Disk Yagi

Description of Antenna:

The geometry of a dipole driven disk yagi is shown in the diagram below, with relevant parameters.

Design Curves:

The following design curves enable the design of a dipole driven disk yagi with respect to element spacing, element number (boom length,1 lambda-8 lambda), element diameter. The curves are presented in terms of constant director spacings for any particular antenna (as opposed to individual director spacing). Reflector spacing and reflector diameter remain constant at 0.35 lambda and 0.6 lambda respectively. The fed element is a resonant dipole of length 0.475 lambda and the reflector and director disks are assumed to be thin (~0.001 lambda).

Figure 1 shows the maximum gain achievable (optimised element parameters) as a function of boomlength. This curve may be used to either find the required boom length for a specified gain, or vice versa.
	Figure 1: Gain vs Boomlength

The table below shows the required director parameters to either maximise gain or maximise bandwidth, for boomlengths of 1 lambda, 2 lambda, 4 lambda and 8 lambda. If a boomlength is required that does not match the discrete values in the table, the next longest length should be used (eg. for a boomlength of 6 lambda, use the values for 8 lambda).

Figure 2 shows the percentage bandwidth (calculated using equation 1) as a function of director spacing. Again, the curve for the longer antenna should be used in cases where a different boomlength is required. BW(%) = (upper freq - lower freq) / (upper freq)...........1 The number of elements needed to construct the antenna can now be calculated using equation 2 as boomlength, reflector spacing (0.35 lambda) and director spacing are known. Elts - 1 = (Boomlength - 0.35 lambda) / (dir. spacing).............2
	Figure 2: Percentage Bandwidth versus Director Spacing
Figure 3 shows the impedance bandwidth characteristics of the antenna (for a 75 ohm transmission line with VSWR < 2.5). The impedance bandwidth can be converted to show the useful frequency range using equation 3. FreqRange = (%VSWR x CentreFreq[MHz]) / 100........3
	Figure 3: Percentage Impedance Bandwidth versus Director Spacing
Figure 4 illustrates the directional capabilities of the antenna. The beam width is expressed in degrees, with respect to reflector spacing.
	Figure 4: Beamwidth vs Director Spacing
Figure 5 shows the front-to-back ratio, or the ability of the antenna to suppress reverse direction radiation.
	Figure 5: Front-to-back Ratio vs Director Spacing

Example:

A Disk Yagi antenna of the following specifications is required:

* operating frequency: 900MHz (centre)
* minimum impedance bandwidth 10%
* minimum gain 12 dBi

Using Figure 1, it can be seen that for a gain of 12 dBi a boomlength of at least 3.95 lambda is required. This translates into a physical length of 1.3m.

From table 1, for a boom length of 4 lambda the director spacing and director diameter for maximum gain are 0.15 lambda and 0.3 lambda respectively.

The percentage bandwidth for the antenna is obtained from figure 2, and is approximately 29%. (The curve for 4 lambda is used as 3.95 is sufficiently close to 4).

The impedance bandwidth is determined from figure 3 to be 15%, which is above the required 10%.

The beamwidth of the antenna is 36° and front-to-back ratio is 16dBi, determined from figures 5 and 6 respectively.

The antenna therefore has the following physical characteristics:

* a boomlength of 1.3m (3.95 lambda)
* 24 directors of spacing 49.5mm (0.15 lambda) and diameter 99mm (0.3 lambda)
* a reflector with a spacing of 105mm (0.35 lambda) and a diameter of 198mm (0.6 lambda)
* a resonant dipole (fed element) of length 157mm (0.475 lambda)

as well as the following electrical characteristics:

* centre operating frequency of 900Mhz
* percentage bandwidth of 29%
* percentage impedance bandwidth of 15%
* beamwidth of 36°
* front-to-back ratio of 16dBi

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Reference:
Fourth Year Design Report, "Investigate the Design of a Disk Yagi Antenna" prepared by Jonathan Berger for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.