RF ANECHOIC CHAMBER: AUTOMATED ANTENNA …

Multi-Disciplinary Senior Design Conference Kate Gleason College of Engineering

Rochester Institute of Technology Rochester, New York 14623

Copyright © 2012 Rochester Institute of Technology

Project Number: P12311

RF ANECHOIC CHAMBER: AUTOMATED ANTENNA

RADIATION AND GAIN MEASUREMENT SYSTEM WITH

USER INTERFACE

Sheldon Palmer/ Electrical Engineer Danielle Walters/ Electrical Engineer

ABSTRACT

With the advent of wireless devices, a growing number of Senior Design projects are utilizing in the S

Band frequency range. Prior to this project, Senior Design teams had limited methods of testing performance of the

design. The objective of this project is to design a system capable of detecting, capturing, logging, and displaying

measurements inside an Anechoic Chamber. This anechoic chamber attenuates outside interference but captures

signals from the transmit antenna which are received by the test antenna. This chamber has a special absorber that

also attenuate unwanted transmitted signal that may reflect off surfaces, such as the wall, floor and or additional

metal surfaces that are contained inside it.

NOMENCLATURE

Anechoic: non-echoing

DAC: Digital to Analog Converter

DB: decibel

DBi: decibel isotropic

DUT: Device Under Test

FAR-FIELD: region of electromagnetic radiation where the shape of the antenna pattern is independent of distance

LabVIEW: graphical programming environment

RF: Radio Frequency

Rx: Receive

S BAND: 2-4GHz frequency range in the microwave band of the electromagnetic spectrum

Tx: Transmit

INTRODUCTION

An Anechoic chamber is an isolated test chamber used to measure the gain and radiation pattern of the

DUT. The most common chamber, and the one used here, is the rectangular chamber. The Tx antenna is fixed to

one of the end walls, and the DUT is located down the length of the chamber as far as possible while still leaving

room for it to spin 360 degrees. This ensures far-field conditions are met. These chambers are isolated, so any

outside signals cannot enter and any inside signals cannot leave. All inside surfaces are lined with RF absorbent

material to prevent reflections so only the main lobe of the antenna signal is being measured. This helps to provide

conditions of simulated open space. The DUT is on a rotating platform with its 0 degree position facing the door

and perpendicular to the Tx antenna. The 90 degree point is when the DUT is facing the Tx antenna and the 180

degree point is when the antenna is facing opposite the door. The Rx antenna begins the measurement process at the

0 degree point, and rotates through to the 180 degree point with power measurements being taken periodically.

Proceedings of the Multi-Disciplinary Senior Design Conference Page 2

Project P12311

These measurements create the radiation pattern of the antenna. The gain measurement is taken when the antennas

are facing each other.

Two critical systems of this project was a rotating antenna platform for the DUT and a LabVIEW program

to control the antenna rotation and capture, log and display measurements, for gain and power. The frequency range

given was 2-4GHz (S Band), with the focus being on 2.4GHz given the number of wireless devices using this

frequency. This is the carrier frequency used by Bluetooth, Zigbee and Wi-fi devices. The isolated chamber used is

8’ wide by 8’ tall by 12’ long. Placement of the Tx antenna should be in the center of the back wall. This

minimizes the number of reflections off the 4 side walls. Therefore, the center of the DUT must sit at 4’ from each

side wall and 4’ off the floor. The DUT must rotate in 0.5 degree and 1 degree increments depending on the test the

user is running. After each rotation increment, the spectrum analyzer must measure the power being received by the

Rx antenna. LabVIEW is responsible for controlling the motor rotation increment, controlling the frequency

generator and spectrum analyzer, and correlating the received power to the angle the DUT has rotated. Once test

data is captured, LabVIEW will then create a linear radiation measurement plot with power in dB. A normalized

linear plot, logarithmic plot, and normalized logarithmic plot are also created. A text file with power and frequency

values is created and can be exported to MATLAB for further manipulation.

PROJECT DESIGN

Rotating Platform

For this chamber, measurements will be taken in the far-field region. To be in the far-field, the Rx and Tx

antenna must be at least a certain distance away. This distance is given by:

(1)

D is the effective aperture size of the antenna being used. In the case of the parabolic reflector being used for this

project, the aperture size is:

√ (2)

where w is the width and h is the height of the grid reflector. λ is the wavelength of the given frequency and is

shown by:

(3)

where c is the speed of light [m/s] and f is the frequency [Hz]. Normally, standard gain horn antennas are used in

Anechoic Chambers; however the horn for this frequency was almost 2’ in length. Having 2 horns of this size in the

system would cut down on the distance between the Rx and Tx antennas available, which could result in far-field

conditions not being met. Figure 1 shows the set-up of the measurement equipment outside the chamber. The

Spectrum Analyzer being used on the right is the Hewlett Packard 8565E 9kHz-50GHz Spectrum Analyzer. The

frequency generator used on the left is the Agilent N5181A 100kHz-6GHz MXG Analog Signal Generator. Figure 2

shows the set-up within the chamber. The antenna on the right is the Tx antenna and the antenna on the left is the

Rx antenna. The blue foam is the RF absorbent foam lining the inside walls of the chamber.

Figure 1: Instrumentation Set-Up

Figure 2: Measurement Set-Up

The Rx antenna is attached to the rotating platform. The center of the Rx antenna must be in line with the

Tx antenna to ensure the direct beam is being received. The platform must be able to accommodate the reflector

antenna and various sized microstrips. The platform should consist of as little metal as possible. RF will reflect off

metal, while plastic is invisible to RF so it will go right through the plastic. If there is any metal in the construction



of the platform, it should be located as low as possible and should be covered with the RF absorbent foam. The base

of the platform is made of wood with 2 parallel rails which the platform sits on. These rails allow the distance

between the 2 antennas to be changed because the Rx antenna can move towards or away from the Tx antenna. This

is shown in Error! Reference source not found.

Figure 3: Base of Rotating Platform

Figure 4: Side View of Rotating Platform

A piece of acrylic 12” wide by 12” long by ½” thick sits on top of these rails. The motor is screwed into

this and there is a hole in the middle to allow the motor shaft to protrude. On top of the acrylic sits a plastic

turntable lazy susan. On top of this is another acrylic sheet with the same dimensions. Attached to the motor shaft

is a threaded metal rod. This metal rod screws into the threaded center hole on the top acrylic sheet. A PVC flange

is screwed into the top side of the top piece of acrylic. Into this a PVC pipe is inserted. Figure 4 shows the side view

of platform put together. The PVC pipe was cut so that the reflector antenna would be at exactly 4 feet high when

assembled.

An antenna mount slides over the top of the PVC pipe. As Figure 5 shows, this mount has threaded

hardware holes in the top for the bracket of the reflector antenna. The antenna mount also has a plexiglass mounting

surface on the front face with Velcro to attach the microstrip antennas. The Velcro allows the microstrip antennas to

be adjustable (i.e. higher, lower, left, right) to ensure the center is at the same position as the center of the reflector

antenna. The mount with the microstrip configuration is shown in Figure 6.

Figure 5: Reflector Antenna on Mount

Figure 6: Microstrip Antenna on Mount

Automation System

The automation of the measurement program uses LabVIEW for a user interface. This controls a frequency

generator for the transmit antenna, a spectrum analyzer to capture the receive power of the test antenna and lastly, a

stepper motor that turns the test antenna through either 180 or 360 degrees. For most lab instruments one can find

drivers, which are programs that specifically control each of the instrument’s functions and can easily be

incorporated into LabVIEW. The programming language used in LabVIEW, also referred to as G, is a dataflow

programming language. Execution is determined by the structure of a graphical block diagram (the LV-source code)

on which the programmer connects different function-nodes by drawing wires. These wires propagate variables and

any node can execute as soon as all of its input data become available. Since this might be the case for multiple


Project P12311

nodes simultaneously, G is inherently capable of parallel execution. The interface contains driver programs for the

Hewlett Packard 8565E spectrum analyzer, Agilent MXG 51581A signal generator and the USB NI 6008 digital

analog converter. The digital analog converter is used to generate a constant DC voltage that is used by the motor

driver’s direction input. The system also uses the parallel port of the computer to generate an approximately 60 Hz

square wave that is used by the clock input of the motor driver. The motor and driver were purchased from

Anaheim Automation and the motor is used to rotate the platform that the test antenna sits on. The axis of rotation

for the platform is directly around the shaft of the motor, which has a maximum torque as specified on the data sheet

attached to this document. The system is turned on and off by a control box that has a fuse with a 2 Amp limit.

Since the system is fully automated, all the user has to do is turn the power on for all instruments, open the

LabVIEW interface, turn on the power to the control box and then click the run button on the interface. The general

sequence execution of the LabVIEW GUI involves the user choosing the degree of rotation of the test antenna and

the degrees per turn. Based on that information, LabVIEW sets the axis of the GUI and sends those values to the

respective blocks that will be executed. The motor block sends a series of pulses to the motor driver, the platform

turns, the power of the receive antenna is measured by the spectrum analyzer, and the data of the power intensity is

captured on the GUI at every angle. The sequence is repeated until the chosen degree of rotation is achieved. The

LabVIEW interface then saves the data and allows the user to look at the data using either linear or logarithmic

scale. The program is closed by pressing the done measurement button. The GUI is shown in Figure 7.

Figure 7: LabVIEW Gain Measurement Program

The stepper motor is controlled using LabVIEW through the parallel port of the computer. The D0 pin of

the parallel port outputs a 60 Hz, 3.5 Volt peak to peak square wave that is used by the motor driver clock input. To

access the parallel port on the computer, the address to the port was assigned to the LabVIEW block controlling the

motor operation. The port address and the number of steps to the motor are placed in a while-loop block that is

executed each time this block is called. The parallel port cable is connected to the motor control box, which is then

connected to the motor driver that is also used to create the ground for the system. (i.e. the parallel port, the motor

driver and the digital analog converter all share the same ground.) The motor driver is set to operate in the micro-

step mode. (i.e. one eighth of a full step which makes it turn 0.0225 degrees per step.) With this fine resolution the

system is able to do 0.5 degrees per turn, which is a requirement of the interface. Lastly, the digital analog converter

is used to generate a 5 or 0 Volt DC signal when the block is called. The DC signal is used by the motor driver

direction input. When the signal is high (5 Volts), the motor turns counterclockwise. When the input is 0 volts, the

motor turns clockwise. The system does not require any additional signals to operate except for the power to the

motor driver that is transmitted through a 12- 24 Volts variable DC power cord. The system is operated at 15 Volts

DC and because the stepper motor uses the applied voltage to excite its windings, the induced current causes the

motor to become very hot. The design recommendation is for the system to operate at this voltage level unless more

torque is needed by the motor. Also, the motor system must be turned off immediately after all tests are completed,

so as to allow the motor to cool down.



The GUI was written using LabVIEW 2011, and because of compatibility issues between different

versions, it is recommended that any changes to the GUI be done using this version. Otherwise, convert all files to

the newer version before making any changes. The GUI is used to implement the motor control mentioned above

and the figure below shows the block used to implement this.

Figure 8: Motor Control Block

With the system being fully automated, the GUI also controls the signal generator and the figure below shows the

block used to implement this.

Figure 9: Block Used to Control the Signal Generator

The figure shows the series of drivers that were downloaded from the National Instrument website. The

blocks were aligned in series starting with the initialization block, followed by the setting for the frequency, the

power setting and lastly the RF on or off. The initialization block contains the instrument address and once the

instrument is on, the configuration blocks allow the user to change the various settings of the instrument. The front

of the GUI contains all the control settings that the user can change. The frequency generator is used to power the

Tx antenna and also sets its operating frequency. The maximum frequency of the generator is 17 dBi and most of

the tests were conducted using 10 or 15 dBi.

Figure 10: Block Used to Control the Spectrum Analyzer and Block Used for Plotting the Graph

As mentioned in the sequence of execution, the system should also acquire the received power from the

spectrum analyzer and this data is plotted in real time on the GUI. This block is shown above. The block operates


Project P12311

similar to that of the signal generator, with the only exception being that all default settings are used and the user is

not allowed to change them. The initialization block is used to open communication with the instrument and the

configuration blocks determine the modes and settings that the spectrum analyzer operates in. The peak at the

operating frequency setting is used to acquire the power intensity and a plot of this value is done for each angle of

the radiation plot. The plot default is a linear-linear scale. The GUI also allows the use of markers to look at a

specific power intensity value and the corresponding angle, which can be seen on the front panel of the GUI.

Figure 11: Block Used to Save the Data and the Block for Calculating Gain

Once all the measured readings are taken, the block above will be executed and the data will be saved. The

gain of the test antenna is then calculated. The data is saved as a text file in the form of a matrix and this data can

then be manipulated later using MATLAB if needed. The gain is calculated using the Friis transmission equation:

(

)

(4)

Where Pr is the power received, Pt is the power transmitted, Gor is the gain received, Got is the gain transmitted,

and R is the distance between the Tx and Rx. The result is output to the front of the GUI in the assigned box.

Figure 12: Block Used to Allow the User to Use the Append Graph Feature

The user will be able to look at the result of the data using the graph append feature. This feature of the

GUI allows the user to change the power intensity scale from linear to logarithmic. Also, the graph can be

represented as normalized or as measured. The buttons to operate this feature are located on the front panel of the

GUI.

The block shown in Figure 8-Figure 10 are replicated to implement the calibration program. The

calibration program is used to characterize the chamber. Two reflector antennas are used while facing each other.

The calibration program uses the known gain from the antenna specification sheet and the power received to

calculate an offset that is used in the radiation program for the gain calculations. This offset equation is shown

below:

(5)

The “test” subscripts represent the gain and power being received from the DUT. The “standard” subscript

represents the gain and power taken from the calibration program. In the calibration program, direct power

measurement over all the specified frequencies of the antenna data sheet and this data is saved similar to how data is

saved in the radiation program. The GUI for the calibration program is shown in Figure 13.



Test Results

The rotating platform set-up was placed in the chamber, and radiation measurements were taken for each

type of antenna. First, the calibration was run using the microstrip antenna as shown below:

Figure 13: Microstrip Calibration Program Results

These values were then used as offset values in the gain measurement test to ensure that the gain being recorded was

accurate. Next, the gain measurement program was run using the microstrip antenna and the results are below:

Figure 14: Microstrip Measured Radiation Pattern

Figure 15: Microstrip Normalized Radiation Pattern

Error! Reference source not found. shows the measured data using a linear scale. From this plot, the gain of the

antenna can be determined.

shows the measured data using a linear scale after it has been normalized. From this plot, the half power beam

width can be determined. Below are the results from the gain measurement program when the reflector antenna was

attached.


Project P12311

Figure 16: Reflector Measured Radiation Pattern

Figure 17: Reflector Normalized Radiation Pattern

The system is able to make radiation measurements comparable to those given by the antenna datasheets. A 0.5

degree and 1 degree increment can be chosen to give fine or course resolution. Both a 180 degree and 360 degree

test can be completed. This system is fully automated which allows users with no background to successfully run

the measurement program. The automated test system to be placed inside of an Anechoic Chamber was a success.

ACKNOWLEDGMENTS

The team would like to express its sincerest gratitude to those who have made invaluable contributions to

the project. Many thanks to Mark Hinman of Kodak, who generously donated the isolated chamber that was the

base of the project. Also thanks to Don McPherson and Lance Bradsteet for absorber and monetary donation, along

with technical expertise. Thanks to Professor George Slack for guidance, support, and an extra hand during

construction. Thanks to Dr. Jayanti Venkataraman for theoretical expertise and for being the driving force behind

the project.

REFERENCES

[1] Constantine A. Balanis, Antenna Theory Third Edition, Hoboken, NJ: John Wiley & Sons, 2005.

[2] Leland H. Hemming. Electromagnetic Anechoic Chambers A Fundamental Design and Specification Guide.

Piscataway, NJ: IEEE Press, 2002.

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RF ANECHOIC CHAMBER: AUTOMATED ANTENNA …