A Yagi–Uda antenna, or Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods (or discs) acting as half-wave dipoles. Yagi–Uda antennas consist of a single driven element connected to a radio transmitter or receiver (or both) through a transmission line, and additional passive radiators with no electrical connection, usually including one so-called reflector and any number of directors. It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, with a lesser role played by his boss Hidetsugu Yagi. (This was the preface and notice in advance for a series of eleven papers, of the same title, by Uda, between 1926 and 1929, on the antenna. However, it seems that Uda's pre-announcement caused his invention to lose its novelty and become unpatentable. He would not have been informed by Professor Yagi about those.)
Reflector elements (usually only one is used) are slightly longer than the driven dipole and placed behind the driven element, opposite the direction of intended transmission. Directors, on the other hand, are a little shorter and placed in front of the driven element in the intended direction. These parasitic elements are typically off-tuned short-circuited dipole elements, that is, instead of a break at the feedpoint (like the driven element) a solid rod is used. They receive and reradiate the radio waves from the driven element but in a different phase determined by their exact lengths. Their effect is to modify the driven element's radiation pattern. The waves from the multiple elements superpose and interfere to enhance radiation in a single direction, increasing the antenna's gain in that direction.
Also called a beam antenna and parasitic array, the Yagi is widely used as a directional antenna on the HF, VHF and UHF bands. It has moderate to high gain of up to 20 dBi, depending on the number of elements used, and a front-to-back ratio of up to 20 dB. It radiates linearly polarized radio waves and is usually mounted for either horizontal or vertical polarization. It is relatively lightweight, inexpensive and simple to construct. The bandwidth of a Yagi antenna, the frequency range over which it maintains its gain and feedpoint impedance, is narrow, just a few percent of the center frequency, decreasing for models with higher gain, making it ideal for fixed-frequency applications. The largest and best-known use is as rooftop terrestrial television antennas, but it is also used for point-to-point fixed communication links, radar, and long-distance shortwave communication by broadcasting stations and radio amateurs.
Origins
The antenna was invented by Shintaro Uda of Tohoku Imperial University, Japan, in 1926, with a lesser role played by Hidetsugu Yagi.
However, the name Yagi has become more familiar, while the name of Uda, who, through experiment, developed and put into practice Yagi's initially vague conception, is often omitted. This appears to have been due to the fact that Yagi based his work on Uda's pre-announcement and developed the principle of the absorption phenomenon Yagi had announced earlier. Yagi filed a patent application in Japan on the new idea, without Uda's name in it, and later transferred the patent to the Marconi Company in the UK. Incidentally, in the US, the patent was transferred to RCA Corporation.
Yagi antennas were first widely used during World War II in radar systems by Japan, Germany, the United Kingdom, and the United States. After the war, they saw extensive development as home television antennas.
Description
The Yagi–Uda antenna typically consists of a number of parallel thin rod elements, each approximately a half wave in length. Rarely, the elements are discs rather than rods. Often they are supported on a perpendicular crossbar or "boom" along their centers. Usually there is a single dipole driven element consisting of two collinear rods each connected to one side of the transmission line, and a variable number of parasitic elements, reflectors on one side and optionally one or more directors on the other side. The parasitic elements are not electrically connected to the transmission line and serve as passive radiators, reradiating the radio waves to modify the radiation pattern. Typical spacings between elements vary from about to of a wavelength, depending on the specific design. The directors are slightly shorter than the driven element, while the reflector(s) are slightly longer. The radiation pattern is unidirectional, with the main lobe along the axis perpendicular to the elements in the plane of the elements, off the end with the directors.
Conveniently, the dipole parasitic elements have a node (point of zero RF voltage) at their centre, so they can be attached to a conductive metal support at that point without need of insulation, without disturbing their electrical operation. They are usually bolted or welded to the antenna's central support boom. The most common form of the driven element is one fed at its centre so its two halves must be insulated where the boom supports them.
The gain increases with the number of parasitic elements used. Only one reflector is normally used since the improvement of gain with additional reflectors is small, but more reflectors may be employed for other reasons such as wider bandwidth. Yagis have been built with 40 directors and more., VE1FA transatlantic Yagi
The bandwidth of an antenna is, by one definition, the width of the band of frequencies having a gain within 3 dB (one-half the power) of its maximum gain. The Yagi–Uda array in its basic form has a narrow bandwidth, 2–3 percent of the centre frequency. There is a tradeoff between gain and bandwidth, with the bandwidth narrowing as more elements are used. For applications that require wider bandwidths, such as terrestrial television, Yagi–Uda antennas commonly feature trigonal reflectors, and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands.Common TV Antenna Types Wider bandwidth can also be achieved by the use of "traps", as described below.
Yagi–Uda antennas used for amateur radio are sometimes designed to operate on multiple bands. These elaborate designs create electrical breaks along each element (both sides) at which point a parallel LC (inductor and capacitor) circuit is inserted. This so-called trap has the effect of truncating the element at the higher frequency band, making it approximately a half wavelength in length. At the lower frequency, the entire element (including the remaining inductance due to the trap) is close to half-wave resonance, implementing a different Yagi–Uda antenna. Using a second set of traps, a "triband" antenna can be resonant at three different bands. Given the associated costs of erecting an antenna and rotator system above a tower, the combination of antennas for three amateur bands in one unit is a practical solution. The use of traps is not without disadvantages, however, as they reduce the bandwidth of the antenna on the individual bands and reduce the antenna's electrical efficiency and subject the antenna to additional mechanical considerations (wind loading, water and insect ingress).
Theory of operation
Consider a Yagi–Uda consisting of a reflector, driven element, and a single director as shown here. The driven element is typically a λ dipole or folded dipole and is the only member of the structure that is directly excited (electrically connected to the feedline). All the other elements are considered parasitic. That is, they reradiate power which they receive from the driven element. They also interact with each other, but this mutual coupling is neglected in the following simplified explanation, which applies to far-field conditions.
One way of thinking about the operation of such an antenna is to consider a parasitic element to be a normal dipole element of finite diameter fed at its centre, with a short circuit across its feed point. The principal part of the current in a loaded receiving antenna is distributed as in a center-driven antenna. It is proportional to the effective length of the antenna and is in phase with the incident electric field if the passive dipole is excited exactly at its resonance frequency. Now we imagine the current as the source of a power wave at the (short-circuited) port of the antenna. As is well known in transmission line theory, a short circuit reflects the incident voltage 180 degrees out of phase. So one could as well model the operation of the parasitic element as the superposition of a dipole element receiving power and sending it down a transmission line to a matched load, and a transmitter sending the same amount of power up the transmission line back toward the antenna element. If the transmitted voltage wave were 180 degrees out of phase with the received wave at that point, the superposition of the two voltage waves would give zero voltage, equivalent to shorting out the dipole at the feedpoint (making it a solid element, as it is). However, the current of the backward wave is in phase with the current of the incident wave. This current drives the reradiation of the (passive) dipole element. At some distance, the reradiated electric field is described by the far-field component of the radiation field of a dipole antenna. Its phase includes the propagation delay (relating to the current) and an additional 90 degrees lagging phase offset. Thus, the reradiated field may be thought as having a 90 degrees lagging phase with respect to the incident field.
Parasitic elements involved in Yagi–Uda antennas are not exactly resonant but are somewhat shorter (or longer) than λ so that the phase of the element's current is modified with respect to its excitation from the driven element. The so-called reflector element, being longer than λ, has an inductive reactance, which means the phase of its current lags the phase of the open-circuit voltage that would be induced by the received field. The phase delay is thus larger than 90 degrees and, if the reflector element is made sufficiently long, the phase delay may be imagined to approach 180 degrees, so that the incident wave and the wave reemitted by the reflector interfere destructively in the forward direction (i.e. looking from the driven element towards the passive element). The director element, on the other hand, being shorter than λ, has a capacitive reactance with the voltage phase lagging that of the current. The phase delay is thus smaller than 90 degrees and, if the director element is made sufficiently short, the phase delay may be imagined to approach zero and the incident wave and the wave reemitted by the reflector interfere constructively in the forward direction.
Interference also occurs in the backward direction. This interference is influenced by the distance between the driven and the passive element, because the propagation delays of the incident wave (from the driven element to the passive element) and of the reradiated wave (from the passive element back to the driven element) have to be taken into account. To illustrate the effect, we assume zero and 180 degrees phase delay for the reemission of director and reflector, respectively, and assume a distance of a quarter wavelength between the driven and the passive element. Under these conditions the wave reemitted by the director interferes destructively with the wave emitted by the driven element in the backward direction (away from the passive element), and the wave reemitted by the reflector interferes constructively.