Yagi-Uda dipole array

Image of the Yagi-Uda dipole array.

This popular linearly-polarised, medium-gain end-fire array consists of a number of linear dipole elements, one of which is driven directly, with the rest having currents induced by mutual coupling. It is a practical radiator in the HF, VHF and UHF ranges. The antenna was invented by Uda in Japan in the 1920’s and popularised in the English-speaking world by his colleague Yagi.

The driven element is resonant at slightly less than λ/2, typically 0.45 – 0.49λ. The parasitic elements in the direction of the radiation (directors) are slightly shorter than the feed element at around 0.4 – 0.45λ. The reflector element is slightly longer than the feed element. The element spacing is not usually much more than around 0.3λ. Element lengths must be increased to compensate for a supporting boom and decreased to compensate for an increase in element diameter.

This type of antenna can be optimised for a variety of requirements, e.g. gain, impedance or bandwidth. However, there is a trade-off between the performance characteristics, e.g. optimisation for increased bandwidth reduces the obtainable gain. Yagi-Uda arrays usually have low input impedance and narrow bandwidth but these characteristics can be improved at the expense of others such as gain and sidelobe levels. A folded dipole feed element is often used to step-up the input impedance. For optimum designs, the director spacing and lengths are not uniform. Such designs were initially accomplished experimentally [Viezbicke, P. P.] but are now optimised using numerical techniques. The antenna designed here by Magus is optimised for high gain rather than input impedance.