[amsat-bb] Re: yagi design
gzook at yahoo.com
Sun Mar 22 14:40:42 PDT 2009
I am very familiar with how a yagi or quagi works. You can use an optimizing computer program like YagiMax to vary any, or all, of the parameters and the program will tell you things like gain, front-to-back ratio, the SWR at various frequencies, the E plane and H plane radiation patterns, and the impedance/reactance of the driven element.
What Overbeck has done with the quagi is to optimize the antenna at various frequencies including a direct feed of the loop driven element to match 50 ohms without the need for anything except a direct connection of the 50 ohm coaxial cable. The result is a "cookbook" antenna that when built according to the directions works each, and every, time.
As for using a folded dipole driven element: If the spacing between the upper and lower sides is correct, and if you use a 2:1 ratio of the diameters of the tubing used for the upper and lower sides of the driven element you can easily obtain a feed point impedance of 200 ohms. This then can be "matched" to 50 ohm coaxial cable by utilizing a 4:1 balun made from a 1/2 wave or odd multiple of 1/2 wave coaxial cable. In fact, that is what I do with my primary 6-meter yagi and my primary 2-meter yagi. Also, my primary 432 MHz antenna which has 27 elements and a simulated plane reflector also uses a folded dipole driven element with a coaxial balun.
Now my 6-meter yagi and 2-meter yagi are both fed with 1/2 inch Andrew Heliax and my 432 MHz yagi is fed with 7/8ths inch Heliax. In fact, virtually all of my antennas are fed with 1/2 inch Heliax including my 160 meter antennas. However, over the years I have obtained quite a few 100 foot lengths of Heliax with connectors for absolutely no cost. Therefore, to use an inferior quality feedline I would actually have to spend money! Therefore, being that I firmly believe in the principles of the conservation of my finances and labor (you can say "cheap and lazy" if you wish) I go with using the Heliax that I have on hand (there are 10 Heliax runs up my main tower and 4 up my "short" tower). There are photographs of my antennas at
As for the design of the late Clarence Moore, W9LZX, the quad (first used at shortwave broadcaster HCJB in Quito, Ecuador, in the late 1930s), modern experiments as well as computer calculations have shown that the quad doesn't really have any more gain than a yagi with a similar number of elements at heights above 1/2 wavelength above ground. Like the yagi the spacing between elements, the size of the elements, and so forth, definitely have an effect on the gain, front-to-back, and the bandwidth of the antenna. The experiments of the 1940s and early 1950s which showed a slight edge of the quad have pretty much been set aside by modern techniques.
The quad is a good antenna. However, due to its volume and materials needed to construct, many operators definitely prefer the yagi. I prefer yagi antennas although I did make a 4-element quad which my eldest daughter used in some radio astronomy experiments at 137 MHz when she was in middle school and high school. About 35 years ago I obtained a tri-band (20, 15, 10) "spider" quad in a trade and put it together in my back yard. Then I looked around and decided that due to the placement of power lines at the house in which I was living that it was not safe to put up the antenna. About that time my wife walked out into the yard and told me in no uncertain terms that I was NOT going to put up THAT antenna. She pointed to the yagi that I already had in the air and said that type of antenna was fine. To this day she still believes that she got her "way" about the quad.
--- On Sun, 3/22/09, Art McBride <kc6uqh at cox.net> wrote:
How a Quagi or Yagi works is dependent on element spacing which can give narrow band higher gain performance at the expense of F/B ratio and very low driven element impedance. An un-normalized impedance Yagi can have an impedance of less than 20 Ohms. The distance of the first director has the most control over reducing the antenna driven element impedance.
A folded dipole can be used for an initial impedance of greater than 200 Ohms, or an inductor (hair pin match) can be used to raise the dipole impedance. The distance of the first director has the most control over the antenna impedance.
The initial impedance of a loop is 100 Ohms (Quagi), and the Dipole is 75 Ohms (Yagi) hence there are some cases that the Quagi will provide a closer to 50 Ohm un-normalized impedance than a Yagi. All require some means of transformation of balanced to unbalanced when using a coax feed line.
A quad type Yagi has slightly more gain per number of elements and would be the best choice for gain VS boom length.
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