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the performance and dimensional data to choose a Yagi design.
The main-menu option print database prints out the content of
the entire database, either in a tabular format (only the key
characteristics) or it can generate a full-blown data sheet for
all the Yagis (with two designs per printed page. This represents
a little booklet of 50 pages).
In the ANALYZE module you can specify some key
characteristics, such as boom length (expressed in either
wavelengths, feet or meters), minimum gain, minimum F/B,
maximum Q factor, etc. The software will automatically select
the designs that meet your criteria.

3.2. Generic Dimensions
Select the SELECT DESIGN module. After having
chosen a proper design from the system database, the screen
will display all the data relevant to this design—gain, F/B,
impedance, etc, at the design frequency and 6 other
frequencies spread up to ±1.5% of the design frequency.
You must now enter the design frequency (such as,
14.2 MHz). The screen now displays all the generic
dimensions of the Yagi for the chosen design frequency.
“Generic” means that the element lengths given in inches as
well as centimeters are valid for an element diameter-to­
wavelength ratio of 0.0010527. These are not the dimensions
we will actually use to construct the Yagi, since the element
will be made of tapered sections. The screen display also
shows the amount of reactance that the driven element
exhibits at the design frequency. The element positions
along the boom are those that will be used in the final
physical design.

3.3. Element Strength
Before we calculate the actual lengths of Yagi elements
with tapering sections, we must first see which taper we might
use. What are the required diameters and taper schedule that
will provide the required strength at minimal cost, weight and
element sag?
The ELEMENT STRENGTH module helps you build
elements of the required strength at the minimum weight. Up
to 9 sections of varying diameters can be specified (that’s
enough sections even for an 80-meter Yagi). Given the lengths
(and overlap) of the different sections and the wall thickness
entered from the keyboard, the program calculates the bending
moments at the critical point of every section. The module lets
you specify wind speeds and ice loading as well as a vibration­
suppression internal rope and several types of aluminum

3.4. Element Taper
It’s time now to calculate the exact length of the tapered
elements. We follow the taper schedule we obtained with the
ELEMENT STRENGTH module. An improved version of the
well-known W2PV algorithm is used to calculate the exact
length. A wide range of boom-to-element clamps (flat, square,
L, rectangular, etc) can be specified. These clamps influence
the eventual length of the tapered elements.

3.5. Mechanical Yagi Balance
This mechanical design module performs the following

3.5.1 Boom strength
This calculates the required boom diameter and wall
thickness. An external sleeve (or internal coupler) can be
defined to strengthen the central part of the boom. If the boom
is split in the center, the sleeve or the coupler will have to take
the entire bending moment. Material stresses at the boom-to­
mast plate are displayed. Any of the dimensional inputs can be
changed from the keyboard, resulting in an instantaneous
display of the changed stress values.
3.5.2 Weight balance
Many of the newer computer-optimized Yagis have non­
constant element spacing, and hence the weight is not
distributed evenly along the two boom halves. The WEIGHT
BALANCE module shifts the mast plate (attachment point) on
the boom until a perfect weight balance is achieved. It is nice
to have a weight-balanced Yagi when laboring to mount it on
the mast!
3.5.3 Yagi wind load
This program calculates the angle at which the wind area
and wind load are largest. In most literature the wind area and
wind load are specified for a wind angle of 45º (wind blowing
at a 0º angle blows along the boom; at 90º it blows right onto
the boom). This is incorrect, because the largest wind load
always occurs either with the boom broadside to the wind or
with the elements broadside to the wind. With large low-band
antennas, it is likely that the elements broadside to the wind
produces the largest wind area. With higher-frequency long­
boom Yagis having many elements (eg, a 5- or 6-element 10­
or 15-meter Yagi), the boom is likely to produce more thrust
than the elements. The wind load is calculated in increments
of 5º, given a user-specified wind speed.
3.5.4. Torque balancing
Torque balance ensures that the wind does not induce any
undue torque on the mast. This can only be achieved by a
symmetrical boom moment. When the boom-to-mast plate is
not at the center of the boom, a “boom dummy” will have to
be installed to compensate for the different wind area between
the two boom halves. The program calculates the area and the
position of the boom dummy, if required.

3.6. Yagi Wind Area
Specifying the wind area of a Yagi is often a subject of
great confusion. Wind thrust is generated by the wind hitting
a surface that is exposed to that wind. The force is the product
of the dynamic wind pressure multiplied by the exposed area,
and with a so-called drag coefficient, which is related to the
shape of the exposed body. The “resistance” to wind of a flat
body is obviously different from the resistance of a round­
shaped body. This means that if we specify or calculate the
wind area of a Yagi, we must always specify the equivalent
wind area for a flat plate (which should be the standard) or if
the area is simply the sum of the projected areas of all the
elements (or the boom). In the former case we must use a drag
coefficient of 2.0 according to the latest EIA/TIA-222-E
standard, while for (long and slender) tubes a coefficient of
1.2 is applicable. This means that for a Yagi consisting only of
tubular elements, the flat-plate wind area will be 66.6% lower
(2.0/1.2) than the round-element wind area. The WIND AREA
Antenna Design Software



3/2/2005, 2:15 PM