Analyzing Recent Advances in Antennas

June 13, 2012
Antenna technology is moving forward as a result of improved printed circuits and mechanical designs, and even through the use of arrays backed by advanced signal processing.

Antennas are the most visible components of communications systems. They come in many forms, including large parabolic dishes pointing towards space and rectangular sector antennas on cell towers. But in some cases, they also must be designed not to be seen, such as for use inside cell phones and other wireless products.

In terms of antenna advances, few commercial companies can match the recent efforts of the US Army's Communications-Electronics Research, Development, and Engineering Center (CERDEC) at Fort Monmouth, NJ, including body-wearable antennas (BWAs) and distributed antenna arrays (DAAs). BWAs include plate structures built into a soldier's protective armor as well as antennas fabricated as part of a helmet. Both approaches have been demonstrated with wideband radios capable of operating through 1 GHz.

One of the challenges in designing such antennas is making them small enough to operate at military wavelengths within very-high-frequency (VHF) and ultra-high-frequency (UHF) bands, since wavelengths grow larger at those lower frequencies, and an antenna's physical size is a function of the operating frequency wavelength. Another issue in having antennas that are part of a soldier's uniform is the effects of electromagnetic (EM) radiation on the human body.

CERDEC is also studying DAAs, with an eye toward using them to jam improvised explosive devices (IEDs) in the field. In one study, directional DAAs were mounted on both sides of a military vehicle, with the vehicle given a clear zone between the two antennas for its own communications. By transmitting broadband signals with circular polarization, remote communications to the UEDs are prevented. Since most IEDs are assembled with whip antennas, which may use vertical or horizontal polarization, the circular polarization helps enhance the jamming effectiveness of the DAA-based jamming transmitter.

Growing needs for small, broadband antennas for surveillance and monitoring purposes have driven the development of some extremely wideband antennas, such as the model 3164-05 open boundary quad-ridged horn from ETS-Lindgren, which is capable of operating from 2 to 18 GHz. The unique appearance of the antenna is due to the absence of side plates and a configuration that is similar to having a pair of Vivaldi printed-circuit-board (PCB) antennas positioned orthogonally. The antenna's orthogonal input feeds allow it to generate both linear and circularly polarized patterns across the full frequency range. Since it is physically small, its phase center changes very little across the broad frequency range.

In addition to its novel antennas, the company also offers the model AMS-8050 antenna measurement system, a portable anechoic chamber that provides over-the-air (OTA) performance measurements of small wireless devices and mobile handsets. It is built on a movable cart for convenience and features a fully anechoic test chamber measuring 30 x 30 in. (76.2 x 76.2 cm), complete with RF shielded door. The shielded chamber has a dual-polarized antenna installed in the enclosure ceiling, with frequency range of 700 MHz to 10 GHz. It provides connections to outside test equipment, including vector network analyzers (VNAs) and spectrum analyzers. The measurement chamber can be used to analyze two-dimensional (2D) and three-dimensional (3D) antenna patterns, total radiated power (TRP), and effective isotropic radiated power (EIRP). It is shipped with the company's model EMQ-100 Antenna Pattern Measurement Software to simplify measurements.

Some antennas are as simple as radiating cables, especially for certain applications that require coverage within long, narrow confines, such as within aircraft and mines. Notable in this category are the GORE cable-based antennas for aircraft and mining applications from W.L. Gore & Associates. These lightweight radiating cables can be fit within small-radius bends and over long distances, supporting antenna lengths of 65 m or longer and frequency ranges of 400 MHz to 6 GHz.

For the most part, microwave antennas are designed for clearly defined operating bands, and often for extremely narrow frequency bands. PCTEL, for example, offers a variety of antenna types for military and government-service applications, including the model 2225NW Global Positioning System (GPS)/aviation antenna. It is a wide area augmentation system (WAAS) antenna that can withstand winds to 100 mph. It employs right-hand circular polarization and is available for use in L1 band at 1575.42 MHz, L2 band at 1227.60 MHz, and L3 band at 1176.45 MHz, with -3 dBic gain at 90-deg elevation in each of the three bands.

For broadcast applications, Radio Frequency Systems (RFS) has achieved significant antenna advances by paying close attention to mechanical design. The company recently unveiled "R variants" of its PCP, PHP, and PVP broadband panel array antennas, using a cylindrical radome to cut wind load by one-half compared to conventional panel antennas (Fig. 1). With their small cross sections and low wind loads, these antennas can reduce structural requirements for broadcast towers and also withstand ice and snow buildup.

The PCP panel antennas support digital television (DTV), analog television, and multiple-input, multiple-output (MIMO) system configurations. They are available from 500 to 700 MHz with horizontal, vertical, circular, and elliptical polarization. The PVP panel antennas are designed for DTV applications from 470 to 860 MHz using vertical polarization. The PHP panel antennas also operate from 470 to 860 MHz, but with horizontal polarization.

The firm also recently announced that its RADIAFLEX radiating cables, which support indoor applications from 698 to 2700 MHz, would be used for in-tunnel wireless service in China's Hangzhou Metro Line 1 transportation system. The in-tunnel radiating cables will be used to support the GSM, CDMA, DCS, UMTS, and LTE standards.

Broadband operation has long been a challenge for antenna designers, whether for communications or other applications . Many of these broadband applications work with directional antennas. For emerging applications in ultrawideband (UWB) communications systems, antennas must not only cover a broad range of frequencies, but they must do so with consistent, omnidirectional radiation patterns. The frequency range for UWB communications in the United States as approved by the Federal Communications Commission (FCC) is 3.1 to 10.6 GHz.

An example of an antenna with this type of performance is the model QOM0.8-40KL from Q-par Angus Ltd., which spans 0.8 to 40.0 GHz with vertical polarization. It is 108 mm long by 100 mm in diameter and weighs 720 g, so it is small enough for monitoring and surveillance applications, yet also suitable for airborne and harsh environments. Bandwidth-wise, it more than meets the needs of UWB systems, with -2.2 to +6.9 dBi gain from 1 to 40 GHz, 3.50:1 maximum VSWR, and 2.50:1 typical VSWR across the full frequency range. It uses a male K coaxial connector and can handle as much as 40 W transmit power. The low-profile antenna is constructed from aluminum and engineered plastic materials and achieves typically less than 1 dB azimuth ripple when measured on the horizon. Another supplier of antennas for UWB use is SkyCross, which offers antenna meeting the 7-GHz bandwidth requirements in a footprint as small as 15 x 15 mm.

Another area of growth for antenna developers lies in the millimeter-wave frequency range. Millitech, a company name synonymous with millimeter-wave frequencies, offers a wide range of antennasincluding planar antennas and arrays, monopulse antennas, reflector antennas, and multiband antennas. The firm's ODA series of omnidirectional antennas includes models from 18 to 140 GHz with as much as 5% bandwidths. As an example, model ODA-15 has a center frequency at 60 GHz, while model ODA-10 operates at a center frequency of 94 GHz (both with vertical polarization and nominal gain of 2 dBi).

These antennas support high-data-rate transfers from cellular base stations, as well as automotive distance-measuring and collision-avoidance systems. Several years ago, Radio Waves, Inc. launched its models HPCPE-42 and HP2-42 antennas for use from 40.5 to 43.5 GHz for short-haul European communications applications. The trend is increasing in the use of millimeter-wave signals, with a growing need for compact antennas capable of supporting these applications.

Andrew-Commscope has been an innovator in a variety of different antenna technologies. The company even offers a free white paper on its unique active antenna technology for cellular communications, notably in Long-Term-Evolution (LTE) systems. Essentially, an active antenna system (AAS) is actually an array of antenna elements and supporting electronic components. By using electronic tuning and extensive digital signal processing (DSP), these active or "smart" antennas (Fig. 2) can respond to changing conditions, providing extended cellular communications coverage with fewer antennas and cell sites.

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