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160 km/h winds (+30% gusts). I have my doubts that an
80-meter 3-element Yagi, with elements that are 20% longer
than for a 40-meter reflector, can be built for a total weight of
only 120 kg.
As an example, I modeled the elements of the old KLM
80M-3 Yagi to assess its wind-survival speed. The element
mechanical data were taken from the assembly manual of the
80M-3 antenna. The safe wind survival speed turned out to
be 90 km/h, without a 30% higher gust factor. An element
stress-analysis shows a very unbalanced design: While sec­
tions 2 (2-inch OD) and 3 (1.75-inch OD) are loaded to the
limit, the three next sections are only loaded to about 60% of
their capabilities.
The tip section is only loaded 25%. This does not neces­
sarily mean that the element will disintegrate at 90 km/h, since
this assumes that the wind blows at a right angle with respect
to the elements. Putting the boom into the wind (perpendicular
to the wind direction) will take all the stress off the elements.
Provided that the side bracing of the boom is well done, it is
likely that the Yagi boom will survive wind speeds above
90 km/h. Using the guidelines explained by Leeson in his
book (Ref 964), sections 2, 3 and 4 can be reinforced by
doubling the wall thickness to increase the wind survival
speed to 123 km/h. In any case, you should add side guying of
the central 3-inch section of the elements. Short boom exten­
sions will be required to do this.
5.1.3 Low losses
A very interesting point was brought up by W6ANR.
Often the weak point of a design is the lack of long-lasting,
low-resistance electrical contacts. Lossy contacts ruin the
gain and the pattern of any array. Invest in some good contact
grease, Parker screws and heat-shrink tube for assembling the
Yagi. Make sure all is done to prevent corrosion in the linear­
loading wires. Better still, stay away from these wires and
invest in high-Q loading coils.
5.1.4 High-Q coil-loaded Yagis
Creative Design Co, Ltd, manufactures a coil-loaded
shortened 80-meter antenna. Their 3-element array has both
elements driven in a ZL-Special configuration with 135°
phasing. The element spacing is λ/8 (9 meters). The elements
are 24 meters long (or approximately 62% of full size), and the
loading is done with high-Q coils and a small capacitance hat
about 2/3 out on the elements. The elements are also loaded at
the center with hairpin loading coils, which allows precise


Chapter 13.pmd

matching to the phasing line and the coaxial feed line. The
array weighs only 80 kg. This is a very popular 80-meter
antenna in Japan. Judging from its weight, it is probably not
an antenna to put up where I live, though! As described in
Section 3.5.1, W6ANR and K7ZV make high-Q loading coils
for 80-meter Yagis.
5.1.4 Gain figures
Be very careful when comparing published gain figures.
The only thing that really makes sense are free-space dBi gain
figures, but the sales and marketing guys like to inflate these
low figures and add ground reflection gain, which could be
anything up to 6 dB, depending on ground quality and antenna
height. Don’t let these guys fool you!
I have withheld from publishing a list of commercial
low-band antennas as I fear that I might not list all of them.
And I worry that doing so might indicate some kind of
endorsement on my part. If you plan to buy a commercial low­
band antenna, I suggest you get a reference list from the
manufacturer, and contact some of the customers—Or better
yet, ask around on the Internet.

5.2. 40-Meter Yagis
It is amazing that none of the major antenna manufactur­
ers advertise 2 or 3-element full-size 40-meter Yagis! That
fortunately leaves a place for the real hams, the home builders
to excel! From the poll I did with over 266 active and success­
ful low-band operators, it appears that an important number
use commercially made rotatable Yagi of some kind. Of those
that listed the make they’re using the breakdown is:
Cushcraft 40-2CD
Creative Design (Japan)

Undoubtedly the most popular commercial low band
Yagi remains the 40-meter Cushcraft 40-2CD Yagi. It appears
to the best value (excellent performance/price ratio) on the
40-meter shortened Yagi market.
D. Leeson, W6NL (formerly W6QHS), calculated the
wind survival speed of an unmodified 40-2CD as 108 km/h.
(Ref 967). The referenced article describes how to increase
the wind survival speed to 150 km/h by using internal boom
and element reinforcements.

Chapter 13



2/17/2005, 2:50 PM