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Nest Watch:

Proximity Sensor Antenna

Catherine Boosales

Spring 2001

Faculty Supervisor: Dr. Thomas Weller

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Abstract

The Nest Watch Wireless Sensor group project set out to design and build a sensor to

monitor activity in a birds nest. This paper gives a brief description of the overall

project and then directs focus to the 2.45 GHz patch antenna that was part of the

proximity sensor of the system. Basic theory centered on the design improvements of an

aperture couple-fed microstrip patch antenna is presented followed by an outline of the

design steps. Lastly, data comparisons are made and conclusions drawn.

Introduction

With an overall goal of producing a wireless sensor that will monitor activity in a

birds nest and transmit the data to a remote location for collection, two teams of students

from two different universities collaborated on the project. Each student at the University

of South Florida (USF) had two roles to play in terms of the overall project. One of these

roles involved a system level responsibility of some type that would require substantial

interaction with other team members, some of which were from the other university

involved, Tennessee Technological University (TTU). The second role involved the

analysis and design of one part of the overall sensor. The production of the antenna for

the proximity sensor and the integration and calibration of the same sensor will be

presented in this report.

Before further details about the antenna or the proximity sensor are discussed,

some description of the overall project, The Nest Watch Wireless Sensor, is necessary.

USF was responsible for the production of three main parts of the sensor, the sensor card

itself, the transmitter and the receiver. The sensor card is simply a proximity sensor with

a temperature sensor and a light intensity sensor mounted to it. The proximity sensor

works at a frequency of 2.45 GHz and will be described in more detail in the next section.

The data from the sensor card is sent via a 915 MHz signal from the transmitter card,

which would be attached to the sensor card, to the receiver. Once the signal is received,

the information is processed and then displayed on a web page designed by TTU. The

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student team members at TTU also designed the signal processing parts of the transmitter

and receiver as well as the software that is used for the data collection. Students from

both universities were responsible for inter-university communication where necessary.

As an illustration of team roles, figure 1 demonstrates how the different parts fit together

to form the collective project. The boxes in green represent USF and those in purple are

for TTU.

Figure 1: University Team Assignments

On a slightly smaller scale, the baseline proximity sensor was assembled and built

by a team of 4 students and then tested to obtain calibration data. This first sensor card

consisted of a surface mount voltage controlled oscillator to generate the 2.45 GHz

signal, a quadrature hybrid coupler to equally split the signal into two (a reference signal

and a target signal), a surface mount mixer to serve as a phase detector and a patch

antenna to send and receive the target signal. In addition, an amplifier was also produced

to be used in the event that one of the signals wasnt strong enough for detection thereby

requiring amplification. This card was the safe approach first design produced to

ensure that a working sensor would be produced within the time frame of a semester.

Further analysis and design of many of the individual components of the system were to

be completed by the students as their individual projects.

Proximity

Baseband

DC ower

Temperature

Light Intensity

RF TX Antenna

USF USF USF

USF

TTU

TTU

TTU

Baseband

DC Power

Signal

ProcessingRF RXAntenna Web

Posting

TTU TTU TTUUSF USF

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The portion of the project presented in this paper contained two initial goals and

the addition of a third, as the semester progressed. The first and foremost goal was that

of producing a suitable antenna for the proximity sensor. This antenna was to transmit

and receive at 2.45 GHz with a return loss of better than 10 dB. The safe-approach

version of this antenna was to be probe-fed from the edge of the coupler on the sensor

card through the ground plane of the antenna. Once the baseline antenna was produced,

additional modifications to improve performance were to take place. The specifications

of the second antenna were to be determined during the semester via feedback from

various sources particularly including the calibration team with the overall goal of

improving sensor performance. One such improvement suggested at the onset, was an

antenna that was feed via a magnetically coupled feed line through a small aperture in the

ground plane of the sensor card as pictured in figure 2.

Figure 2: Example drawing

of an aperture-coupled feed

antenna.

Once the initial probe-fed design was produced and tested, the motivation for

producing this type of couple-fed antenna changed from that of improving polarization

purity and return loss (by improving input impedance match) to the necessary goal of

bringing the antenna closer to the sensor card for logistical reasons which will be

explained later. The production approach for the antennas began with the performance of

lab exercise 12 from the Wireless Circuits and Systems Design Course1 with

modifications to feed characteristics and resonant frequency. The second design began

with calculations by hand and then continued or altered in ADS-Linecalc and PCAAD.

These calculations were then utilized to create a layout in ADS to allow for a Momentum

Simulation. Lastly, the layouts were submitted for milling and then tested, and tuned as

necessary, after production.

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The second initial goal was to produce valid calibration data for the proximity

sensor so that analysis could be performed. This data was to help determine how to

distinguish between different types of activities in the nest. This calibration was to be

performed initially on the coaxial part sensor that was set up and then on each successive

sensor card to be produced. Calibration involved the testing and recording of output

voltages for a range of set-ups selected to simulate bird nest activity such as the presence

of eggs or that of a bird or two. This information was then relayed to TTU for reference

when designing the baseband part of the transmitting system.

As the second stage antenna design depends very highly on the placement of the

feed line on the circuit, it was necessary to become considerably more involved with the

sensor integration team than originally intended. In order to determine if the antenna

design to be produced was going to function as planned, it was necessary to produce the

new sensor card as well. As the semester progressed, additional time was spent on the

production of the second and third stage sensor cards. The successful fabrication of these

two cards quickly developed into the third goal of this particular project.

Theory

The Nest Watch Wireless Sensor utilizes three different antennas to operate. Two

of these antennas are designed to operate at a frequency of 915 MHz for transmission and

reception of data. The third antenna is part of the proximity sensor and as such will

simultaneously transmit and receive a signal at a frequency of 2.45 GHz. The VCO on

the sensor card generates a signal that is split into two equal signals. Both of the new

signals are of the same frequency and phase. One of the signals gets sent directly to the

mixer as the reference signal. The second signal is transmitted from the antenna towards

the target (in this case, a resident of the nest). The signal will bounce off the target (if

there is one present) and return to the antenna still at the same frequency, but with a

different phase. This new signal is received and then sent back through the coupler to the

other input of the mixer. The mixer will combine the two signals (the reference signal

and the target signal) to produce a DC voltage, as is always the case when two signals of

the same frequency are mixed, that will vary based on the phase difference. Since the

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sensor card will be placed below the birds nest, a microstrip patch antenna was chosen as

the design of the antenna because it will radiate a relatively broad beam that is normal to

the surface of the patch. This will direct the signal in the correct direction for occupant

detection. In addition, the patch antenna will resonate effectively at /2, which at 2.45

GHz would be of a fairly small size and therefore as unobtrusive to the bird as possible.

Due to simplicity and the need for a baseline antenna to operate with, a probe-fed patch

was designed. The equivalent circuit for such a patch is depicted in Figure 3, where the

parallel RLC represents the patch and the series i