DEVELOPMENT OF SIMPLE AFFORDABLE BEAMFORMERS FOR ARMY PLATFORMS

Development of Simple Affordable Beamformers for Army Platforms

Steven Weiss, Steven Keller, and Canh Ly

Army Research Lab

Adelphi, MD

301 394 1987

sweiss@arl.army.mil

Abstract

(useful for both COTM and SOTM) is an ALGaAs SP8T PIN

Diode Switch available from M/A-COM. This switch operates

In this paper we describe some Rotman Lens development software

from 50 MHz to 40 GHz with a maximum insertion loss 2 dB and

developed for the Army Research Lab by Remcom Corp under a

a switching speed of 20 ns. As such, it is ideally suited for both

Phase II SBIR. After software simulation, the Rotman Lens was

COTM and SOTM applications.

fabricated and measured with impressive results.

Keywords

Rotman Lens

1. Introduction

Affordable Electronic Scanning Arrays (ESAs) have

long been a desirable technology for Army requirements such as

Satellite on the Move (SOTM) and Communications on the Move

(COTM). One way to fabricate an electronically scanned array is

to use Commercial off the Shelf (COTS) phase shifters. For

COTM C-band systems, these shifters tend to have insertion

losses on the order of 3 or 4 dB and they require a shifter for each

element of the array increasing the complexity (and cost) of the

design. For SOTM (generally at X, Ku, or Ka bands) the

insertion losses become higher.

Building on its historical work with Rotman Lenses [1] [3], the

Army Research Lab awarded an SBIR to REMCOM Inc. to

develop software that would allow rapid designs of Microstrip

Figure 1 - A Microstrip Rotman Lensdeveloped with REMCOM

and Stripline Rotman lenses, Figure 1. By the nature of the

software.

design, a Microstrip Rotman lens is an inherently simple structure

2. Development and Measurement of the Lens

that achieves electronic scanning while avoiding such problems as

beam squint. The cost of a Rotman lens implemented on

microwave material is primarily driven by the cost of the material

Our design objectives were to develop a simple Rotman

itself and the price of photo etching. This results in an affordable

beamformer fabricated for operation at 4.6 GHz to support

ESA that avoids the complexities of individual phase shifters for

terrestrial communications. Specifications required +/- 60

each element of the array with insertion losses comparable to the

Degrees in azimuth scanning and no scanning in elevation. The

discrete phase shifters.

REMCOM Rotman Lens Design (RLD) software was used as the

software design tool. This initial prototype was fabricated using a

The array ports (shown in red) are connected to the radiating

router to realize a first cut design. The Rotman was then attached

elements. Since the Rotman is realized on planar microwave

to an array of patch antennas and the patterns measures, Figure 2.

material, an obvious radiator is a patch antenna array. Such

We found surprisingly good performance in comparison with

elements can be integrated onto the substrate producing a simple

theory. Photographs of the front and back of the prototype are

and affordable ESA. Scanning is achieved by selecting an

presented in Figure 3. Simple patch antennas are used for the

appropriate beam port (shown in gray) requiring a low loss

radiating elements.

switch matrix. A number of excellent COTS switches have

become available that are ideal for this ESA. One good example