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critical aspects of designing a sound and lasting 40-meter
Yagi. The Yagi described also happens to be the Yagi I
have been using successfully over the past several years on
40 meters (it has brought several new European records in
major contests on 40 meters). The design criteria for the
Yagi are:
• Low Q, good bandwidth, F/B optimized.
• Survival at wind speeds up to 140 km/h with the elements
broadside to the wind.
• Maximum ice load 10 mm at 60 km/h wind.
• Lifetime greater than 20 years.
• Boom length 10.7 meters maximum (only because I hap­
pened to have this boom)
3.3.1. Selecting an electrical design
Design number 10 from the database of the YAGI
DESIGN software program meets all the above specifications.
Fig 13-2 shows a copy of the TLW main screen for my 40­
meter Yagi. While I could have selected another design with
up to 0.5 dB more gain, I selected this design because of its
excellent F/B pattern and wide bandwidth for SWR, gain and
F/B.
I mounted this Yagi 5 meters above my 20-meter Yagi
(design number 68 from the database), 30 meters above ground.
The combination of both antennas was modeled once more
over real ground at the final height using a MININEC-based
modeling program, to see if there would be an important
change in pattern and gain due to the presence of the second
antenna. The performance figures (gain, F/B) and directivity
pattern of the 40-meter Yagi changed very little at the 5-meter
stacking distance.
3.3.2. Principles of Mechanical Load and Strength
Calculations for Yagi Antennas
R. Weber, K5IU, brought to our attention (Ref 958) that
the variable-area method, commonly employed by most Yagi
manufacturers, and used by many authors in their publications
as well as software, has no basis in science, nor is there any
experimental evidence for the method.
The variable-area method assumes that the direction of
the force created by the wind on an element is always in line
with the wind direction, and that the magnitude is proportional
to the area of the element as projected onto a plane perpendicu­
lar to the wind direction (proportional to the sine of the wind
angle).
The scientifically correct method of analyzing the wind­
force behavior, called the “cross-flow” principle, says that
the direction of the force due to the wind is always perpen­
dicular to the plane in which the element is situated and that
its magnitude is proportional to the square of the sine of the
wind angle.
Fig 13-3 shows both principles. It is easy to see that the
cross-flow principle is the correct one. The experiment de­
scribed by K5IU can be carried out by anyone, and should
convince anyone who has doubts: “Take a 1-meter long piece
of aluminum tubing (approximately 25 mm in diameter) for a
car ride. One person drives, while another sits in the passenger
seat. The passenger holds the tube in his hand and puts his arm
out the window positioning the tube vertically. The tube is
now perpendicular to the wind stream (wind angle = zero). It
is easy to observe a force (drag force) that is in-line with the
13-4

Chapter 13.pmd

Fig 13-2—Free-space performance data for full-sized 3­
element Yagi design number 10 from the YAGI DESIGN
software suite. This was created by the YW (Yagi for
Windows) program.

Fig 13-3—Most amateur literature uses the “variable
area” method shown at A for calculating the effect of
wind on an element. The principle says that the
direction of the force created by the wind on an
element is always in-line with the direction of the wind,
which is clearly incorrect. If this were correct, no plane
would ever fly! The “cross-flow” principle, illustrated at
B, states that the direction of the force is always
perpendicular to the element, and is the resultant of
two components, the drag force and the cross force
(which is the lifting force in the case of an airplane
wing). See text for details.

Chapter 13

4

2/17/2005, 2:49 PM


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