Table Top Shielded Antenna Anechoic Chamber for Bluetooth Applications

By Dan Pulito and Jason Blackman

MotivationKodak desired an environment in which data

rate and power measurements were capable of being measured

Extended in order take automated antenna measurements

Finished design is to provide an environment for other senior design teams to test their wireless projects for antenna radiation and signal power.

Overview, What has been done?An aluminum box was donated by Kodak and adjustments

were made to prevent RF radiation from leaking from outside to inside and vice versa

Calculations were done in order to determine if the size of the box was adequate for the application and also to determine antenna placement

The box was then padded with RF absorbent foamA platform and motor were implemented to perform antenna

measurementsSoftware was written to synchronize motor angle and

measurements taken on a spectrum analyzerA known antenna with known gain was measured to calibrate

loss in the system and a sample antenna was measured

Project RequirementsEngineering Specifications

Tasks Added at DDR

Engr. Spec. #

Importance Specification (description) Unit of Measure

Marginal Value Ideal Value

ES1 1 Leakage Out of the Box dB <-25 <-60ES2 1 Leakage into the Box dB <-25 <-60ES3 1 Absorption of Material dB/Reflection > 15 > 30ES4 1 Additional Power Readings Due to Reflections dB < -20 < -50ES5 2 Frequency range of box GHz 2-3 1-6 ES6 2 Frequency range of receive antennas GHz 2-3 1-6ES7 3 Overall Weight of Box Pounds <50 <20ES8 1 Area Effected by EMI leakage from USB cables Inches <12 <3ES9 1 Loss Connection loss dB/Connection <2 <1

Engr. Spec. #

Importance Feature

ES10 1 Incorporation of a Motor for Angle MeasurementsES11 1 Synchronize Measurements Between Motor Controller and Spectrum AnalyzerES12 1 Calibrate Out Losses due to Free space and CablesES13 1 Map Data into Normalized Radiation PatternEngr.

Spec. #Importance Specification (description) Unit of

MeasureMarginal Value Ideal

ValueES14 1 Minimum Angle Step Degrres ≤1 ≤0.5

Size and Position AnalysisThe analysis of the position of the antennas, and

the verification that the size of the box meets the requirements was done in the DDR

The box dimensions are: 9”x 27” x 37”It was found that at a frequency of 2.4 GHz, the

antenna separation should be at minimum 21 inches

To reduce skipping waves, the antennas should be placed very close to the minimum distance based on the far-filed criteria so as to reduce shallow angles

Finally, to meet the criteria, the antennas were placed 4.5 inches from the bottom, 13.5 inches from the two side rails, and 21 inches apart.

Platform (Thank You Professor Slack)

• A Platform was built to encase the motor and also to lift the box up in order to allow the motor to be installed from beneath the box.

• 37”x 24”• Features compartment for the motor• Two wooden pieces and L-brackets hold the box in place

while on the platform

Wideband AntennaA wideband Antenna

was designed with a return loss of -15 dB between 2 to 4.8 GHz.

The design features two resonances allowing a good frequency response over a wide range

Utilizes a distinct patch geometry as well as etching in the ground plane

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00Freq [GHz]

-40.00

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

dB(S

(1,1

))

Ansoft LLC PlanarEM1_1_2Return Loss ANSOFT

Curve Info

dB(S(1,1))Setup_1 : Sw eep_1

Rotating PlatformVERTICAL PLATFORM

Rx ANTENNA

Tx ANTENNA

STARTING POSITION

VE

RTI

CA

L P

LATF

OR

M

Rx

Ant

enna

Tx ANTENNA

HALFWAY POSITION

VERTICAL PLATFORM

Rx ANTENNATx ANTENNA

ENDING POSITION

• The antenna to be tested is initially facing 90° to the right, where directly facing the transmit antenna is 0°

• It rotates counter clockwise in steps of about 1°

• At each step, a power measurement is taken from the spectrum analyzer

• This continues for half of a revolution, until the antenna is facing the opposite direction that it started in

Motor Controller• The PC (LabVIEW) sends a signal

(via the DAQ) to the MCU telling it to rotate the motor

• The MCU sends a serial command to the motor controller to get the motor to rotate, then monitors the encoder feedback until the next step is reached

• The MCU then signals to the PC (via the DAQ) that the next step has been reached

• The PC acquires the power reading from the spectrum analyzer and displays it graphically and also writes it to a text file

• The MCU signals to the PC when a full cycle (180°) is complete

MOTOR

ENCODER

MOTOR CONTROLLERMCU

DAQ

PC SPECTRUM ANALYZER

LabVIEW The instrument,

frequency, span and ref level are selected before running

The full span is displayed on the upper smaller graph at each step

The peak power value for each step is plotted on the lower graph

After a full cycle is complete, a text file with all of the peak power measurements is saved for use in Matlab or Excel

Project StatusThe programming of the motor and the spectrum analyzer

are synchronized, meaning, the Tektronix Spectrum analyzer is programmed to make a measurement every time the motor makes a movement

The antenna has been designed and simulated in software. The design of the patch has been etched and four samples were produced. However, due to the manufacturing process, the ground plane has to be etched by hand. This has yet to be completed.

The four aluminum plates to cover the preexisting holes have been machined and are attached to the box

The chamber has been padded with RF absorbent foamTests comparing the unaltered chamber against the

modifications have been completed and the results are tabulated

Test Plan The Transmit antenna was powered with

the Atheroes software via a USB cable The transmit antenna was placed at the

geometric center of the box The Transmit antenna pattern is

omnidirectional, allowing antenna position to be ignored

The transmit antenna was placed upright as well as the receive antenna, thus positioning the antennas to be polarization matched

The transmit antenna was secured on a cardboard platform to prevent any electrical properties of the stand from interfering with test measurements

The lid was then secured and measurements were taken for each side of the box. Each time, the antennas were placed one meter apart.

Measurements were taken with no modifications to the box in order to compare and quantify any solutions that were implemented to the box

Isolation Test resultsPeak Power Measurements 1m from Transmitter

Side Free Space(dBm) Original Box (dBm) Box w/ Foam (dBm) Box w/ Foam and Covers (dBm)1 -35 -52 -792 -35 -43 -713 -35 -48 -67 -934 -35 -39 -89

IsolationSide Original Box (dB) Box w/ Foam (dB) Box w/ Foam and Covers (dB)

1 17 442 8 363 13 32 584 4 54

Isolation (from outside to in)Side Box w/ Foam and Covers (dB)

1 352 353 314 39

Unmet and met Specifications Specification (description) Marginal

Value MetIdeal Value

MetMarginal

Value Ideal Value Comments

Leakage Out of the Box Yes No <-25 <-60 Secured lid and shielded USB cable will improve isolationLeakage into the Box Yes No <-25 <-60  Absorption of Material Yes Yes > 15 > 30  Additional Power Readings Due to Reflections - - < -20 < -50 Unable to devise a test plan to evaluateFrequency range of box Yes No 2-3 1-6 Based on seperaton of antennas, 2-6 GHz can be achievedFrequency range of receive antennas Yes No 2-3 1-6 Antenna has been designed for 2-4.8 GHzOverall Weight of Box Yes Yes <50 <20  Area Effected by EMI leakage from USB cables No No <12 <3 USB cable acted as an antenna, poor shieldingConnection loss Yes Yes <2 <1  

Incorporation of a Motor for Angle Measurements Yes -   

The motor has been implemented and the code to control the motor has been completed

Synchronize Measurements Between Motor Controller and Spectrum Analyzer Yes -

   The motor controller and the tektronix spectrum analyzer are syncronized with labview

Calibrate Out Losses due to Free space and Cables No -     An antenna with a standard gain is required to do this. More time is required to finish.

Map Data into Normalized Radiation Pattern No -     Calibration Must be performed first

Minimum Angle Step No No ≤1 ≤0.5 Friction, creates energy loss which leads to unpredictable motor displacement

Additional Suggestions to Improve Isolation Vast Majority of leakage problems

occurred because the lid did not make a flush contact to the edges of the wall.

A cross section of a possible solution is shown in the upper right

The lid and the wall interlock with each other in addition to being padded with gasket material

The JRE 1618 shown here implements this solution

Latches should also be used to fasten the lid down on the box

Another source of poor isolation was the USB cable.

The USB cable was picking up the transmitted signal and acting as an antenna outside the box

A USB filter can be used, but performance usually deteriorates after 1 GHz

Thus, the outside shield of the cable must somehow be completely grounded to the box

Questions ?