20 September 2016
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.
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
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.
(Graphic courtesy of the American Radio Relay League.)