSN applications. In WSN, the ground plane size must be decreased as well as the height of the antenna. This requires new designs to reduce both factors and keep the antenna highly functional.
Second, in WSN/SmartDust applications, a large amount of transceiver nodes are distributed randomly. These transceiver nodes, as well as the antennas associated with them, are oriented in various directions and form an autonomous communication network. Each communication node in this network is a complete self powered transceiver node, which requires the antenna to have a radiation pattern as omnidirectional as possible to transmit and receive signals from all directions due to the random orientation of the nodes.
Third, there is no need for a base station in WSN/Smart Dust applications. Any node in the network may serve as a base station. These nodes cover a large communication range by multi-hops. The communication distance is determined mainly by the separation of nodes, and can range from 1 to 10 m. Therefore, the gain of antenna is traded against the volume requirement.
Thus there is a need in SmartDust WSN applications for an antenna which occupies a volume no larger than 20 mm×25 mm×8 mm, which is 0.06λ×0.076λ×0.024λ (for a particular operating frequency of 916 MHz), and which has an omnidirectional a radiation pattern in order to transmit to and detect signals from random directions. The desired compact antenna also must be optimized for maximum efficiency and bandwidth, since small antennas inherently have high Q or low efficiency.
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