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    EEC 560: Engineering Electromagnetics

    Term Paper Report

    The work presented in this report represents only my own work. I have not

    consulted with anybody or used somebody elses work, nor have I shared my

    work with anybody else

    Rizwan Ahmed Shaik

    (2619170)

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    INTRODUCTION

    There are several navigation systems such as Global Positioning Systems (GPS),Global

    navigation Satellite Systems (GLONASS),Galileo and Beidou among which GPS is most

    preferred navigation system due to its compatibility, cost effectiveness and coverage. The Global

    Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation

    of satellites that continuously orbit the earth. The Clustering of many neighboring GNSS

    channels requires better coding schemes for this satellite systems, hence each GPS satellite has

    on board several atomic clocks that are precisely synchronized to signals which are coded. These

    Coded signals are broadcast by each of the satellites with the exact time and position of the

    satellite. All GPS receivers use an antenna to receive these signals. By using a GPS receiver

    optimized for time and not position it is possible to get extremely precise time synchronization

    with the satellites atomic clocks.

    The GPS signal is very weak so an Antenna usually amplifies the signal to drive it through the

    cable to the receiver. Antenna cable however offers some resistance and the GPS signal strength

    will attenuate as it travels down the cable. GPS receiver sensitivity is finite so if the cable length

    is too long the signal will be too weak for the receiver to detect it. Consequently it is very

    important to know the distance in advance between the antenna and the receiver so that the

    proper cable solution can be installed.

    The conventional GPS antenna used was bowtie dipole or spiral Antenna type which offered

    good bandwidth to the signals but was bulky in size. The above problem is overcome by using

    the Compact Dual-Band based Antenna which can operate in two different tuning frequencies L1

    and L2 with more reliable and accurate positions with a compact size by underlying the

    parameters of size, scalability and bandwidth .

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    Problem Description and Assessment

    Due to the presence of several navigations systems apart from GPS there shall be more

    frequency bands with an adequate and advanced coding scheme for the Satellite signals.These

    advanced coding scheme need a wider bandwidth but the existing small L1/L2 GNSS/GPS

    antennas have relative narrow bandwidth in the range of (10 MHz), and thus are not adequate for

    supporting advanced GPS codes. Though some GNSS/GPS antennas adopting wideband designs

    such as bowtie dipoles or spiral antennas have good bandwidth, but are relatively large in size. In

    order to overcome this limitation A Dual-band GPS antenna is proposed which not only can

    operate in two frequency bands, but can also provide more reliable and accurate positions when

    used with proper GPS receivers that combine information received at both

    frequencies.Therefore, the dual-band GPS Antenna design presented in this letter attempts to

    address size, bandwidth, manufacturability, and scalability, i.e., easily redesigned for otherfrequencies.

    The compact dual-band Antenna which was proposed initially used circular polarization (CP)

    and the antennas included stacked-patch designs specially shaped patches and slot-loaded patch.

    The Stacked patch design was not easy to manufacture due to its bonding issue at the metal di-

    electric and di-electric- di-electric interfaces hence a new proximity probe design which requires

    an external probes on the external probes is proposed in this paper.

    Unlike conventional stacked-patch designs, this design does not have an internal conducting

    patch. The dual-band coverage is achieved by operating the patch mode in L2 band and slot

    mode in L1 band. The low-loss, high-dielectric substrate and the meandered-slot designs are

    employed to increase the antennas electrical size. This design also adopts external proximity

    probes. This design also utilizes a small 00-90

    0hybrid chip to reduce the size of the feeding

    network and to achieve good right hand circular polarization over the frequency range.

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    Antenna Specifications:

    1) L1=1575 Mhz

    2) L2=1227 Mhz

    3) Diameter D=25.4 mm

    4)Thickness = 11.27 mm

    Antenna Structure and Operational Principles:

    The proposed antenna is composed of a single slot-loaded conducting patch design on top of two

    stacked dielectric layers. The top layer includes the slot-loaded patch design that is fabricated on

    a Rogers TMM10i board ( h1=1.27 mm,r = 9.8,tan=0.002) using standard printed circuit board

    (PCB) fabrication processes. The bottom substrate is a high-dielectric ceramic puck( h 2=10mm,

    r=45, tan=0.002). The two substrates are bonded together using ECCOSTOCK dielectric paste

    (r=15) to avoid air gaps and low-dielectric bonding layer formed by common glues, both

    causing detuning of resonant frequencies.

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    This new design is also mechanically superior to conventional stacked-patch designs where the

    presence of the middle con- ducting patch weakens the bonding between the top and bottom

    layers when the patch size is relative compared to the diameter of the dielectric layers. Two

    conducting strips (width=2 mm, height= 9.8 mm) on the side serve as proximity feeds for the

    new design. The bottom ends of these strips are connected to theoutputsofa090 hybrid to obtainRHCP property.These two feeding strips are located in the middle of two adjacent longer

    meandered slots at 90 azimuth angle from each other.

    The above figure shows computed magnitude of equivalent currents on the patch at 1227 MHz

    (left) and 1575 MHz (right).It shows that resonant current distribution occupies the entire patch

    in L2 mode and is mostly concentrated around the meandered slots in L1 mode. The meandered

    slots, the center circular hole, and the high-dielectric substrate help to establish L2 mode

    resonance within the physically small antenna volume. The concentration of fields only around

    slots in L1 band also makes it possible to tune the L1 frequency independently by adjusting the

    length of the inner tuning stubs.

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    Design Procedure:

    The design procedure of the compact dual band Antenna begins with selecting the diameter D

    based on physical constraints and two desired resonant frequencies of a specific application such

    as GPS. The three-step design procedure is discussed as follows.

    Step 1: The first step is to determine the dielectric constant and thickness of the two stacked

    dielectric materials according to the desired lower resonant frequency. The effective dielectric

    constant of two stacked dielectric layers can be estimated using a double-layer parallel-plate

    capacitor model that gives

    r =

    Where (1,h1) and (2,h2) are the dielectric constant and thickness of top and bottom dielectric

    layers,respectively.

    The resonant frequency in the lowest mode can be estimated from

    F0 =

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    Step 2: The second step is to design the length and width of the meandering slots(seeFig.1)to

    tune the resonant frequency of the lower mode. Figs. 3 and 4 plot simulated input impedance as a

    function of slot length and width,respectively. Fig. 3 shows that increasing the slot length from 9

    to 10 mm effectively lowers the resonant frequency of both low- and high-frequency modes.As

    is shown in Fig.4,changing the slot width from 0.51 to 0.76 mm shifts the higher resonant

    frequency from 1.48 to 1.6GHz,but only shifts the lower resonant frequency slightly.

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    Measurement Results:

    Table I summarizes the optimized design parameters of the final L1/L2 GPS antenna element

    design. Fig. 8 shows a fabricated antenna element was then mounted on a 117.2 117.2mm FR4

    board containing the feeding circuitry.

    The simulated and measured broadside gain for the Fig. 8(a) configuration are plotted in Fig. 9,

    which shows an excellent agreement. The RHCP antenna gain is around 3.2 dB at 1.227 GHz

    and 3.5 dB at 1.575 GHz. The RHCP-to-left-hand-CP (LHCP) isolation is 20 dB at L2 band and

    15dB at L1 band. The axial ratio is found to be 1.3dB at 1.227 GHz and 1.9dB at 1.575 GHz.The

    3-dB band width of lower mode is 45 MHz from 1200 to 1245 MHz, and high mode is 50 MHz

    from 1545 to 1595 MHz at zenith. Such band widths are sufficient to support modern codingschemes.

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    The mutual coupling between closely spaced antenna array element soften affects impedance

    matching condition, resonant frequency,and radiation pattern of the element.To examine these

    effects,afour-element GPS array was assembled as shown in Fig.8(b).The distance between

    adjacent elements is 62.5mm.

    Notice that these patterns are slightly tilted and not completelysymmetric due to the finite ground-plane scattering effect. Reasonable RHCP-to-LHCP isolation

    is also preserved in the four-element array.Therefore,we concluded that the mutual coupling

    impact on antenna performance is not significant compared to a single element forth is antenna

    design.Also,the measurement data are in good agreement with the simulation data.

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    Conclusion and Future work:

    The Compact Dual Band Antenna proposed in the above paper overcomes the deficiency of the

    commercial Antennas which operate at narrow bandwidth of about 10 MHz and the Bowtie

    Dipole or Spiral Antennas which are very large in size. Thus the proposed model provides

    operating bandwidth and reduced size and it attempts to address manufacturability and

    scalability in broader aspect.

    The Dual Band Coverage is achieved by operating L1 in the patch mode and L2 in the Slot mode

    of operation. A three-step design procedure of this new antenna design was discussed and can be

    used to design for different operating frequencies. The RHCP feeding circuitry was implemented

    using a small 090 hybrid chip that provides desired power splitting and stable quadrature phase

    difference at its two outputs.Simulation results indicated that 90% radiation efficiency is

    achieved by using the low-loss dielectric materials in this design.

    The Di-electric materials used as the layers for mounting the Slot loaded conducting path should

    have a minimum value of tan so that there is no conduction in the middle layer and the two

    layers hence in the future design aspect the two layers could be efficiently stacked by choosing

    the value of tan.

    In this proposed model tuning is done by varying the length of the conducting channel but the

    tuning done was not precise as the Control Knob adjustment was not in steps hence efforts couldbe made to design a model in which the Tuning is in Steps and with uniform distribution of

    current patterns in both L1 and L2 Bands respectively.