YAGI Antennas
The Yagi, one of the most popular directional antennas
Gain is the motivation to build the Yagi antenna. Long and slider is the look as the number of elements and boom length the biggest factor in gain. Just a three elements are required; Reflector, Drive Element (dipole or loop) and a Director gives about 5 dB gain. As Directors are added more boom length is needed so more gain is linked to both. Element spacing is the second factor in gain. The most important element spacing is the reflector and first director. The rest of the Directors spacing has less impact. Overall wide Director spacing gives more gain than more closely spaced Directors. These are the general characteristics of the Yagi! Use this calculator as a tool to understand and planning of your Yagi Antenna. Like other calculators the results will vary as there are many designs each with variations in formulas since Uda and Yagi invented the antenna we now know as the Yagi in 1926. The results from this calculator are in line with current commercial practice and amateur radio design in use today. This means the calculations are based on mid-range values for dimensions, nothing here is pushing the envelope and is appropriate for the 6 to 10 dB Yagi builder. Use this calculator to understand the scope of your antenna project, if you want more gain but play it safe by using a design that has been built and tested. You will find the best information on ultra-high gain Yagi antennas published on Earth Moon Earth (EME) Yagi Antennas.
My DIY experimentation with different YAGI antenna design
The QUAGI is a case where results will vary as there are variations in design and formulas; matching and gain and has been optimized by the designer. The QUAGI, a variation on the YAGI antenna, uses a driven loop with a loop reflector instead of dipoles resulting in better matching impedance to a 50 Ohm source. Design credit goes to the Antenna Handbook 18th 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. Based on the Antenna Handbook it should have about 12 db gain. The half-power beamwidth appears to be about 25 degrees.
My 5-element 70 cm QUAGI
The picture above is a 5-element 70 cm QUAGI is constructed from 1/16" copper or brass 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" brass rod (brazing 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
All Dimensions are shown in Inches
Better view of 1st Director,
Driven Element & Reflector
