Multi-beam antenna design for broadband wireless communications

Multi-beam antennas improve wireless communication capabilities with enhanced spectrum efficiency and higher quality of service. One of the methods for designing such antennas involves spatial division multiple access (SDMA) technology. The SDMA method can provide higher user capacity within a limited spectrum without any major technology changes.

Many wireless service providers use SDMA technology to optimize the use of available spectrum, which is generally limited to three sectors within a 360-degree coverage area. But with a multi-beam antenna system, the coverage sector can be increased to as many as 48. Because the system's beamforming network can reuse available frequencies and reduce interference, it can serve more users and have better service quality for the wireless network service area.

该系统可在多个方向长距离传输数据、语音和视频信号且不需中继站。这样，就把网络的运营成本降至最低且显著提升了可靠性、质量并增加了用户数。用长距离(高增益)窄束定向天线取代短距离(低增益)全向天线。通常，长距离天线会增加单一方向上的用户数，但不允许其它方向上的用户使用该系统。本文建议的系统通过采用既可同时又可顺序重复利用高增益窄束天线的多束技术解决了该问题，该技术有效实现了全向天线的球面型覆盖范围从而显著增加了各个方向的用户数。采用频率再用技术可进一步增加容量。

The multi-beam system is a hardware solution based on phased array antennas and the Optibeam proprietary beamforming network developed by Electromagnetic Technologies Industries (ETI). Because this hardware solution does not require software programming and external power supply, it is very suitable for use in harsh environments.

The main components of the multi-beam antenna system discussed here are the antenna and the beamforming network. The antenna consists of small antenna elements such as dipoles or patch antennas, which are combined into an array. The beamformer provides the required signal phase to all antenna patches to form beams in various directions. The design parameters of both elements are critical to achieve the desired performance of the multi-beam antenna system.

The antenna used in the system discussed in this article is based on patch antennas arranged in a matrix. Patch antennas are based on the proven microstrip high frequency printed circuit technology. The advantages of using patch components in such a matrix arrangement are small size, low manufacturing cost, light weight, easy installation and high reliability. Depending on the desired electromagnetic radiation direction, different signal amplitude and phase excitation are fed to each patch. The different phases of the radiating components are combined with the antenna far field to form a narrow beam. The antenna discussed in this article is designed as a linear phased array antenna system, where the patches are equidistant and progressive phase shifting is used throughout the matrix.

The spacing of each patch is maintained at half the wavelength of the center frequency (λ/2). The centerline of the patch is initially selected as the feed point, but the actual exact location of the feed point is determined empirically from input reflection measurements made with a high-frequency vector network analyzer (VNA). In addition to the feed point, the shape of each patch is carefully selected to obtain a voltage standing wave ratio (VSWR) of less than 1.50:1 over the frequency range of interest. The feed point is selected slightly higher than the center point to improve performance over the frequency range of interest. Other design parameters for the patch antenna component include: resonant frequency = 3.7 GHz; substrate height = 0.030 inches; substrate dielectric constant = 2.2; patch antenna length = 1.575 inches; patch antenna width = 0.710 inches; feed point location slightly higher than the center point of the patch; polarization = vertical.

Many patch antennas are designed by arranging patch elements in a linear fashion on a single dielectric substrate to achieve 15 degree azimuth beamwidth and 7 degree vertical beamwidth, respectively. A four-beam antenna design requires a minimum of four patch antenna elements. A four-beam system using the proposed technique was designed to have 26 dB antenna gain, a front-to-back ratio greater than 30 dB, and a sidelobe level of 20 dB (less than the main lobe level). The performance of a four-beam antenna design was measured using a commercial microwave VNA over a full frequency sweep range of 2.0 to 4.5 GHz and the results are shown in Figure 1. The antenna system operates from 3.2 to 4.2 GHz with a VSWR less than 1.50:1.

Beamformer Design

A beamformer is a complex network of passive microwave components. It is used to provide the desired phase and amplitude between the antenna and the system transceiver. The beamforming network forms the beam from the antenna matrix and steers the beam direction electronically without mechanical motion. Such an electronically steered beamforming network can be designed using time or frequency domain analysis of the antenna elements and associated electrical components. For the multi-beam antenna system discussed, frequency domain analysis is used when designing the beamforming network for broadband applications.

To minimize RF signal losses and maintain signal properties such as phase and amplitude, the beamforming network is typically placed close to or integrated into the antenna assembly. In this example, the beamformer is placed close to the antenna and phase-matched cables are used to match the phase across the matrix. These phase-matched cables provide ±1 degree phase matching accuracy over the desired frequency band. Each 36-inch cable length contributes less than 0.5 dB of insertion loss.