Also called the Yagi Uda antenna after two japanese engineers, Yagi and Uda, who researched how parasitic elements affect the directional properties of an antenna. An array of parallel, straight antenna elements, one or more driven and one ore more parasitic, was originally called a Yagi-Uda array. In recent years this has been shortened toYagi. Sometimes an antenna of this kind is called a beam.
Two Yagi antennas, one low band and one multiband.
Still most yagis are for tv-reception and they are also used for more specialized purposes with transmitting too. It is the most popular directional array antenna on the radio amateur bands from 14MHz through VHF and UHF. It is sometimes seen on the 30 and 40 meter bands too. Most hams use rotators with their Yagi arrays so that the antenna can be pointed in any horisontal direction.
Where low transmitting power is used, like down to 1 Watt or less, as when a qrp transceiver is used, a yagi antenna can somewhat compensate for that. And since a yagi gives such improved receiving performance, other qrp stations can be heard better too.
Single mast mount yagi
antenna in a garden.
Yagi wifi antenna
Build a Yagi antenna
There are some resources in the menu to the left, and here is another one;
How to Build an Antenna
However, sometimes an antenna rotor is needed so let's take a little look at that.
An antenna rotor
is rotating the antenna in the desired direction with a control box indoors. There is a rotor cable between the control and rotor.
Here is an overview of different many antenna rotors with reviews.
Now let's take a look at the design of the yagi antenna and the elements. Again, in an antenna with parasitic elements, those elements connected to the transmission line are called driven elements. In most parasitic arrays, there is one driven element, one reflector, and one or more directors.
The driven element
The driven element(s) is/are connected to the feed line. In yagi antennas for ham radio, there is usually just one driven element. This element is sometimes physically shortened by inductive loading. It might contain traps so that it resonates on more than one band. It is half wave resonant, is center fed, and by itself would be a dipole antenna.
The driven element is a dipole.
With a gamma match driven element, the elements may be, or not be electrically connected.
The driven element can be either as an electrically separate dipole or together with the boom and the other elements, as a gamma match.
When several parasitic antennas are operated together, such as in a collinear or stacked array, there are several driven elements. Each driven element receives a portion of the output power of the transmitter. Generally, the power is divided equally among all of the driven elements. [In some phased arrays, all of the elements are driven.]
The driven element in a parasitic array is always resonant at the operating frequency. The parasitic elements are usually (but not always) slightly off resonance; the directors are generally tuned to a higher frequency than that of the driven element, and the reflector is generally set to a lower frequency (thus longer). The impedance of the driven element, at the feed point, is a pure resistance when the antenna is operated at its resonant frequency. When parasitic elements are near the driven elemt, the impedance of the driven element is low compared to that of a dipole in free space.
For the purpose of providing an impedance match between a driven element and a transmission line, the driven element can be folded or bent into various configurations. Among the most common matching systems are the delta, gamma and T networks. Sometimes the driven element is a folded dipole rather than a single conductor.
Dipole as the driven element
The basic design uses a dipole as a driven element. It is resonant when the electrical length is 1/2 of the wavelength, for the used frequency, applied to the feed point. Folded Dipoles are commonly used in Yagi design.
Gamma match on the driven element
A gamma match makes it easier to adjust the antenna. It matches the impedance of the feedpoint which is rarely is 50 Ohm, to the 50 Ohm coax. On Yagi antennas with more than 4 elements, the impedance at the feedpoint becomes low (like 20 Ohms).
Parasitic elements - Reflector - Director
A parasitic element is an element that is not directly connected to the feed line. Parasitic elements are used for the purpose of obtaining directional power gain. Generally, parasitic elements can be classified as either directors or reflectors, hence they work in opposite ways.
Parasitic elements operate by electromagnetic coupling to the driven element. Parasitic elements are parallel to a radiating element, at a specific distance from it, and of a certain length, causes the radiation pattern to show gain in one direction and loss in the opposite direction. For many years now, Yagi's principle is used at high frequencies by amateur and commercial radio operators.
At high frequencies, parasitic elements are often used in directional antennas. The most common of these are the quad antenna and the Yagi.
The forward gain, or power gain of a Yagi is measured relative to a half wave dipole (dBd) in most cases. Power gain is more important in terms of transmitting; the front-to-back ratio is of more interest in terms of receiving directivity.
Front to back ratio
The front-to-back (F/B) ratio is specified in decibels, and is the ratio of signal strength in the favored direction to the strength 180 degrees opposite the favored direction.
It is one expression of the directivity of an antenna system. The term applies only to antennas that are unidirectional , or directive in predominantly one direction, as for Yagi antennas. Mathematically, the front-to-back ratio is the ratio, in decibels, of the field strength in the favored direction, given an equal distance. In terms of power, if P is the power gain of an antenna in the forward direction and Q is the power gain in the reverse direction, both expressed in decibels with respect to a dipole or isotropic radiator, then the front-to-back ratio is given simply by P - Q.
A two-element Yagi can be formed by adding either a director or reflector alongside the driven element. The optimum spacing for a driven-element/director Yagi is about 0.1 to 0.2 wavelength, with the director tuned to a frequency 5 to 10 percent higher than the resonant frequency of the driven element. The optimum spacing for a driven-element/reflector Yagi is about 0.15 to 0.2 wavelength, with the reflectror tuned to a frequency 5 to 10 percent lower than the resonant frequency of the driven element.
The gain of an optimally built, full-size two-element Yagi is about 5 dBd using either a director or reflector, If the elements are shortened by inductive loading or with traps, the gain is somewhat compromised.
A Yagi with one director and one reflector, along with the driven element, increases the gain an F/B ratio, compared with a two-element beam. An optimally designed three-element Yagi that can be scaled for universal construction purposes. Optimization must be done after the antenna is built. Matching schemes and construction details are not included.
The gain and F/B ratio of a Yagi increases as more elements are added. This is usually done by placing extra directors in front of a three-element Yagi. Some Yagis have 10, 15 or even 20 elements. The extra directors tend to have optimum spacings that increase as the number of elements increases. Thus, to make a 10-element Yagi into an 11-element Yagi, a director is added about 0.4 wavelength in front of the 10-element Yagi for optimum performance. The directors alse tend to get shorter and shorter, converging to a minimum limit. It is rare to see a Yagi more than 20 elements. An ideal 20-element Yagi can theoretically give 19 dBd forward gain. If the gain needed in an antenna requires more than 20 elements on a single boom, two or more Yagis canbe fed together in pahse, and placed end-to-end (collinear) or sacked, or both. Other antennas, such as the dish, become easier to construct and tune than the Yagi at high UHF and microwave frequencies, when large gain and F/B ratio is wanted.
Traps can be placed in the elements of a Yagi, allowing operation on more than one band. Extra elements for some bands can be placed in between the elements for other bands, because of the differences in optimum element spacing. A multiband Yagi often takes on a complex appearance. The most common multiband Yagis work the bands from 14MHz through 29.7MHz (20 through 10 meters). A few also work other combinations such as 40/30/20 meters or 30 through 10 meters.
Variations on the Yagi
The driven element of a Yagi can be a full wave loop, fed so that its polarization is parallel to that of the other elements. This antenna is called a quagi because it is a hybrid between a Yagi and a quad. Sometimes the reflectors is also formed into a loop to make a cuagi.
A pair of Yagis can be oriented at right angles on a single boom, and fed 90 degrees out of phase to get circular polarization. This is favored when working ham satellites.
Updated September 18 2011