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This page has a variety of calculators that can be used to calculate dimensions for practical antennas for 6, 10, 11, 12, 15, 17 and 20 Meter Yagi Antennas. While you may not have the space for a large array, there is a lot to be gained from simple, 2 and 3 Element arrays. Yes, I do include 11 meters. While it is not a ham band, a 2 or 3 element yagi for this band would provide better communication than strapping on a amplifier.
All of the equations used on this page are based on the Basic Wavelength Formula, which is shown on the right. But that is quite a bunch of numbers to remember, so the formula is approximated, as shown under the heading the Approximate Wavelength Formula.
But we are really only interested in a "1/2 wavelength" for use as a dipole element. So the formulas would be adjusted, as shown on the right under 1/2 Wavelength Formula.
That is assuming an antenna in "free space". There are velocity effects of the length/diameter ratio, end capacitive effects, and effects due to external objects. All of these things can change the actual length of a half-wavelength dipole. These things generally cause the length to be shortened anywhere between 3% and 7% giving us the formula under 1/2 Wavelength Formula (Adjusted).
For the purposes of this web page, I will be using a slightly different formula that takes the length/diameter ratio into account.
The designs presented on this page are what William Orr, W6SAI, refers to as "pretuned" arrays. By that he means that, the formulas used will create a yagi design that can be adjusted while the antenna is on the ground, and then placed in operation, without the need for re-tuning. Adjustments to the parasitic elements, made after the antenna is in place, could yield a slight improvement in F/B Ratio, but rarely produces any increase in Gain. Its best to simply settle for reasonable Gain and F/B Ratio figures. With the designs produced on this page there will be a F/B Ratio of approximately 12 dB, for two element arrays and 20 dB for three element arrays.
The original information on these antennas did not include data for the WARC bands (12 meters and 17 meters) and only included US Imperial dimensions (feet and inches). I have expanded on this to include Metric dimensions and information for the WARC bands. If you want to design for a frequency that is different than the ones I provide, you can use the frequency that is closest to your needs and then use my web page on Antenna Scaling. Making small changes in the element lengths, without changing the element diameters, will not cause a problem. The antenna will still perform well.
You might notice that the drawings only show the Driven Element as solid element, the same as the parasitic elements. While the Driven Element requires a matching device, it is not shown to keep the drawings uncluttered. For information on the matching device, see the section at the bottom on Impedance Matching. The Driven Element is marked with a Center Line (℄) that can be used as a reference point in the section on Impedance Matching.
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1-1/2" (38.1 mm)
The next step up from a Two-Element Yagi is the Three-Element Yagi. This antenna contains one Driven element plus two parasitic elements, a Reflector and a Director. The Reflector is located on one side of the Driven element and the Director is located on the other side of the Driven Element. The Direction of Maximum Signal is then outward, on the Director side of the yagi
The calculator on the right illustrates the Three-Element Yagi. It shows the relationship between the elements.
In the space provided on the top right, enter the center frequency for your proposed 3-element Yagi antenna. Then just click your mouse anywhere on the the page. The details of the 3-element Yagi antenna will be calculated using the equations above, and then annotated on the drawing.
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