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  • 8/3/2019 Antennae Boo Sales


    Nest Watch:

    Proximity Sensor Antenna

    Catherine Boosales

    Spring 2001

    Faculty Supervisor: Dr. Thomas Weller

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    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.


    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.



    DC ower


    Light Intensity

    RF TX Antenna







    DC Power


    ProcessingRF RXAntenna Web



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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


    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.


    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