Yagi beam antennas

The Yagi beam antenna (more correctly, the Yagi–Uda antenna, after both of the designers of Tohoku University in Japan 1926) is unidirectional. It can be vertically polarized or horizontally polarized with little difference in performance (other than the polarization!). The Yagi antenna can be rotated into position with little effort. Yet the Yagi antenna shows power gain (so it puts out and receives a stronger signal), reduces the interfering signals from other directions, and is relatively compact.

COMPOSITION OF A BEAM ANTENNA

The Yagi antenna is characterized by a single driven element which takes power from the transmitter (or is connected to the receiver), plus one or more parasitic elements. The parasitic elements are not connected to the driven element, but rather receive their power from the driven element by indirect means. The indirect means is that they intercept the signal, and then re-radiate them.

The minimalist two element beam antenna may be composed of either a driven element and a reflector, or a driven element and a director. The reflector and directors are known as parasitic elements.

The parasitic reflector is three to five per cent longer than the half wavelength driven element. It provides power gain in the direction away from itself. It is inductive in reactance and lagging in phase.

The parasitic director is three to five per cent shorter than the half wavelength driven element. It provides power gain in its own direction. It is capacitive in reactance and leading in phase.

The factors that affect the phase difference between the direct and reradiated signals is determined principally by the element length and the spacing between the elements. Proper adjustment of these factors determines the gain and the front-to-back ratio that is available.

The presence of a parasitic element in conjunction with a driven element tends to reduce the feedpoint impedance of the driven element for close spacings (<λ/2) and increase it for greater than λ/2 spacing. In general, beam antennas have element spacing of 0.1λ to 0.25λ (with 0.15λ to 0.19λ being common), so the impedance will be lower than the nominal impedance for a half wavelength dipole.

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