domingo, 25 de julio de 2010

Microwaves & RF: Small Antennas Are Big On Efficiency

Antennas must be large to have gain and high efficiency. At least that has long been the traditional view of antenna designers, until the wireless industry spurred interest in electrically small (ELS) antennas in the early 1990s. If small efficiency antennas could be developed for handsets and other wireless applications, fewer burdens would be placed on active components within the system, such as amplifiers. Well, it is time to change the traditional view, thanks to work by startup Dockon on innovative antennas that use both the transverse electric (TE) and transverse magnetic (TM) modes of the antenna to achieve high gain, high efficiency (often more than 90 percent), and even broad bandwidth in designs that are 13 x 11 mm or less for 2.4-to-2.5-GHz use.

The company's compound field antennas bring benefits to almost any wireless application, including those that are tight on space or that use multiple- input, multiple-output (MIMO) techniques. Named CPL, an acronym that stands for compound P x M (cross product of electric and magnetic dipole moments) loop, the technology is suitable for almost any wireless application. The CPL technology promises increased radiation efficiency, intensity, directivity, and radiated power compared to other microstrip antennas. The CPL antennas can be manufactured using standard printed-circuit-board (PCB) techniques using a variety of PCB substrate materials, even low-cost FR-4 substrates, with excellent results. Dockon licenses its designs to its customers.

According to Patrick Johnson, Chief Executive Officer (CEO), "CPL technology is the culmination of over 20 years of prior research and rewrites 60 years of assumptions about electrically small antennas. After years of development, testing, and refinement of the development process, the CPL delivers on the promise of compound antenna technology in an easy to manufacture format. CPL is a revolution in antenna design offering a more powerful and efficient radiator over a wider band in a smaller footprint than before possible."

A CPL antenna can be thought of as two antennas fabricated on a common substrate, generating orthogonal electric and magnetic wave patterns that together form a dense RF wave near the surface of the planar circuitry prior to propagation. The approach generates more net energy from an input signal than individual antenna elements generating electric or magnetic field patterns separately. In fact, CPL antennas are the first commercially available compound field antennas that use both magnetic loop radiators and co-located electric field radiators. Simultaneous excitation of both radiators results in an effective cancellation of reactive power, improving the overall performance and efficiency of the design and enabling efficiency of 90 percent or more.

In a conventional antenna, an electric field wave is first generated and, by its propagation generates a magnetic wave, with some of the original electric field strength lost parasitically by the creation of the magnetic field component. In the CPL antenna designs, the antenna elements are fed separately and generate wave components separately, but with phase tightly controlled for constructive addition of the field components.

The CPL technology involves colocating a series resonant electric dipole and small magnetic loop. The dipole is located along the z axis and the magnetic loop lies in the yz plane of a threedimensional design. The dipole moments must be orthogonal to prevent the two radiating elements from operating as two independent elements. If the electric and magnetic elements are driven with in-phase signals, they will operate independently, and their patterns will be combined destructively. So, the way that signals are delivered to the antenna elements is critical in a CPL antenna design.

The CPL approach can be used for both narrowband and broadband antennas, but required development of several high-performance passive components. Two innovations helped make the CPL technology viable: a low loss, high isolation RF combiner/splitter for multielement arrays and a phase tracker. Both passive components allow CPL antennas to maintain proper current magnitude ratio and phase quadrature over very wide bandwidths from a single power source. The RF combiner/splitter achieves high isolation on all ports for optimum impedance matching of CPL antenna arrays in either broadband or narrowband use. Its low loss contributes to the high gain possible with CPL antennas. The phase tracker always maintains proper quadrature between the electric and magnetic radiating elements over very wide frequency ranges. Together, the two components enable adjustable antenna patterns for a wide range of applications, with requirements ranging from omnidirectional to directional antenna patterns. They also support dual linear (horizontal and vertical) polarization to overcome the effects of multipath environments, and can be used for wideband, multi-element spatial diversity for increased receiver sensitivity.

Dockon has developed a number of reference designs that demonstrate the small size and excellent radiation efficiency of CPL antennas. For example, its model ECA2C-240 design operates from 2400 to 2500 MHz with linear polarization, 11 dBi gain, and better than 90-percent efficiency. Designed as an external antenna for WiFi applications, it measures 4.25 x 4.25 x 0.9 in. (108 x 108 x 23 mm) and is the largest of the reference designs. The multi-element array provides spatial diversity to combat the effects of multipath environments, achieving a 45-deg. azimuth beamwidth and 45-deg. elevation beamwidth (Fig. 1). The reference design was fabricated on low-cost FR-4 substrate material.

Working over the same frequency band but in a smaller footprint, the model ECA1C-240-RevA4 reference design is suitable for embedded WiFi applications. It operates from 2400 to 2500 MHz with linear polarization, 3.5 dBi peak gain, and 1.6 dBi average gain (Fig. 2). It measures just 0.51 x 0.44 in. (13 x 11 mm) and boasts 97 percent efficiency. The omnidirectional 50-O design takes advantage of the high 10.2 dielectric constant of RO3010 laminate from Rogers Corporation to achieve the miniature size with two PCB layers. It is ideal for mobile telephones, PDAs, and other compact, portable wireless products.

For lower-frequency use, the model ECA1C-824-960-RevC2 reference design is an omnidirectional planar antenna measuring just 1.45 x 1.10 in. (37 x 28 mm). It yields 3.5 dBi maximum gain and 1.9 dBi minimum gain from 824 to 960 MHz with linear polarization. The antenna exhibits better than 6 dB return loss with 90-percent minimum, 93-percent maximum, and 92-percent average efficiency. Although fabricated on Rogers RO3010, it could also be fabricated on lower-cost materials, such as FR-4.

Morales R. Karelis
CI 18089995

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