Fall 2002 Issue

A 435-MHz circular-polarized log periodic.

By Kent Britain, WA5VJB
 

The log periodic is part of a family of antennas designed to work over a broad range of frequencies. The elements are mounted along the transmission line with increasing length. The radio wave travels along the boom and picks out the element it likes best. That element resonates, becoming the driven element. The shorter element in front acts like a Yagi director, and the longer element in the rear of the antenna acts like a Yagi reflector. I like to think of a log periodic as a three-element close-spaced Yagi. It’s just a three-element Yagi on whatever frequency you choose.

The log periodic in photo 1 operates from 140 to 1400 MHz. Can we operate a log periodic over an even wider band of frequencies? Yes. It’s just a matter of physical size on the low end as well as precision for the smaller and smaller high-frequency elements.

As I mentioned earlier, the booms are transmission lines. You should space the booms far enough apart to make them look like a section of twinlead transmission line, approximately 100 to 125 ohms impedance (figure 1). If you try to make the booms down around 50 ohms (one of my first mistakes), the booms may radiate. Also, the elements have a loading effect on the impedance of the booms. While the impedance of the booms may be 100 or 125 ohms, it is possible to use the loading effects of the elements to pull the impedance of the antenna down to 50 ohms.

As you can see in figure 2, the booms look a lot like a cross between a J-pole antenna and a folded dipole. If you are not careful, the booms will radiate mainly off the sides of the antenna, which really messes up the antenna radiation pattern. Later, the loading effects of the elements can be used to load the booms from 125 ohms down to 50 ohms. (My thanks to Roger Cox, WBØDGF, for straightening me out on that one.)

For a Yagi to work, the current in the reflector must be out of phase with the current in the driven element. Carefully choosing the length and spacing between the driven element and the reflector takes care of this (figure 3).
 

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