Two-way mobile communications require tunable antennas for best performance. But mechanically tuned antennas lack the tuning speed and reliability demanded of security, public safety, and first-responder teams. Fortunately, high-frequency innovator AK Electromagnetique, Inc. (Les Coteaux, Quebec, Canada) has developed a low-profile antenna system that can scan electronically in both azimuth and elevation angles.
The innovative antenna system Fig. 1> (originally reported in terms of technology in Microwaves & RF, September 2006, p. 88) is a low-profile adaptive array that has been designed for mobile use in the satellite-communications band from 1.520 to 1.665 GHz but can be used for other frequency ranges and applications. Current designs include three-ring and five-ring adaptive arrays. The five-ring configuration contains 72 parasitic monopole elements with a ground-plane diameter of 54 cm (Table 1). The center of the design contains an active monopole, which is connected to an RF source through a low-loss coaxial cable.
Identical parasitic elements in the five-ring adaptive array are mounted perpendicular to the ground plane formed by an electrically conductive brass plate. Each parasitic element incorporates a PIN diode and an RF bypass capacitor. Rohacell material (with relative dielectric constant of 1.05) has been added between parasitic elements to increase the rigidity of the antenna. Parasitic monopole elements are connected by cylindrical brass outer and inner conductors, which form a coaxial line electrically shorted at one end with a shorting plate. A feedthrough capacitor is mounted on the ground plane, which holds the parasitic element perpendicular to the brass plate.
One end of the center conductor to the feedthrough capacitor is connected to the inner connector of the coaxial line section. A PIN diode is connected between the coaxial line's outer conductor and the ground plane. A bias voltage applied from a power supply by means of a biasing resistor and the feedthrough capacitor connected to the center conductor can switch a PIN diode between conducting and non-conducting states, thus activating or deactivating a parasitic element.
A microprocessor-manipulated controller circuit fabricated on printed-circuit board (PCB) is used to send the biasing voltage to different combinations of parasitic elements, controlling the reflection properties of the parasitic elements in the five-ring antenna array. The microcontroller allows different radiation patterns to be generated and stored for instant recall when different antenna beams are required.
A number of different radiation patterns of variable directivity and orientation in both azimuth and elevation planes can be generated by applying suitable bias to the appropriate parasitic elements. For example, it has been possible to set the biasing for the antenna to generate low-, intermediate-, and high-elevation antenna beams suitable for satellite communications in high-, intermediate-, and low-latitude countries. The five-ring antenna in Fig. 2 has been field tested for mobile satellite communications in the receive and transmit range of 1.520 to 1.665 GHz. As a comparison, data have been included in Table 2 for a three-ring variation of the antenna.
AK Electromagnetique, Inc., 30 Rue Lippee, Les Coteaux, Quebec J7X 1H5, Canada; (514) 620-3717, FAX: (450) 267-1144, e-mail: [email protected].