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• Whether modifiers or safety factors are recommended (see
EIA/TIA-222-E standard).
• Whether you will expose the element to the wind or put the
boom into the wind (see Section 3.3.4).
• Whether you have your Yagi on a crank-up tower, so that
you can nest it at protected heights during high wind
storms.
Fig 13-5 shows the 3-element full-size 40-meter Yagi
placed 5 meters above my 5-element 20-meter Yagi, which
has a similar taper design. Note the very limited sag on the
elements. The telescopic fits are discussed in Section 3.3.7.
Figs 13-6 and 13-7 show the section layout of the 40-meter
reflector element, calculated for both metric and US (inch)
materials.

3.3.3.1. Element sag
Although element sag is not a primary design parameter,
I included the mathematics to calculate element sag in the
ELEMENT STRENGTH module of the YAGI DESIGN soft­
ware. While designing, it is interesting to watch the total
element sag. Minimal element sag is an excellent indicator of
a good mechanical design. Too much sag means there is
somewhere along the element too much weight that does not
contribute to the strength of the element. The sag of each of the
sections of an element depends on:
• The section’s own weight.
• The moment created by the section(s) beyond the section
being investigated (toward the tip).
• The length of the section.
• The diameter of the section.
• The wall thickness of the section.
• The elasticity modulus of the material used.
The total sag of the element is the sum of the sag of each

section. The elasticity modulus is a measure of how much a
material can be bent or stretched without inducing permanent
deformation. The elasticity modulus for all aluminum alloys
is 700,000 kg/cm2 (9,935,000 lb/inch2). This means that an
element with a stronger alloy will exhibit the same sag as an
element made with an alloy of lesser strength.
The 40-meter reflector designed above has a calculated

Fig 13-5—Three-element 40-meter Yagi at ON4UN. The
Yagi is mounted 5 meters above a 5-element 20-meter
Yagi with a 15-meter boom, at a height of 30 meters.
Note the very limited degree of element sag, which is
proof of a good physical design.

Fig 13-6—Mechanical layout of a 40-meter full-size reflector using metric materials.

Yagis and Quads

Chapter 13.pmd

7

2/17/2005, 2:49 PM

13-7


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