Yagi-Uda Antenna Design

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The design of a Yagi-Uda antenna is actually quite simple. Because Yagi antennas have been extensively analyzed and experimentally tested, the process basically follows this outline:

  • Look up a table of design parameters for Yagi-Uda antennas

  • Build the Yagi (or model it numerically), and tweak it till the performance is acceptable

    As an example, consider the table published in "Yagi Antenna Design" by P Viezbicke from the National Bureau of Standards, 1968, given in Table I. Note that the "boom" is the long element that the directors, reflectors and feed elements are physically attached to, and dictates the lenght of the antenna.

    Table I. Optimal Lengths for Yagi-Uda Elements, for Distinct Boom Lengths

  • d=0.0085wavelength


    Boom Length of Yagi-Uda Array (in )
    D2 0.4240.4200.4150.4200.424
    D3 0.4280.4200.4070.4070.420
    D4 0.4280.3980.3980.407
    D5 0.3900.3940.403
    D6 0.3900.3900.398
    D7 0.3900.3860.394
    D8 0.3900.3860.390
    D9 0.3980.3860.390
    D10 0.4070.3860.390
    D11 0.3860.390
    D12 0.3860.390
    D13 0.3860.390
    D14 0.386
    D15 0.386
    Spacing between directors,    (SD/)
    Gain (dB)9.2511.3512.3514.4015.5516.35

    There's no real rocket science going on in the above table. I believe the authors of the above document did experimental measurements until they found an optimized set of spacings and published it. The spacing between the directors is uniform and given in the second-to-last row of the table. The diameter of the elements is given by d=0.0085 . The above table gives a good starting point to estimate the required length of the antenna (the boom length), and a set of lengths and spacings that achieves the specified gain. In general, all the spacings, lengths, diamters (including the boom diameter) are design variables and can be continuously optimized to alter performance. There are thousands of tables that further give results, such as how the diamter of the boom affects the results, and the optimal diamters of the elements.

    As an example of Yagi-antenna radiation patterns, a 6-element Yagi antenna (with axis along the +x-axis) is simulated in FEKO (1 reflector, 1 driven half-wavelength dipole, 4 directors). The resulting antenna has a 12.1 dBi gain, and the plots are given in Figures 1-3.

    gain in the E-plane for a yagi antenna

    Figure 1. E-plane gain of Yagi antenna.

    h-plane gain for yagi antennas

    Figure 2. H-Plane gain of Yagi-Uda antenna.

    3d three-dimensional radiation pattern for yagi-uda antenna

    Figure 3. 3-D Radiation Pattern of Yagi antenna.

    The above plots are just an example to give an idea of what the radiation pattern of the Yagi-Uda antenna resembles. The gain can be increased (and the pattern made more directional) by adding more directors or optimizing spacing (or rarely, adding another refelctor). The front-to-back ratio is approximately 19 dB for this antenna, and this can also be optimized if desired.

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