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.milAbstract(useful for both COTM and SOTM) is an ALGaAs SP8T PIN
Diode Switch available from M/A-COM. This switch operates
from 50 MHz to 40 GHz with a maximum insertion loss 2 dB anda switching speed of 20 ns. As such, it is ideally suited for both
COTM and SOTM applications.

In this paper we describe some Rotman Lens development softwaredeveloped for the Army Research Lab by Remcom Corp under aPhase II SBIR. After software simulation, the Rotman Lens wasfabricated and measured with impressive results.

Keywords
Rotman Lens

1. Introduction
Affordable Electronic Scanning Arrays (ESAs) havelong 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 isto 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 eachelement of the array – increasing the complexity (and cost) of thedesign. For SOTM (generally at X, Ku, or Ka bands) theinsertion losses become higher.
Building on its historical work with Rotman Lenses [1] – [3], theArmy Research Lab awarded an SBIR to REMCOM Inc. todevelop software that would allow rapid designs of Microstrip
and Stripline Rotman lenses, Figure 1. By the nature of thedesign, a Microstrip Rotman lens is an inherently simple structurethat achieves electronic scanning while avoiding such problems asbeam squint. The cost of a Rotman lens implemented on
microwave material is primarily driven by the cost of the material
itself and the price of photo etching. This results in an affordable
ESA that avoids the complexities of individual phase shifters foreach element of the array with insertion losses comparable to thediscrete phase shifters.

Figure 1 - A Microstrip Rotman Lensdeveloped with REMCOM
software.

2. Development and Measurement of the Lens
Our design objectives were to develop a simple Rotman
beamformer fabricated for operation at 4.6 GHz to support
terrestrial communications. Specifications required +/- 60
Degrees in azimuth scanning and no scanning in elevation. TheREMCOM Rotman Lens Design (RLD) software was used as thesoftware design tool. This initial prototype was fabricated using arouter to realize a first cut design. The Rotman was then attached
to an array of patch antennas and the patterns measures, Figure 2.
We found surprisingly good performance in comparison withtheory. Photographs of the front and back of the prototype arepresented in Figure 3. Simple patch antennas are used for theradiating elements.

The array ports (shown in red) are connected to the radiatingelements. Since the Rotman is realized on planar microwave
material, an obvious radiator is a patch antenna array. Suchelements can be integrated onto the substrate producing a simple
and affordable ESA. Scanning is achieved by selecting anappropriate beam port (shown in gray) – requiring a low loss
switch matrix. A number of excellent COTS switches havebecome available that are ideal for this ESA. One good example

1Figure 2 – Comparison of measured and theoretical Radiation
patterns for the Terrestrial Link Rotman Lens.

As seen in the measured data presented in Figure 2, the REMCOM
RLD, based solely on Geometric Optics, gave measured results very
consistent with the theoretical predictions. A slight loss of power isnoted for the two beams oriented toward broadside – which we
attribute to side reflections from the lens attenuating the beam. TheRLD software is for obtaining a first cut model of the lens. It is
understood that the model will then be exported and optimized using
REMCOM’s XFDTD software. The measured system gain of thebeamformer and the antenna array was 6 dBi which includes aninsertion loss of 10 db for the Rotman lens and the connectors.
This Rotman lens was extensively measured on a network analyzer
over a frequency range of 4.0 to 5.0 GHz. As the targetedfrequency of operation was 4.6 GHz, we now present measured datafor input ports 1 to 8 (seen on Figure 3), showing the progressive
phase shift needed to form the beam at various scan angles at theoutput ports, Figures 4 to 7. Note the good measured linearity –even at the most pronounced scan angle of Figure 7.

Figure 3 - Prototype of Rotman
communications with patch array.

2lensforTerrestrial

Figure 4 – Progressive phase shift for array ports exciting beamports 4 and 5.

Figure 7 - Progressive phase shift for array ports exciting beam ports
1 and 8.

As a Rotman lens is supposed to be a wideband “true-time delay”
beamformer [4], we expect to observer a linear phase shift at eachoutput port as a function of frequency. Such a plot is presented in
Figure 8. For this plot, we observe the phase shift measured between
beam port #2 and the array port #9. Again we note good linearity indicating a stable beam position over a wide frequency range.
Similar plots are obtained for the matrix of other beam and arrayports.

Figure 5 - Progressive phase shift for array ports exciting beam ports
3 and 6.

Figure 8 – Measured phase between beam port #2 and the array port
#9 versus frequency.

Figure 6 - Progressive phase shift for array ports exciting beam ports
2 and 7.

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3. Conclusions

References

We have used newly developed Software by REMCOM to realize
an affordable Rotman lens. The software (RLD) is based onGeometric Optics and gives a first cut for the design. We found thisto give very satisfactory results without using the morecomputationally intensive XFDTD code.
Measured results
validated the simulations. Follow on work will integrate active
components (amplifiers) between the lens and the radiating elements
as well as a switch array for selection of the input ports.

1.

S. Weiss, R. Dahlstrom, “Rotman Lens Development at
the Army Research Lab,” Proceedings of the AESS
conference, Big Sky, MT., March 2006.

2.

O. Kilic, S. Weiss, “Dielectric Rotman Lens Design for
Multi-Function RF Antenna Applications,” Proc. 2004 IEEE
AP-S/URSI Int. Symposium, Monterey, CA.

3.

R. Dahlstrom, A. Bayba, “A Rotman Lens
Implementation for Multifunction RF Applications,”
Proc. 2002 Antenna Application Symposium, Allerton
Park, Monticello, Illinois.

4.

W. Rotman, and R. F. Turner “Wide-Angle
Microwave Lens for Line Source Applications,” IEEE
Trans. on Ant. and Prop. 1963 pp. 623-632.

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