Mumbai University > Electronics and Telecommunication > Sem 5 > RF Modeling and Antennas

Marks: 10M

Year: May 2015

Yagi-Uda antenna consists of a number of linear dipole elements, as shown in Figure, one of which is energized directly by a feed transmission line while the others act as parasitic radiators whose currents are induced by mutual coupling. A common feed element for a Yagi-Uda antenna is a folded dipole. This radiator is exclusively designed to operate as an end-fire array, and it is accomplished by having the parasitic elements in the forward beam act as directors while those in the rear act as reflectors. Yagi designated the row of directors as a “wave canal.”

The parasitic elements of the Yagi antenna operate by re-radiating their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. As a result these additional elements are referred to as parasitic elements. In view of the fact that the power in these additional elements is not directly driven, the amplitude and phase of the induced current cannot be completely controlled. It is dependent upon their length and the spacing between them and the dipole or driven element. As a result, it is not possible to obtain complete cancellation in one direction. Nevertheless it is still possible to obtain a high degree of reinforcement in one direction and have a high level of gain, and also have a high degree of cancellation in another to provide a good front to back ratio. The Yagi antenna is able to provide very useful levels of gain and front to back ratios. To obtain the required phase shift an element can be made either inductive or capacitive.

Inductive: If the parasitic element is made inductive it is found that the induced currents are in such a phase that they reflect the power away from the parasitic element. This causes the RF antenna to radiate more power away from it. An element that does this is called a reflector. It can be made inductive by tuning it below resonance. This can be done by physically adding some inductance to the element in the form of a coil, or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element.

Capacitive: If the parasitic element is made capacitive it will be found that the induced currents are in such a phase that they direct the power radiated by the whole antenna in the direction of the parasitic element. An element which does this is called a director. It can be made capacitive tuning it above resonance. This can be done by physically adding some capacitance to the element in the form of a capacitor, or more commonly by making it about 5% shorter than the driven element. It is found that the addition of further directors increases the directivity of the antenna, increasing the gain and reducing the beam width. The addition of further reflectors makes no noticeable difference.

In summary:

Reflectors - longer than driven element = Inductive

Directors - shorter than driven element = Capacitive

To achieve the Unidirectional end-fire beam formation, the parasitic elements in the direction of the beam are somewhat smaller in length than the feed element. Typically the driven element is resonant with its length slightly less than λ/2 (usually 0.45–0.49λ) whereas the lengths of the directors should be about 0.4 to 0.45λ. However, the directors are not necessarily of the same length and/or diameter. The separation between the directors is typically 0.3 to 0.4λ, and it is not necessarily uniform for optimum designs. It has been shown experimentally that for a Yagi-Uda array of 6λ total length the overall gain was independent of director spacing up to about 0.3λ. A significant drop (5–7 dB) in gain was noted for director spacing greater than 0.3λ. For that antenna, the gain was also independent of the radii of the directors up to about 0.024λ. The length of the reflector is somewhat greater than that of the feed. In addition, the separation between the driven element and the reflector is somewhat smaller than the spacing between the driven element and the nearest director, and it is found to be near optimum at 0.25λ. Since the length of each director is smaller than its corresponding resonant length, the impedance of each is capacitive and its current leads the induced emf. Similarly the impedances of the reflectors is inductive and the phases of the currents lag those of the induced emfs. The total phase of the currents in the directors and reflectors is not determined solely by their lengths but also by their spacing to the adjacent elements.

Thus, properly spaced elements with lengths slightly less than their corresponding resonant lengths (less than λ/2) act as directors because they form an array with currents approximately equal in magnitude and with equal progressive phase shifts which will reinforce the field of the energized element toward the directors. Similarly, a properly spaced element with a length of λ/2 or slightly greater will act as a reflector. Thus a Yagi-Uda array may be regarded as a structure supporting a traveling wave whose performance is determined by the current distribution in each element and the phase velocity of the traveling wave.