Microwave Antennas Come In Many Forms

Microwave/RF antennas are used in a wide range of applications, dictating the design of structures as physically large as space-based arrays to devices as small as semiconductor chips.

Antennas for RF and microwave applications form a group of products and technologies that is as diverse as any in the industry. After all, any system that operates by means of the transmission and reception of electromagnetic (EM) energy relies on transmit and receive antennas, and this means designing antennas that meet the frequency, power-handling, and size requirements of that system.

Antenna configurations range from a wire to a complex phased-array system with a large number of individual antenna elements. Smaller antennas for portable devices are usually fabricated on printed circuit boards (PCBs) using stripline or microstrip transmission lines. Particularly in large PCB-based antennas, the choice of laminate substrate material is critical to maintain consistent dielectric constant across the physical dimensions of the antenna and minimize frequency shifts due to temperature changes. In support of antenna designs, for example, materials specialist Rogers Corp. recently introduced its RO4730 LoPro antenna-grade laminates with low dissipation losses, low coefficient of thermal expansion (CTE), and excellent dimensional stability.

The basic idea behind any antenna, first developed by Marconi around 1897, is to tune to a desired resonant frequency. For example, an antenna element for a receiving antenna can be as simple as a length of wire cut to a fractional or multiple wavelength of the operating frequency. Larger antenna structures handle longer wavelengths and, thus, lower frequencies. In a transmit antenna, energy is supplied to the antenna structure to generate an EM field for propagation through free space. An antenna's radiation pattern is selected for a particular application. For example, broadcast systems employ antennas with omnidirectional patterns, while radar systems rely on highly directional focused beams.

Phased-array antennas can be electronically aimed by changing the relative phase of the signal at each antenna element. Antenna arrays have been used, for example, in radar systems as well as in conjunction with three-dimensional (3D) imaging software for such unusual applications as nondestructive evaluation (NDE) of concrete and steel rebar in buildings following earthquakes.

Wireless applications have influenced many of the developments in antennas for both base station and portable applications. Last year, for example, Electromagnetic Technologies Industries (ETI) introduced a series of scalable antennas for WiMAX base-station equipment based on dual polarization in order to cover multiple sectors. The antennas, which operate to 11 GHz, can be specified with 2 to 12 beams per unit with vertical and horizontal polarization. In addition, Radio Waves recently announced a log periodic dipole array antenna capable of 7-dBi gain from 1 to 18 GHz in a single antenna.

For portable antennas, in addition to the need for smaller embedded designs, cellular and other wireless systems have driven requirements for multiple-band coverage and flexibility. For example, Antenova has developed its patented High Dielectric Antenna (HDA) technology for multiband coverage in wireless handsets. Antenna designers generally rely on EM simulation software for modeling their circuits. For example, the High- Frequency Structure Simulator (HFSS) from Ansoft has been used in conjunction with some of the firm's other modeling programs for cosimulation of antenna designs. In addition, Ansoft has developed a new

Antenna Design Kit to complement HFSS and simplify geometry creation. Also, Computer Simulation Technology (CST) developed its Antenna Magus software tool to help engineers speed and simplify the antenna modeling process.

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