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Antenna Test Range

Final ReportSpring Semester 2013

Team Members:

Brent AtkinsonDaryl FreemanAaron HallerKaiyan Sheng

Department of Electrical and Computer EngineeringColorado State University

Fort Collins, Colorado 80523

Project advisors: Professor Branislav Notaros, and Olivera Notaros

APPROVED BY: Professor Notaros

Abstract

The purpose of this project is to build an anechoic test chamber to allow for the character-ization of microwave antenna and devices here on the Colorado State University campus. Thisfacility will allow for both near-field and far-field antenna characterization, development of novelantenna concepts and designs, verification of modeling and simulation results, analysis of elec-tromagnetic and human body interaction and atmospheric scattering, and most importantly avaluable resource for research and teaching at CSU.

This is a continuation project that has evolved over several years from the initial constructionof the chamber, fabrication of the antenna positioners, development of hardware, software, andfirmware to control the positioner systems and perform RF measurements utilizing an AgilentPNA network analyzer. The project is truly multi-disciplinary incorporating electromagnetics,electronics, hardware design, software development, firmware development, microcontrollers, RFsystem and component design, control systems, and mechanical systems.

The primary goal of this years team was to incorporate the work of previous teams withthe construction of the chamber and fabrication of the positioner systems, but truly focus onthe improvement of the electrical hardware, firmware, and software design in order to have anaccurate, robust, and reliable system.

For the first time in the five years this project has been going, we have successfully broughtthe system to a state where the true heart of the project has been realized: the characterizationof microwave antenna and devices.

The system is now capable of performing automated measurement scans to determine themaximum gain and directivity, radiation efficiency, HPBW, and VSWR of an antenna undertest from the single click of a button. In addition, the user can analyze individual 2-D radiationpattern slices and create 3-D radiation patterns to provide additional visualization in the analysis.

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Contents

1 Introduction 5

2 Applications 5

3 Overview of the System 5

4 Hardware 64.1 Mechanical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1.1 Speed Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 Electrical Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.2.1 Background information on stepper motors . . . . . . . . . . . . . . . . . . . . . . . . 74.2.2 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.2.3 Arduino Microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2.4 Stepper Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2.5 Electrical hardware management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2.6 Limit switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Firmware 11

6 Software 116.1 Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.2 Current Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.2.1 Menu dropdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.2 System connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.3 Motor control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.4 Motor position display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.5 Text display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.2.6 Graph display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.2.7 Antenna parameter display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.2.8 Measurement scan control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.2.9 3-D Radiation Plotting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2.10 Solutions to control issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7 RF System 167.1 Agilent E8364B PNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.2 Software integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.3 Low-loss RF Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.4 Agilent Sponsorship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

8 Conclusions and Future Work 178.1 Radiation pattern distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188.2 Near-field to far-field conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188.3 Vertical motor speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198.4 Measurement scan time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208.5 Motor movement accuracy and resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218.6 Polarization motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218.7 Impact of exposed metallic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

9 Acknowledgments 22

10 References 23

Appendix A Budget 24

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Appendix B Analysis and characterization of antennas 25B.1 VSWR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.2 Reflected Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.3 Received Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.4 Transmitted Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.5 Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.6 HPBW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26B.7 Directivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Appendix C RF chain 27

Appendix D Purchased Antenna Specifications 28D.1 X-band horn antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

D.1.1 Product Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28D.1.2 Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28D.1.3 Gain vs. Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28D.1.4 Radiation Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

D.2 Ku-band horn antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29D.2.1 Product Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29D.2.2 Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29D.2.3 Gain vs. Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30D.2.4 Radiation Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

D.3 Links to company website and full datasheets . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Appendix E Stepper motor specifications 31E.1 NEMA-17 bipolar stepper motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

E.1.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31E.1.2 Torque vs. Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31E.1.3 Dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

E.2 NEMA-23 bipolar stepper motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32E.2.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32E.2.2 Torque vs. Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32E.2.3 Dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

E.3 NEMA-34 bipolar stepper motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33E.3.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33E.3.2 Torque vs. Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33E.3.3 Dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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List of Figures

1 Diagram of antenna test chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Motor Positioner Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Spherical stepper driver PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Toshiba TB6560 3-axis microstepper controller . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Management of electrical hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Previous version of ATR software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Current version of ATR software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 2-D radiation pattern slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Gain vs. frequency plot in X-band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 3-D radiation pattern plot created from measured data . . . . . . . . . . . . . . . . . . . . . . 1511 Comparison of measured data between old and new antenna . . . . . . . . . . . . . . . . . . . 1712 Radiation pattern distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1813 Vertical motor rotary stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1914 Proposed reinstallation of vertical motor rotary stage . . . . . . . . . . . . . . . . . . . . . . . 2015 RF chain diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2716 LB-90-15 dimensions [mm] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2817 LB-90-15 gain vs. frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2818 LB-90-15 radiation pattern at 10.0 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2919 LB-62-15 dimensions [mm] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2920 LB-62-15 gain vs. frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3021 LB-62-15 radiation pattern at 15.0 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3022 NEMA-17 torque vs. speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3123 NEMA-17 dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3124 NEMA-23 torque vs. speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3225 NEMA-23 dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3226 NEMA-34 torque vs. speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3327 NEMA-34 dimensions and wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

List of Tables

1 Bipolar stepper motor phase configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Cable attenuation at 40.0 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Budget for the Fall 2012/Spring 2013 academic year . . . . . . . . . . . . . . . . . . . . . . . 244 NEMA-34 bipolar stepper motor specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 275 LB-90-15 X-band horn antenna specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 LB-62-15 Ku-band horn antenna specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 297 NEMA-17 bipolar stepper motor specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 318 NEMA-23 bipolar stepper motor specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 329 NEMA-34 bipolar stepper motor specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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1 Introduction

RF anechoic chambers are widely used by manufacturers of both defense and commercial products. Some-times their purpose is to keep extraneous signals from interfering with a measurement, and at other timesthey are used to protect sensitive information from outside sources. Usage examples include research in elec-tromagnetic characteristics of new synthetic materials, radar cross-sectional (RCS) performance, validationof theoretical designs, or implementation of new computationally derived designs.

Antenna test ranges are a subset of general anechoic chambers. As commercially available systems arevery expensive, the goal of the Antenna Test Range senior design project at Colorado State University wasinitially to design and construct an affordable, functional, antenna test range to be used as a classroomtool in various applications without the financial impact. The project has integrated numerous advancedresearch utilities and knowledge bases in the fields of electromagnetics, communications, circuit theory,power, mathematics, computational algorithms, and programming. The research conducted in and for thischamber will likely be in design and verification of developmental antenna configurations, probably on higherfrequencies and on student constructed antennas, wireless networks, or communication systems.

In order to achieve the absorption or isolation of electromagnetic waves, special cone shaped absorbersline the walls of ours and most RF anechoic chambers. When the incident wave from the transmittingantenna hits the cone it will be reflected into the wall, absorbed, and will not reflect back into the chambercausing interference with the measurements

Test range classifications generally fall into one of three categories: far field, near field, and compact. Thenear and far field ranges differ in the electrical distance at which the measurements are taken. The majorityof systems intend to measure the far-field regions due to the frequencies of interest in the consumer electronicsdevices. However, once a characterization of the near-field patterns of an Antenna Under Test (AUT) havebeen obtained, the far-field approximation can be computed using Bessel functions and the higher orderterms can generally be neglected due to their greatly reduced impact at electrically large distances.

Wireless telecommunications systems in consumer electronics is an enormous field. With so many prod-ucts, services, situations, and companies engaging in disputes over spectrum allocations and nearby interfer-ence from competitors, accurate, safe, and affordable antenna designs are necessary. Antenna designs withoptimized gain, directivity, and intensity are naturally preferable.

2 Applications

The Antenna Test Range facility will allow for both near-field and far-field antenna characterization, devel-opment of novel antenna concepts and designs, verification of modeling and simulation results, analysis ofelectromagnetic and human body interaction and atmospheric scattering, and most importantly a valuableresource for research and teaching here at Colorado State University.

3 Overview of the System

The anechoic chamber is approximately 12’ by 18’ wide. The floor, walls, door, and ceiling are all linedwith aluminum sheeting to prevent electromagnetic interference from outside of the chamber. Absorbtioncones are installed along all surfaces of the chamber in order to absorb electromagnetic radiation that maybe reflected from the surfaces of the chamber and interfere with measurements. A diagram of the chamberis given in Figure 1.

There are two mechanical positioner systems: a planar (cartesian) and spherical system. Each system hasmovements with two degrees of freedom. For instance, the planar system allows movements in the horizontaland vertical directions, while the spherical system allows for azimuthal and elevation type movements. TheAUT is mounted to one of the two mechanical positioner systems. Typically this is the spherical system.Therefore the transmitting antenna will be a previously characterized antenna mounted to the planar system,while the AUT will be mounted to the spherical system.

Each system will also have an additional degree of freedom through the implementation of polarizationmovements. These will allow for on-axis rotations of the antenna and will add further depth and analysiscapabilities to the system. Details on the polarization motors will be discussed in Section 8.6

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Figure 1: Diagram of antenna test chamber

Outside of the chamber are the Agilent PNA, electrical hardware to control the bipolar stepper motorsfor the positioner stages, as well as the PC used to control the system.

The whole system can be observed via web cameras installed in the chamber and connected to the maincomputer via USB. This allows the user to observe the system while measurements are being conductedwithout being in the chamber and interfering with the measurements.

The cameras are Logitech QuickCam Orbit AF web cameras and have automated exposure and focuslevels. The cameras can be controlled via a PC application with tilt, pan, and zoom capabilities to viewthe internals of the chamber with minimum medium disruption. The cameras have an extented mountingsystem to position the cameras beyond the absorbers, but keep the majority of the electronics below theabsorbers in the chamber. This reduces the inevitable (if minor) electrical interference of the cameras withthe characterization tests occurring in the chamber.

Lastly, each positioner system also has optical limit switches installed in order to ensure safe operationof the system. These prevent any unsafe movements that may cause damage to the absorption cones, RFand control cables, or to the positioners themselves.

4 Hardware

4.1 Mechanical Systems

Prior teams were involved in the design and fabrication of the motor positioner systems. These were fabri-cated in-house here at CSU in the Mechanical Engineering department machine shop by previous mechanicalengineering senior design students. Commercial anechoic chambers can be extremely expensive to construct,and so a major goal of the project was to build a cost-effective alternative to these commercial “out ofthe box” chambers that can cost upwards of $500, 000. Having the mechanical systems fabricated here oncampus was not only cost-effective, but also valuable hands-on experience for the mechanical engineering

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students involved.

(a) Spherical (b) Planar

Figure 2: Motor Positioner Systems

Figure 2 shows the two positioner systems. The movement of each axis of the positioner systems areaccomplished via bipolar stepper motors. For the horizontal, vertical, and azimuth motors, NEMA-23 bipolarstepper motors are used. Due to the increased torque exerted on the elevation motor stage caused by themoment arm created by having the antenna attached to a 42.5” long extension, a larger NEMA-34 steppermotor was used. Each stepper motor is mated to a 5” rotary stage which provides a 72:1 gear conversion forthe NEMA-23 motors, and an 80:1 conversion on the elevation motor stage which is mated to an 8” rotarystage. The rotary stages allow for extremely high movement resolution, as well as increased holding torquecapacity.

NEMA-23 bipolar stepper motors are designed to optimally run with a drive current amplitude of 2.7 A,and NEMA-34 motors are designed to run with a drive current amplitude of 3.14 A. Specifications for eachstepper motor type are provided in Appendix E.

4.1.1 Speed Issues

Upon taking over the project, our team quickly encountered problems with the mechanical systems. Oneof the main causes of concern was with the speed of the vertical-axis movements. Previously it would takeapproximately 12 min. to move the motor 1 inch. This was an unacceptable rate, and it became a primaryfocus of our team to determine both the cause of this problem, and develop solutions to bring the movementrate on par with the other axis. These solutions will be discussed later in sections 4.2 and 8.3.

4.2 Electrical Hardware

4.2.1 Background information on stepper motors

Stepper motors require three control signals to operate: an enable signal, direction signal, and a digitalpulse to move a single step. Bipolar stepper motors require two step signals that are applied to the twopoles with a phase difference between them. This allows for several different ways to operate the motor:

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One-phase, Two-phase or Hi-Torque, and Half-step. These are representative of the phase overlap betweenthe two signals. Table 1 attempts to illustrate how these phase overlaps occur.

—+—+ –++ –++–+- -++- –+-

One-phase -+– Two-phase ++– Half-step -++-+— +–+ -+-—+ –++ ++–

+—

Table 1: Bipolar stepper motor phase configurations

One-phase is the simplest configuration and is used for high-speed, low-torque situations. This config-uration has a 180◦ phase difference between the two clock signals, however this configuration suffers fromnon-continuous power transfer between the poles due to the shared zero between the signals.

In contrast, a Two-phase configuration ensures that there is always a continuous power transfer beingdelivered to the coils by having a 90◦ phase difference between signals. This allows for smooth torque deliveryat the cost of lower speed.

Lastly, the Half-step configuration is a combination of the two which allows for smooth torque deliveryand high speed.

4.2.2 Previous Work

Previous teams designed two stepper driver boards to control each positioner system. The spherical positionerboard was controlled by an Atmel AtMega168 microcontroller running at 20MHz. The program was writtenprimarily in C using WinAVR and Eclipse Integrated Development Environment (IDE). The program wasuploaded to the controller with the AVR mkII In-System Programmer (ISP). The movement of the motorswas handled entirely by interrupt service routines after timer prescaling calculations were done. The planarpositioner controller board was similar in design but was controlled by an Arduino Uno microcontroller.

The printed circuit boards (PCBs) were designed and implemented using CadSoftEagleCAD and eachcircuit board had four microstepping DMOS driver chips, each of which controlled a single motor. Figure 3displays an image of the PCB layout for these boards.

Figure 3: Spherical stepper driver PCB layout

When our team took over the project we encountered numerous problems with both of these boards. Inboth cases, only three out of the four axis worked. In addition, the spherical system suffered from seriousconnectivity issues. The board would typically work for approximately ten minutes, after which it wouldlose all connectivity. This would last anywhere from 15 minutes to several hours before working again.

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The planar board, while not suffering from the same connectivity issues, had some serious design flaws.In particular, after some research it was discovered that the board was actually designed with a NEMA-17motor in mind, which are designed to operate with a maximum drive current of 1.0 A. The output of theboard was measured to be 900 mA, confirming these suspicions.

Therefore the board was seriously under specced to drive the NEMA-23 motors that are actually used,which require at least 2.7 A. Lastly, soon after taking over the project the board suffered from major electricalfailure with two of the DMOS driver chips shorting out, most likely caused by excessive strain on the electricalcomponents.

At this point, our team was faced with several options: We could either redesign both boards, sendout to have the PCB’s fabricated, test the new boards and reiterate as needed, or we could look into somecost-effective commercial alternatives in order to move ahead with the project. In order to stay on trackwith our projected time-line, we decided to go with a commercial alternative purchasing a Toshiba TB6560Microstepper Controller board and an Arduino MEGA microcontroller.

4.2.3 Arduino Microcontroller

The Arduino MEGA microcontroller is used to process commands from the PC and send the appropriatecontrol signals to the stepper motors. The speed of the motors is determined by the frequency of the clocksignal sent out by the Arduino, however the speed and torque of each motor is inversely proportional. Forthe high torque demands of the elevation stage, the motor is operated at a lower speed of about 40% of themaximum speed allowed. All other motors are running at 66% of the maximum speed. This seems to be agood setting for the motors with no slippage having been encountered thus far.

It was found that the duty cycle of the clock signals play an important role in obtaining smooth motormovements. After experimenting with various possibilities, it was found that a 43.5% duty cycle allowed forvery smooth movements with no slippage. In addition, each clock signal is pulse-width modulated to ensureaccurate signal generation.

Lastly, the microcontroller also handles all limit switch signals in order to ensure safe operation of thesystem.

4.2.4 Stepper Drivers

A Toshiba TB6560 4-Axis microstepper controller is used to send the appropriate control signals to themotors. Figure 4 gives an image of the board.

Figure 4: Toshiba TB6560 3-axis microstepper controller

The TB6560 allows for selectable current output for each axis (max 3.0 A), 1,4,8, or 16 step microstepdivision to smooth out the signal being supplied to the motors with a more sinusoidal approximation, andselectable phasing configurations (high-torque, high speed, half-step, etc.). We are currently running theboard at 3.0 A, 8 microstep divisions, and the half-step phase configuration.

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The purchase of the TB6560 effectively solved all of our electrical hardware issues. All motors are nowoperating under their optimal specifications, and this dramatically improved the speed of the planar systemwhich was suffering from the previous under specced stepper driver board. With the current speed settings,the horizontal motor moves at about 1.6 inches/sec. and the vertical motor moves at about 0.03 inches/sec.This means that it currently takes about 33 seconds for the vertical motor to move one inch.

While this is a vast improvement over the previous speed, it is still not satisfactory. It has been determinedthat further improvements will have to be solved from the mechanical end and is discussed in Section 8.3.

Due to the success of the new TB6560, our team went ahead and purchased an additional 3-axis versionof the board which will be used to control the future polarization movements.

4.2.5 Electrical hardware management

At this point, managing the Arduino, two stepper driver boards, two power supplies, and all of the variouswiring, buses, and control cables between the boards and motors became very cumbersome. In order toaddress this issue, our team came up with the idea of using an old desktop computer case to house all of thevarious electrical hardware.

Figure 5: Management of electrical hardware

This proved to be a very effective solution, all while not taking away from the senior design team budgetby having to either purchase commercially available plastic cases, or utilize a 3-D printer to fabricate customunits to house each component.

Figure 5 shows the layout of the electrical components housed in the old computer case. 1) is the 3-axis stepper board to be used for future polarization movements, 2) is the 4-axis stepper board used forthe primary motor axis of the planar and spherical positioner motors, and 3) is the Arduino MEGA. Alsocontained within the case, but not shown in the image, are the two power supplies used to power the stepperdriver boards. These power supplies are mounted at the front of the case where additional cooling fans arelocated.

4.2.6 Limit switches

In order to prevent unsafe motor movements, a number of limit switches have been installed. The limitswitch signals are handled by the Arduino microcontroller.

For the vertical axis, an infrared (IR) proximity detection sensor is used. This returns an analog voltagesignal whose magnitude is related to the distance between the two sides of the device. When the voltageexceeds a set threshold, the Arduino will stop the motor and move the stage in the opposite direction untilthe voltage is once again below the threshold.

For the horizontal, azimuth, and elevation axis, IR interrupt detection sensors are used. These are digitaldevices that read a high or low dependent on whether the IR beam reaches the other side of the device or is

10

interrupted by a flag mounted on each positioner stage. As in the vertical axis, when an interrupt is detected,the Arduino will stop the motor and move the stage in the opposite direction until the signal indicates a safeposition.

5 Firmware

Firmware for the Arduino MEGA was written using the Arduino IDE. The Arduino is an open-sourcedevelopment platform and has several libraries available which help to handle many of the common, low-level functions of the microcontroller such as setting the voltage at the digital I/O pins.

The firmware was written in C++, and communication with the PC is handled through a USB connection.The firmware receives commands from the PC, processes the commands, and sends the appropriate signalsout to the stepper driver boards. The code keeps track of the position of all the motor axis, and communicatesthis information back to the PC so that the software can keep track of the motor positions. In addition, thefirmware handles all limit switch signals during any motor movement.

6 Software

The central hub of the system is the Graphical User Interface (GUI) which controls both the positioner andmeasurement systems. The software is written in C# using Microsoft Visual Studio 2010. The program isa Windows Forms application with multiple classes and forms designed to handle the numerous functionsnecessary.

One of the primary goals when designing this software was to provide all of the necessary functionalityfor characterizing microwave antennas, but also doing it in a way that is simple, intuitive, and user-friendlyso that anyone can sit down and quickly learn how to operate the system and perform measurements.

6.1 Previous work

Previous teams on the project had built a basic GUI that communicated between the PC and both sphericaland planar stepper driver boards using two virtualized serial COM ports through USB ports. In addition,some basic graphing functionality was in place and communication with the PNA through a LAN connectionwas established. However there were some major design flaws from a controls perspective.

While the functionality was theoretically in place, there was no control between the movement side ofthe system and the measurements. They were using a BackgroundWorker thread to handle the processorintensive tasks of performing an automated measurement scan, retrieving data from the PNA, and plottingthe data. The idea was to use a background thread in order to keep the main thread of the program free toupdate the form and plot the data.

However there were serious flaws when it came to cross-thread event handling, in that essentially therewas none. You could run the motors through the sweep range, or you could retrieve information from thePNA, but the two tasks were in no way being performed at once. In addition, while performing a scan therewas no way for the program to receive feedback information from the positioners. Therefore the softwareand firmware could not remain synchronized during a measurement scan.

Lastly, the software was very difficult to work with, being riddled with excessive buttons, text boxes, andextraneous information. An image of the original software is given in Figure 6.

6.2 Current Software

A complete overhaul of the software was performed from the ground up in order to address not only themany control issues inherent in previous versions of the software, but also to address inheritance and codingstructure issues in order to ensure reliable, and robust software.

Thorough testing and debugging was performed in order to address any and all software bug issues.While there are always bugs that can reveal themselves, great care was made to address as many bugs aspossible to ensure a reliable system that would not crash under normal use.

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Figure 6: Previous version of ATR software

Figure 7 displays a screenshot of the current ATR software with the various subsections labeled. Thesubsections are as follows: 1) Menu dropdown, 2) System connections, 3) Motor positioner control, 4)Motor position displays, 5) Text display, 6) Graphs, 7) Antenna parameter display, and 8) Measurementscan control.

6.2.1 Menu dropdown

The menu dropdown gives access to additional controls and functionalities including links to system docu-mentation such as a Quick-Start guide to operating the software and the full system documentation, access toa virtual PNA console to control measurement parameters, saving and loading of radiation pattern measure-ment data, 3-D radiation pattern plotting, curve fitting, and Gain/VSWR vs. frequency data saving/loading.

6.2.2 System connections

This panel provides connections to both the positioner and measurement systems. The positioner systemconnects via USB to the Arduino microcontroller, while the measurement system connects with the PNAvia ethernet connection

6.2.3 Motor control

This panel allows for both single-axis motor movements as well as movement to a specified coordinate. Thegoal was to provide a simple movement control layout that could handle any type of movement but keep thesoftware clean and simple.

6.2.4 Motor position display

This section has both graphical and numeric displays for the position of the various motor axis. The graphicaldisplays are for the azimuth and elevation motors of the spherical system. The azimuth display is a top-down

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Figure 7: Current version of ATR software

view which indicates the direction the antenna is pointing, and the elevation display is a side-view of wherethe antenna is in space.

The purpose of these displays is to help the user visualize exactly where the antenna is located sincevisualizing this is often difficult based on just a number value.

Each motor also has a numeric display that gives the position value for each axis in relation to eachsystem’s origin.

6.2.5 Text display

This is a text box that displays feedback from the software to let the user know the status from variousevents. In addition, the bottom bar displays the connection status of the two subsystems which turn greenwhen the connection is established.

6.2.6 Graph display

This section displays the graphs associated with the various measurements possible with the software. Thereare four different tabs: Cartesian, Polar, Gain, and VSWR.

The first two tabs display the radiation pattern for a given 2-D radiation pattern slice, while the last twodisplay the gain and VSWR of the antenna as a function of frequency.

6.2.7 Antenna parameter display

This section displays the essential characterization parameters for the antenna under test. These includethe maximum Gain, Directivity, Half-power Beamwidth (HPBW), VSWR, and the reflected power at thetransmitting antenna.

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6.2.8 Measurement scan control

Lastly we have the measurement scan control panel. This panel has two different tab panels: Radiationpattern, and Gain/VSWR.

The radiation pattern tab allows the user to first select the type of scan to perform: Spherical, Planar,or a hybrid Cylindrical type scan which will sweep the azimuth and vertical motor axis. Next the user canchoose whether they want to perform a single axis scan (single 2-D radiation pattern slice) or a sweep-typescan (multiple 2-D slices). For a single-axis scan the user can then select which motor axis to sweep.

The user can input the range of the scan for each motor axis as well as the desired movement resolution.While the scan is being performed, the progress bar at the bottom of the panel will display the progress ofthe scan, as well as plot the pattern in real-time in the cartesian graph panel.

Once the scan is complete, a polar radiation plot will be made and is available for viewing in the polargraph panel. In addition, the HPBW and estimated Directivity will displayed. If the measured data isacceptable, the user can then save the data for future analysis. Examples of typical patterns are displayedin Figure 8.

(a) Cartesian plot (b) Polar plot

Figure 8: 2-D radiation pattern slices

This Gain/VSWR panel allows the user to measure the gain and VSWR of the antenna as a function offrequency. There are currently three options for the frequency range of the measurement: X-band, Ku-band,or a dual band analysis from 8.0 to 18.0 GHz.

After the measurements are complete, the data is displayed in the appropriate graphs and the Gain,VSWR and reflected power at the center of the selected frequency band are displayed. An example of atypical gain measurement is given in Figure 9.

Figure 9: Gain vs. frequency plot in X-band

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Gain calculations are all performed using the Friis transmission formula, the details of which are givenin Appendix B.

6.2.9 3-D Radiation Plotting

The software also gives the ability to create a 3-D radiation plot from measured data. This is accomplishedusing a standalone MATLAB executable program written by this years team. The program takes a data fileas an input argument and creates a normalized 3-D radiation pattern from the given data.

Figure 10: 3-D radiation pattern plot created from measured data

This is fully integrated into the software and allows images to be created with a single button click. Inaddition, the program has been written so that there is no need to have MATLAB installed on the PC inorder to create these plots. An example of a typical pattern is given in Figure 10.

6.2.10 Solutions to control issues

As mentioned in Section 6.1, the previous software suffered from control issues between the positioner andmeasurement systems. This was the central focus when rewriting the software, and was built up one pieceat a time.

The primary source of the control issues was in feedback from the positioner systems. Movement com-mands would be sent out, but the application would not be free to listen for feedback from the positionersystem to know when a movement was complete. The result was that the software was never synchronizedwith the positioner systems, and so the software always thought that the motors were located at differentpositions from where they actually were.

This was addressed by using a new thread devoted strictly to serial communication. Whenever a move-ment command is completed, the Arduino sends a confirmation message with the updated position valuesfor each motor axis. This ensures that no matter what else is going on in the software, the positions of allthe various motor axis are constantly updated and the software and firmware are always synchronized.

The next step was in handling automated measurements. Retrieving information from the PNA, pro-cessing the data, graphing the data in real-time, and sending out movement commands is a very processorintensive situation. Again, this was addressed by using a new thread to handle all scan methods, howeverthis is further complicated by the fact that we now need to synchronize three different threads.

After a great deal of research, and numerous techniques tested, a synchronous thread marshalling tech-nique was found that could handle all of these cross-thread events. This allowed for the proper synchroniza-tion between the two separate systems, all while keeping the main thread open to allow real-time positionupdates and graphing.

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7 RF System

7.1 Agilent E8364B PNA

RF measurements are performed using an Aglient E8364B Precision Vector Network Analyzer (PNA). Anetwork analyzer is a powerful measurement device capable of S-parameter measurements, Smith chartanalysis, noise figures, gain compression, time domain analysis, phase analysis, and much more.

The E8364B has a frequency range of 10.0 MHz to 50.0 GHz, and a dynamic range of 104 dB. This allowsthe chamber to have a wide frequency range to test under, and the dynamic range is helpful given that ourchamber is not a perfectly lossless chamber and we do not have a low-noise amplifier in the RF chain.

7.2 Software integration

Communication with the PNA is performed via LAN connection using TCP protocols. Commands are issuedto the PNA using the Agilent VISA-IVI C# library. These are a Visual Basic style command called SCPI.For determining the proper syntax for commands we have been using the Agilent Command Expert softwarewhich has been an invaluable tool for integrating the PNA controls with the software and exploring thenumerous capabilities of the PNA.

A complete overhaul of the previous PNA software code was performed to both add new features andcapabilities, and to improve the efficiency of measurement retrieval. All of the key settings and controls havebeen integrated into the software to form a sort of virtual PNA console within the application. In addition,the software now has the capability to perform measurements at multiple frequencies for a single positionpoint in a measurement sweep in order to create radiation pattern data that spans several frequencies in asingle measurement scan.

7.3 Low-loss RF Cables

A recent addition to the system, which occurred last summer, was the purchase of new low-loss RF cables.These were purchased from MegaPhase and details on the cables can be found in Appendix C . Losses in awireless link can be significant due to both losses in the cables and connections, but also from unavoidablefree space losses. It was important to determine the need for a low-noise amplifier in the system if the totallosses far exceeded the dynamic range of the PNA.

One of the first tasks our team took on at the beginning of the Fall semester was determining theattenuation in the cables and comparing with the manufacturers specifications. Testing the cables resultedin favorable measurements, the results of which are given in Table 2.

Cable # Measured losses Manufacturer reported losses1 27.80 dB 27.82 dB2 14.65 dB 14.78 dB3 4.77 dB 4.68 dB

Table 2: Cable attenuation at 40.0 GHz

Based on these results, we calculated the losses over the entire chain to check that we would still fallwithin the dynamic range of the PNA. The losses in free-space is determined by

AdB = 22 + 20 log( rλ

)−Gt −Gr (1)

where r is the distance between the two antenna, λ is the wavelength of the signal, Gt and Gr are themaximum gain of the transmitting and receiving antenna respectively. In order to be in the far-field werequired that r ≥ 30λ.

As a worst-case scenario at 40.0 GHz with Gt = Gr = 0, the losses in free-space came to 51.54 dB. Thetotal losses in the cables and connectors came out to 55.65 dB. This makes a worst-case scenario total lossin the RF chain of 107.19 dB. The dynamic range of the PNA is 104 dB, putting our worst-case scenariojust outside the range of the PNA.

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7.4 Agilent Sponsorship

Agilent Technologies was kind enough to host a senior design competition, and our team was able to win$1,000 for the project. With this money we were able to purchase two brand new, professional grade antenna.

After researching different possibilities and receiving quotes from several companies we ended up pur-chasing two antenna from A-Info. Both antenna have 15 dB gain. One antenna was designed for the X-band,while the other was designed for the Ku-band. We chose to purchase antenna in two separate frequencybands to expand the range of the system.

The addition of two high-quality antenna to the system made a dramatic difference. Prior to this, we wereusing antenna which we had termed the “beer can” antenna. These were very poor quality, high-loss antennaand measurements from these antenna were characterized by poorly defined lobes and high amounts of noisein the signals. A comparison of the measurements with the old and new antenna is given in Figure 11.

(a) New antenna radiation plot at 10.0 GHz (b) Old antenna radiation plot at 10.0 GHz

Figure 11: Comparison of measured data between old and new antenna

Details of the costs and data sheets for the new antenna can be found in Appendix A and Appendix Drespectively.

8 Conclusions and Future Work

Upon taking over this continuation project, our team was faced with numerous challenges. These includedelectrical, mechanical, and software issues, not to mention the challenges inherent in a multidisciplinaryproject such as this one. However it was felt that all of the basic pieces were in place, and it was up to usto put all of the pieces together in order to get the system up and running.

This presented a prime example of what the engineering design process is all about. Not only did wehave to determine the causes of the issues we were faced with, but also determine the most cost-effectiveand time efficient solutions to the problems in order to move ahead with the project and stay within ourprojected time-line.

We feel that our team has faced these challenges head-on. For the first time in the five years this projecthas been going, we have successfully brought the system to a state where the true heart of the project hasbeen realized: the characterization of microwave antenna and devices.

The system is now capable of performing automated measurement scans to determine the maximum gainand directivity, radiation efficiency, HPBW, and VSWR of an antenna under test from the single click of abutton. In addition, the user can analyze individual 2-D radiation pattern slices and create 3-D radiationpatterns to provide additional visualization in the analysis.

However, despite all of the improvements and success our team has realized, there is still a great deal ofwork to be done on the project. The following sections list some of the improvements we feel can and shouldbe focused on in the future of the project.

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8.1 Radiation pattern distortion

Due to the relatively small size of the chamber, we have encountered an issue that leads to distortion in theshape of the measured radiation patterns. The far-field approximation assumes that the electrical distancebetween the two antenna is relatively constant. However, when performing a far-field measurement scanusing the spherical system this is not the case.

For the spherical positioner system, the AUT is mounted at the end of a 42.5” rod. If the measurementbegins with the positioner at an elevation angle of -60◦ and ends at +60◦, this ends up bringing the AUTover 100 wavelengths closer to the transmitting antenna on the planar system. This leads to a maximumreceived power being achieved at approximately +10◦ instead of at 0◦. An example of this pattern distortionis given in Figure 12

Figure 12: Radiation pattern distortion

In order to correct for this, a more rigorous, localized antenna coordinate system must be developedwhich tracks the true location of the antenna in space relative to the transmitting antenna, and not just thecoordinate of the physical positioner system.

This will add additional constraints to the system in that the physical distance separating the two antennamust first be accurately measured when both positioners are at their respective origins. From this, it canbe determined through some simple trigonometry the change in antenna separation based on the geometryof the spherical system. However, if the spherical system is moved within the chamber, any hard-codedcalculations will be thrown off.

When plotting a radiation pattern, we are measuring the received power. The received power is propor-tional to r2, where r is the magnitude of the separation vector between the two antenna. The correctionfactor will add a ∆r factor from the relative movement of the AUT with respect to the transmitting antenna,making the received power proportional to (r + ∆r)2.

8.2 Near-field to far-field conversion

When characterizing an antenna, one is typically only interested in the far-field characteristics of the AUTsince antenna are typically used for wireless communication over large distances relative to the electricalseparation between the two devices. Exceptions to this are when conducting electromagnetic and humanbody interaction. An example of this would be in medical applications where the device applies a highlylocalized dose of electromagnetic radiation within several inches of the treatment area.

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The current state of the system only allows for far-field measurements, however it would be very beneficialto expand the functional range of the system to include near-field characterization. A former studentof Professor Notaros helped to develop a near-field to far-field conversion algorithm which utilizes Besselfunctions, however this algorithm has never been tested.

The Antenna Test Range provides a perfect medium for testing and verification of this algorithm. Theoriginal algorithm was written using MATLAB but could easily be incorporated within the ATR software.Verification would be performed by taking measurements in the far-field, moving the spherical positioner towithin the near-field regime, performing a new measurement, and comparing the results.

This would not only verify the algorithm for use in numerical analysis and simulations, but also expandthe effective frequency range of the chamber. The chamber was designed for characterization within theX-band, primarily due to the length of the absorption cones that line the chamber. However, this does notexclude testing outside of this frequency band. While the quality of the measurements may suffer from asmall reduction in the efficiency of the absorption cones in suppressing reflected radiation, one could stilltest antenna from outside of the X-band.

For instance, if an individual were to bring in an antenna that was designed to operate at 500 MHz, dueto the size of the chamber no matter where in the chamber the AUT is positioned it would be within thenear-field. The near-field to far-field conversion algorithm would allow for far-field characterization even atthis frequency range.

8.3 Vertical motor speed

As previously mentioned, the speed of the vertical motor movement has been an ongoing issue. The originalmechanical design was focused almost entirely on allowing for extremely high movement precision since theplanar system will be used for near-field measurement. Unfortunately this has also become the main sourcefor the slow movement inherent in the system. Figure 13 displays the current installation of the verticalmotor and rotary stage.

Figure 13: Vertical motor rotary stage

The AUT is mounted on a small platform that has a threaded nut mounted in the center of the platformthrough which a threaded rod is mounted to the center of the vertical motor axis rotary stage as depicted inFigure 13. As the rotary stage rotates, the threaded rod also rotates and pushes the platform up and down.

One of the primary purposes of a rotary stage is to utilize the mechanical advantage from taking a smallrotation of the actual stepper motor and converting it through a gear conversion to a much larger movementat the rotary stage. This particular rotary stage provides a 72:1 gear conversion. However, with the threadedrod being mounted along the center of the rotary stage axis, all of this mechanical advantage is completelylost.

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From an electrical standpoint, we have improved the speed of the vertical axis movements as much aspossible. The stepper motor can only move so fast and still allow for proper torque transfer. We havecurrently set the speed of the motor to the maximum allowable without experiencing slippage in the motormovements due to insufficient torque. At this point a critical analysis must be conducted on the mechanicalsetup to determine if there is another way to utilize the current components to allow for high movementresolution and still provide the necessary torque to move the platform up and down.

A proposed solutions is as follows: Move the rotary stage off to the side and mount the threaded rodas is but with an additional gear attached at the bottom. Install a worm gear to transfer the motion ofthe rotary stage to the new gear in order to utilize the mechanical advantage of the rotary stage. A crudediagram illustrating this idea is given in Figure 14

Figure 14: Proposed reinstallation of vertical motor rotary stage

While this design would still not fully utilize the mechanical advantage created by the rotary stage, itis felt that this would still be a dramatic improvement from the current design. The gear mounted at thebottom of the threaded rod could be designed to realize the desired speed for the vertical motor movementwhile still allowing high movement resolution.

8.4 Measurement scan time

A full spherical measurement scan takes just over two hours, primarily due to the way these scans areperformed. A measurement is taken, then the positioner is moved to the next position point. After amovement there is typically some wobble at the antenna due to the top-heavy nature of the sphericalsystem. We pause for one second to allow the antenna to settle, take another measurement, and the processis repeated. With this move-stop-wait-measure type scan, a lot of excessive time is added to a measurementscan.

The reason for this is to ensure strong correlation between motor position and measured data. We wantto ensure that we hear back from the Arduino and update motor position information before taking the nextmeasurement.

While this is not an excessive amount of time to perform a full characterization of an AUT, it is still feltthat there are improvements to be made in this.

One proposed idea is to perform continuous movement measurement sweeps. In this case the major axisof a sweep (azimuth for a spherical sweep) is swept across its entire scan range in one continuous movementwith measurements being taken at a clocked frequency. The problem here is that there is no certainty inthe correlation of position values to measurements given that depending on the performance of the PC theremay be latency between when these values are registered.

A simple and highly effective solution to this problem would be the installation of optical encoders tomeasure the spherical motor axis positions. Whenever a measurement is performed, the value of the opticalencoder could be read to determine the position of the motor axis. This could be integrated using the

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Arduino microcontroller and would not only allow for this continuous sweep type measurement scan, butalso greatly improve the accuracy of the motor positions.

8.5 Motor movement accuracy and resolution

Currently all motor positions are based on a calculation. Each motor axis requires a certain number of stepsper revolution (200 for all motors), number of microstep divisions from the stepper driver (8 with currentstepper driver settings), and the gear conversion of the rotary stages (72:1 for all motors axis except theelevation). Multiply these numbers together and we find 115,200 clock pulses are required for a full revolutionof the rotary stage.

A problem arises when a given motor movement does not correspond to an integer division of 115,200number of steps. While the error is typically small due to the large number of steps in a full revolution of therotary stage, rounding errors occur between the software and firmware and the local origin for the positionersystem will tend to drift after numerous movements.

This is a major problem that needs to be addressed since over the course of a full measurement scan thisdrift can become significant. The maximum of a given 2-D radiation pattern slice can drift as much as 10◦

by the end of the scan.While this can be corrected for after the fact, it would be much more beneficial to address the motor

position issue through the implementation of a more sophisticated system of measuring the motor positionsuch as the optical encoder mentioned in Section 8.4.

8.6 Polarization motors

Antenna polarization movements still need to be incorporated into the system. These movements will beperformed using NEMA-17 bipolar stepper motors and the Toshiba TB6560 3-axis stepper driver whichhave already been purchased. The firmware and software already have all of the necessary code written tofacilitate these additional motors and have been tested. All that is left is the actual installation of the motorsand a mounting system for the antenna.

When we purchased the new antenna we did so with polarization movements in mind. We went with anendlaunch waveguide adapter on both antenna which allow the RF cable to run along the axis of the antenna,as opposed to a right-angle waveguide adapter. Each waveguide will be mounted directly to a rotary jointwhich will allow the antenna to rotate along its axis without twisting the RF cable.

8.7 Impact of exposed metallic surfaces

Both of the mechanical positioner systems have exposed metallic surfaces. The full impact of how thesesurfaces effect measurements needs to be analyzed, and research into possible solutions needs to be performed.

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9 Acknowledgments

We would like to thank the following people for their assistance throughout the life of the project and thissemester:

• Dr. Branislav Notaros for direction and consultation in all aspects of the project

• Olivera Notaros for keeping the team on track and ensuring a quality project throughout

• Agilent Technologies for their support and contributions

• Sanja Manic for help with cable measurement and layout

• Ana Manic for help with the radiation pattern correction factor calculations

• All of the graduate students in the Applied Electromagnetics Laboratory for all of their help, support,and feedback throughout the year.

Thank you for supporting the project and contributing to the education of the team.

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10 References

[1] B. M. Notaros, Electromagnetics, 2nd ed. Upper Saddle River, NJ: Pearson Prentice Hall, 2010.

[2] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. Hoboken, NJ: Wiley-Interscience, 2005.

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Appendix A Budget

One of the primary goals of this project is building an anechoic test chamber in a cost-effective manner.Throughout the year our team has tried to hold true to these values through careful considerations of allpurchases. Table 3 gives a breakdown of the funding and purchases for the academic year.

Item Amount Total Remaining Funds

Initial budget $400 $400

Agilent funding $1,000 $1,400

4-axis stepper driver $50 $1,350

3-axis stepper driver $48 $1,302

Arduino MEGA $20 $1,282

Power supply (x2) $92 $1,190

X-band horn antenna $355 $835

Ku-band horn antenna $325 $511

Total spent $889

Table 3: Budget for the Fall 2012/Spring 2013 academic year

Each member of the team was allocated $100 by the ECE department towards the project for theacademic year. Funding from Agilent Technologies was pivotal, allowing for the purchase of professionalquality antenna to use as transmitting antenna in the chamber.

In order to quickly move ahead in the design process, the team purchased two commercial stepper driverboards. The 4-axis board was for the major axis of the two positioner systems, and due to the success of thepurchase we also purchased a smaller 3-axis board to facilitate future polarization motor movements.

An Arduino MEGA was purchased due to the extended number of I/O pins available which are necessaryin order to accommodate both the current motors and the future polarization motors. Lastly, two new powersupplies were purchased, one for each stepper driver board.

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Appendix B Analysis and characterization of antennas

B.1 VSWR

The VSWR of an antenna describes how well matched the impedence of the antenna is to the transmissionline. This is based on the reflection coefficient Γ and is given by

V SWR =1 + Γ

1 − Γ(2)

The value of Γ can be determined from an S11 or S22 measurement on the PNA. This is determined from

Γ = 10S11/20 (3)

where S11 is the logarithmic measurement obtained from the PNA.

B.2 Reflected Power

The reflected power is given as a percentage and is also determined by the reflection coefficient. This is givenby

Reflected Power = Γ2 (4)

B.3 Received Power

The value of the received power is determined through an S21 measurement on the PNA. This is typicallyin logarithmic units [dB], and the power in Watts is determined by

Pr = 10S21/10 (5)

B.4 Transmitted Power

The power transmitted by the antenna will always be less than the actual power delivered by the source dueto transmission line reflection. The reflected power is determined from the value of an S11 measurement aspreviously discussed. From this we can determine the actual power transmitted to the antenna by

Pt(dB) = PsourceΓ2 (6)

Typically, the source power while working in the X or Ku-band should be set to 2.0 dBm. Therefore theactual power transmitted, in linear units is

Pt = (10−3)10Pt(dB)/10 (7)

B.5 Gain

Once the values of the transmitted and received powers have been calculated, the gain of the antenna iscomputed using the Friis Transmission equation which is given by

PrPt

= GtGr

(λ

4πR

)2

(8)

where Pt and Pr are the power transmitted and received respectively, Gt and Gr is are the gains of thetransmitting and receiving antennas, λ is the wavelength of the microwave stimulus, and R is the magnitudeof the distance vector between the two antenna. These are in linear units.

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If we rearrange the terms and put in logaritmic units we have

Gt +Gr = 20 log

(4πR

λ

)+ 10 log

(PrPt

)(9)

The first term on the right hand side is the free-space loss which is unavoidable. The second term issimply the ratio of the two powers. If we assume that the two antenna are well matched, than the gain ofthe two antenna are equal and we are left with

G =1

2

[20 log

(4πR

λ

)+ 10 log

(PrPt

)](10)

This is the basis for what we will term the reference antenna. For our case, we have two well matchedantenna and we can determine the gain of each for our chamber. Once this value is known we can determinethe logarithmic gain of any antenna under test by using the following

Gt = GR + 10 log

(PR2

Pr

)(11)

where Gt is gain of the test antenna, GR is the gain of the reference antenna previously determined, PR2 isthe power received by the reference antenna, and Pr is the power received by the antenna under test.

Currently, the gain calculations are all based on Eq. 10, however additional care will need to be taken inthe future when using unmatched antenna.

B.6 HPBW

The half-power beamwidth (HPBW) of an antenna is determined from the analysis of the radiation patternof the antenna under test. This is defined as

HPBW = θ2 − θ1 (12)

where θ1 and θ2 are the angles at which the received power is -3dB from the maximum. The HPBW istypically determined for two characteristic radiation pattern slices where θ = 0 and φ = 0.

These can be determined from two single-axis measurement sweeps: One with the elevation set to zeroand sweeping across the azimuth, and one where the azimuth is set to zero and sweeping across the elevationrange.

B.7 Directivity

The directivity of the antenna under test can be determined either through numerical integration, or anapproximation can be found by

D ≈ 4π

(HPBWφ=0)(HPBWθ=0)(13)

where the values of the two characteristic HPBW values are in radians.

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Appendix C RF chain

New RF cables were purchased from MegaPhase. Table gives the specifications for the cables as reported bythe company.

Step angle Steps/rev. Amps/phase Holding torque [N.m] Max speed [rpm]1.8◦ 200 5.0 12.07 3000

Table 4: NEMA-34 bipolar stepper motor specifications

A diagram displaying the RF chain is given in Figure 15.

Figure 15: RF chain diagram

There are three different cables utilized in the RF chain: a single cable connecting with the antennamounted to the planar system, and two cables connecting to the spherical system that are connected betweentwo rotary joints. The rotary joints are to minimize any cable twisting that may result from the rotation ofthe azimuth motor axis as well as in future polarization movements.

The PNA ports have a 2.4 mm connection, while the antenna both have a female SMA connection. Therotary joints both have K-type connections which is 2.92 mm. 2.4 mm cables cannot mate up with SMAand it is necessary to utilize adapters for these connections. K-type connections, while featuring differentdielectrics can mate directly with the appropriate SMA connection.

Cable #1, which is the longest cable in the RF chain and has 2.4 mm connections on both ends. Thespherical system uses a 2.4 mm connection at the PNA, but has a K-type connection at the other end. Thisis so that the cable can mate up directly with the rotary joint at the base of the spherical positioner. Anadditional K-type cable is fed up through the hollow extension arm of the spherical system to another rotaryjoint which is connected up to the antenna.

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Appendix D Purchased Antenna Specifications

Both new antennas were purchased from A-Info. Below is a summary of the specifications for both antenna.

D.1 X-band horn antenna

D.1.1 Product Specifications

Model # Frequency range [GHz] Waveguide type Typ. Gain Typ. HPBW Connection typeLB-90-15 8.2 - 12.4 WR90 15.0 dB 30◦ SMA-F

Table 5: LB-90-15 X-band horn antenna specifications

D.1.2 Dimensions

Figure 16: LB-90-15 dimensions [mm]

D.1.3 Gain vs. Frequency

Figure 17: LB-90-15 gain vs. frequency

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D.1.4 Radiation Pattern

Figure 18: LB-90-15 radiation pattern at 10.0 GHz

D.2 Ku-band horn antenna

D.2.1 Product Specifications

Model # Frequency range [GHz] Waveguide type Typ. Gain Typ. HPBW Connection typeLB-62-15 12.4 - 18.0 WR62 15.0 dB 30◦ SMA-F

Table 6: LB-62-15 Ku-band horn antenna specifications

D.2.2 Dimensions

Figure 19: LB-62-15 dimensions [mm]

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D.2.3 Gain vs. Frequency

Figure 20: LB-62-15 gain vs. frequency

D.2.4 Radiation Pattern

Figure 21: LB-62-15 radiation pattern at 15.0 GHz

D.3 Links to company website and full datasheets

http://www.ainfoinc.com/en/index.asp

http://www.ainfoinc.com/en/pro_pdf/new_products/antenna/Standard%20Gain%20Horn%20Antenna/tr_

LB-90-15.pdf

http://www.ainfoinc.com/en/pro_pdf/new_products/antenna/Standard%20Gain%20Horn%20Antenna/tr_

LB-62-15.pdf

http://www.ainfoinc.com/en/p_wr_wca_ewca.asp

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Appendix E Stepper motor specifications

The system utilizes three different sizes of bipolar stepper motors. This appendix section describes thespecifications for each of the three types: NEMA-17, NEMA-23, and NEMA-34.

E.1 NEMA-17 bipolar stepper motor

E.1.1 Specifications



E.1.2 Torque vs. Speed

Figure 22: NEMA-17 torque vs. speed

E.1.3 Dimensions and wiring

Figure 23: NEMA-17 dimensions and wiring

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