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  • Antenna Theory

    Wire Antennas Monopole Antenna

    Long Wire or Traveling wave Antennas

    Yagi Uda Antenna

    Reference: C. A. Balanis, J.D. Krauss

    Prof. D. Kannadassan

  • Monopole antenna

  • Image theory, an intro

    A positive charge+Q (or a charged body) is located a height h from perfect ground, then an image as -Q will form at a depth of h.

    A dipole will form, and the filed lines will be closed.

  • Both the poles will contribute the field present at point P,

    located at (x, y, z)

  • Ground or

    Perfect conducting plane

  • Monopole antenna - Theory

    Theory and derivation of

    Monopole and Dipole are

    completely same, but the

    Power output will always

    half to monopole.

    And Radiation resistance

    will be half of that for

    dipole, Rr=36.5

    V

    P

  • V

    Radiation pattern

  • From the ground, at a very small height,

    a /4 antenna is place vertically, called

    monopole antenna.

    AM, FM applications.

    Monopole antenna

  • Examples

    1cm

    Bluetooth dongle

  • Long wire antennas

  • Long wire antennas - Theory

    Consider a thin wire of length L is horizontally placed at a height from

    ground plane. At one end a RF source is connected and other end is

    terminated by the characteristics impedance of the wire Z0.

    When the wire is excited with a sinusoidal signal, the wave should travel

    along one direction and will not create any standing wave so called

    Travelling waves.

    We are gonna see the expression for electric field at a point P - located at a

    distance of r from the wire and angle of with respect to the length of

    wire.

    ~ Z0

    Wave direction

    h

  • Travelling wave structure E field

    The retarded current in the wire shall be described as

    Where: v=p.c or p=v/c

    p is the ratio of the wave velocity at the wire to the free space, called Relative

    phase velocity. Used to vary with the attenuation constant.

    v

    Z

    C

    rtII m

    1sin

    Z0 Wave direction

    z

    P

    r

    Z-axis

    z1

  • Radiation pattern

    We can find the radiation pattern as similar to the dipole case, will result:

    Here, is wave impedance.

    cos1

    2sin.

    cos1

    sin

    2

    0 ppc

    wL

    pR

    pIE

    [Krauss]

  • m or

    Maxima direction: Also called Elevation angel (max, m, , )

    Lm

    37.01cos

  • For 3 and 5

    [Krauss]

  • Minor lob directions, Directivity

    m=0 m=1 m=2

    [Balanis]

  • Bidirectional long wire antenna

    The maxima direction oriented with the direction of wave in the wire, so

    by introducing wave on both the direction we can introduce Bi-

    directional radiation.

    This can be possible by Open circuited long wire or Un-terminated, so

    called Stand wave antenna.

    Wave direction

    P

    R

  • Vee antennas

    =2

    By terminating with Z0, we can get Uni-directional radiation

    Design equation:

    =2 where is max of single long wire

  • Rhombic antenna

    Based on the principle of Traveling wave and Vee antenna, Rhombic

    antenna is a very high directive antenna has Diamond or Rhombus shape

  • Working of Rhombic antenna

    By properly selecting the tilt angle, the

    rhombic antenna will give additive effect

    of radiation pattern of each long wire

    antenna

    The radiation mechanism is basically

    depends on two factors:

    Tilt angle ()

    Height above the ground (h)

    These are design parameters of antenna

    Due to ground effect, the maximum

    radiation is elevated about an angel ()

    Z0

  • Design equations of Rhombic antenna

    BBL field equation: (Bruce, Beck and Lowry 1935)

    From this equation, we can deduce the condition to get the maximum

    power direction with respect to height h and length of line L

    About Dr. Bruce, E: http://ieeexplore.ieee.org/iel5/10933/35478/01685103.pdf

    Original Article: www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdf

    http://ieeexplore.ieee.org/iel5/10933/35478/01685103.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdfhttp://www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdf

  • Maxima with height

  • Maxima with Length L

  • Design formula

    Finally, the design formulae are: (also called BBL formula)

  • Design a rhombic antenna to operate at 20MHz when the angle of

    elevation angle =10o.

    ooo 801090

    173.010sin o

    44.1;36.12 hL

    m15mhmL 795.3;715.5

  • Numerical Problems and Review Questions

    Explain about long wire and Rhombic antenna with its radiation pattern

    Design a rhombic antenna to operate at 20MHz when the angle of elevation

    angle =10o.

    Explain resonant and non-resonant modes of Long Wire Antenna.

    If we assume the average beamwidth of rhombic antenna as 10o, then

    design an antenna system such that it will radiate maximum power over the

    ranges from 10o to 40o for the operating frequency of 10MHz

    Explain the working of Open circuited long wire antenna and V antenna

    with radiation pattern

  • Yagi-Uda Antenna A high frequency and high directive Parasitic array antenna

  • Introduction

    Prof. S. Uda (japan) was invented this antenna by 1927, and collaborated with H.

    Yagi

    S. Uda, "High angle radiation of short electric waves". Proceedings of the IRE, vol. 15, pp. 377-385,

    May 1927.

    After the invention, more than 40 researcher were studied on the improvement.

    Latest article (2011): Application of bacteria foraging algorithm for the design

    optimization of multi-objective Yagi-Uda array

    Shintaro Uda Hidetsugu Yagi

    http://en.wikipedia.org/wiki/Proceedings_of_the_IREhttp://en.wikipedia.org/wiki/File:Hidetsugu_Yagi.jpg

  • Principle A folded dipole or ordinary half wave dipole is centered between two types of parasitic

    elements, called: Directors and Reflectors.

    The coupling (capacitive) effect between the parasitic elements and active element(dipole), the directional properties are improved a high with endfire pattern

    Reflector: about 5% greater length than the active element, will reflects the power radiation at backward direction.

    Directors: 5% lesser length than the active element, will create a converging mechanism and increase the directivity along the forward direction.

    Spacing between the directors and reflector are depending on the optimality, in most of the case, the spacing should from 0.3 to 0.4 (at 1927, the spacing was /10)

    12 to 20 element yagi-uda antennas are optimum and have improved directivity

  • Radiation properties Basically End-fire radiation pattern, with high directivity (less HPBW)

    Due to the ground and parasitic element, the pattern maxima at elevation will not be

    at 90o (along the axis), but 45 to 60 degree elevated so.

  • Radiation properties

    We can show that, while increasing the directors, the gain and directivity

    will improve, however the side lobs will degrade the performance by

    attracting the noise in unwanted direction.

  • Measurements

    Forward gain

    Backward gain (or back gain)

    Front to Back ratio (diff of F.gain and B.gain)

    Magnitude of side lobes

    Input impedance

    Bandwidth, quality factor

  • Simulation 9GHz Yagi Uda

    Atleast 1 or 2

    Dipole=0.5

    Director=0.45

    Reflector=0.53

  • Due to inductive effect at dipole

    and capacitive effect at parasitic elements

    fo=8.8GHz

  • max

    @ 8.8GHz

  • Front gain

    Approx=40dB

    Back gain

    Approx=25dB

    @ 8.8GHz max

    Front to back ratio=40-25 = 15dB

  • Front to back ratio=42-17 = 25dB

    Back gain

    Approx=17dB

    Front gain

    Approx=40dB

  • Optimized Design of N-element Yagi-Uda.

    For the frequency of operation f0, the will be estimated.

    Reflector: (mostly 1 element) length of 0.5 with spacing of 0.25 to 0.3 from dipole.

    Dipole length (active element): 0.475

    Directors (N-2 elements) 0.405 with spacing of 0.3 to 0.4 between each element.

    To match the dipole, usually the QWT section be utilized.

  • Example (from Balanis)

  • Various Yagi Uda antenna

  • Smallest Yagi Uda antenna!! Ivan S. Maksymov et al, Optical Yagi-Uda nanoantennas, http://arxiv.org/pdf/1204.0330.pdf