TWO-ELEMENT YAGI ARRAY

Figure 10.1 shows the two-element Yagi array antenna. This particular one uses a driven element and a director, so the direction of maximum signal is in the direction of the director. The gain of a two-element Yagi is about 5.5 dBd (gain above a dipole) for spacing less than 0.1λ and the parasitic element is a director. For the case where a reflector is used, the gain peak is 4.7 dBd at about 0.2λ spacing.

 

The difference between reflector and director usage is quite profound. The usual curve shows the director with higher gain, but it is more responsive to element spacing. The reflector has less gain, but is more tolerant of spacing errors.

The front-to-back ratio of the beam antenna is poor for two-element antennas. A compromise spacing of 0.15λ provides front-to-back ratios of 5 to 12 dB.

The feedpoint resistance of the antenna is clearly not 73 ohms as would be implied by the use of a half wavelength dipole for a driven element. The feedpoint impedance will vary roughly linearly from about 5 ohms at a spacing of 0.05λ to about 30 ohms for spacings of about 0.15λ. Above 0.15λ the differences between director and reflector implementations takes place. A reflector two-element beam feedpoint impedance will increase roughly linearly from 30 ohms at 0.15λ to about 45 ohms at 0.25λ spacing. The director implementation is a little less linearly related to spacing, but varies from about 30 ohms at 0.15λ to about 37 ohms at 0.25λ spacing.

Element lengths
The element lengths for a two-element Yagi beam are given below:

Where:
Director is the length of the director
D.E. is the length of the driven element in meters (m)
Spacing is the spacing between the elements in meters (m)
FMHz is the frequency in megahertz.

These element lengths will result in 0.15λ spacing, which is considered about ideal.

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