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

Marks: 10 M

Year: Dec 2014

Another very practical radiator in the HF (3–30 MHz), VHF (30–300 MHz), and UHF (300–3,000 MHz) ranges is the Yagi-Uda antenna. This 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 Yagi-Uda array has been widely used as a home TV antenna;

To achieve the 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. It should be noted that the previous discussion on the lengths of the directors, reflectors, and driven elements is based on the first resonance. Higher resonances are available near lengths of λ, 3λ/2, and so forth, but are seldom used. In practice, the major role of the reflector is played by the first element next to the one energized, and very little in the performance of a Yagi-Uda antenna is gained if more than one (at the most two) elements are used as reflectors. However, considerable improvements can be achieved if more directors are added to the array. Practically there is a limit beyond which very little is gained by the addition of more directors because of the progressive reduction in magnitude of the induced currents on the more extreme elements. Usually most antennas have about 6 to 12 directors. However, many arrays have been designed and built with 30 to 40 elements. Array lengths on the order of6λ have been mentioned as typical. A gain (relative to isotropic) of about 5 to 9per wavelength is typical for such arrays, which would make the overall gain on the order of about 30 to 54 (14.8–17.3 dB) typical.

Usually Yagi-Uda arrays have low input impedance and relatively narrow bandwidth (on the order of about 2%).Improvements in both can be achieved at the expense of others (such as gain, magnitude of minor lobes, etc.). Usually a compromise is made, and it depends on the particular design One way to increase the input impedance without affecting the performance of other parameters is to use an impedance step-up element as a feed (such as a two-element folded dipole with a step-up ratio of about 4). Front-to-back ratios of about 30 or15 dB can be achieved at wider than optimum element spacings, but they usually are compromised somewhat to improve other desirable characteristics.

The Yagi-Uda array can be summarized by saying that its performance can be considered in three parts:

1. the reflector-feeder arrangement
2. the feeder
3. the rows of directors