YAGI antennas are one of the most common types of directional antennas.
These antennas send most of feed energy in one direction. This “gained” energy is then ratio compared to an all direction antenna. They work by using a large number of dipole elements to concentrate the signal. Another way to say this is YAGI antennas block unwanted signals and concentrate signals in one direction and are generically called beam antennas. YAGI antennas are generally designed for a narrow frequency range and are used for direction finding and point to point communications. In areas where cable television is not available you can build a YAGI to boost your HDTV. Some used YAGI’s as a WiFi antenna.
The YAGI antenna consists of three types of elements: driven, reflectors and directors. The YAGI has one driven element, one reflector element and any number of directors. The most common form of YAGI antenna uses a series of dipoles for theses elements. With a YAGI antenna all parts of the antenna usually lay on the same plane. YAGI antennas are made to focus in one direction, forward. All waves in the other direction are canceled out.
A basic 3 element YAGI has almost 10 times the energy of a ¼ Vertical.
Example: Driven = ½ WL, Refl=Driven + 5%, Dir=Driven - 5%.
The driven element, which allows for signal to be amplified in one direction, is longer than the director elements. YAGI antennas take advantage of this and have several director elements which hookup to the driven element and allow for increased gain. The director elements then aim the antenna in the correct direction in order for the antenna to have the best possible reception.
It is important to understand the feedpoint impedance is lower than a dipole and the impedance decreases as directors are added so most YAGI antennas use some type matching. Matching can be by a gamma match, using a folded dipole or quad as the driven element.
5-element 70 cm QUAGI
The picture above is a 5-element 70 cm QUAGI is constructed from 1/16" copper rod and 3/8" Plastic Rod (Electric Fence Post). The use of a full loop reflector and driven element provides a 50 Ohm feed point match. A traditional dipole YAGI design would have a low feed impedance and require a GAMMA or other matching interface to get more RF into the antenna.
The boom of the antenna is made from 3/8" Plastic Rod (Electric Fence Post) with the Quad reflector, driver and directors made from 1/16" copper rod (welding rod). The feedpoint is at the bottom of the loop for horizontal polarization to match the dipole element directors.
Details of Element Spacing
All Dimensions are shown in Inches
Design credit goes to the Antenna Handbook 18 th Edition Pages 18-33, Table 18 for a 6-element QUAGI that was adapted to this 5-element design shown here. This is a high gain antenna is cut for 446 Mhz. with about 12 db gain. The half-power beamwidth appears to be about 25 degrees.
Now you are ready to try building your own beam antenna with a little help from these links