Nuts and Volts - September 2016

20 September 2016

en.wikipedia.org/wiki/Amateur_radio_direction_finding.)

This particular antenna’s elements are made from metal measuring tape so they can flex and bend without damage. You can clearly see the reflector at the back, the driven element in the middle, and the director element at the right. Figure 4 shows the radiation pattern typical for such an antenna. Yagi antennas with a dozen elements or more — creating a very narrow beam — are not uncommon for frequencies above 100 MHz. At lower frequencies, the longer elements and boom require more robust construction materials and techniques.

If you would like to experiment with a Yagi antenna to improve your TV reception, pull in a distant FM station, or maybe extend the range of a 900 MHz data link, you can do so inexpensively by building the antenna yourself. Kent Britain WA5VJB has developed an entire line of Yagi antennas that can be constructed from nothing more than spare copper wire and a piece of 1x2 lumber. Kent’s “Cheap Yagis” website ( www.wa5vjb.com/yagi- pdf/cheapyagi.pdf) provides all the details.

Log-Periodics

You may be thinking, “Ah hah, so my outside TV-FM antenna is a Yagi!” Not quite. While common TV antennas look like a Yagi, closer inspection shows that the dipole elements are bent forward in shallow vees and the feed line is connected to every element in a criss-cross pattern down the length of the array. This type of antenna is a log-periodic dipole array (LPDA, or more commonly, a log-periodic or just plain “log.” Figure 4 shows a log-periodic antenna for 50 MHz through 1,300 MHz mounted above my house (the antenna elements are all horizontal — the antenna is not pointing at the sky).

The log-periodic’s name comes from the logarithmic spacing and length of the elements; it is defined entirely in terms of angles, such as the taper of the triangle surrounding the elements. The active region of the antenna “moves” back and forth along the array as the frequency changes. The dipole elements closest to the frequency of the signal radiate it or transmit it while the other elements remain electrically inert.

With a sufficient number of elements, a log can cover a very wide range of frequencies with consistent performance — even on the shortwave HF (high

The Biggest Yagi The largest single amateur radio antenna ever built was a three-element behemoth Yagi for the 160 meter band (1.8 MHz) by the Finnish club, Radio Arcala, OH8X ( vk6ysf.com/Radio_Extremes.htm). It had elements 59 meters long atop a 100 meter tower, and a boom so large a small car could be placed inside of it.

Compare that to a Yagi for Wi-Fi with the longest element approximately 6 cm long! Yet, the antennas operate on exactly the same principle of re-radiation and reinforcement.

Hams have a saying that, “If it stayed up last winter, it’s not big enough!” Well, the OH8X Yagi was certainly big enough, and unfortunately validated the saying by collapsing in the winter of 2013. Ironically, it was not the antenna that failed, it was the supporting tower!

FIGURE 4. The T- 28 log-periodic by Tennadyne covers 50 to

1,300 MHz. The 12 foot boom consists of a pair of square tubes that act as the feed line for the antenna.

(Photo by the author.)

FIGURE 3. A three-element Yagi antenna made for portable direction finding use. The antenna consists of a reflector (left), driven element (middle), and director (right). This antenna has approximately 5 dB of gain in the forward direction (to the right) compared to the maximum signal radiated by a dipole.