Three-Element, Collapsible Yagi
For Two Meters
(And an Introduction to NEC2)
Mac A. Cody / AE5PH
March, 2016
Taking the Challenge(s)
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"Sherman, set the Wayback Machine to October, 2013 ..."
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Reach more repeaters with a 5W handheld
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Roll-up Jpole on a mast could reach a number of repeaters
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How much better could I do with a Yagi?
Construct my first Yagi antenna
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Must switch easily between vertical and horizontal polarization for
future 2M SSB communications
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Must be collapsible for easy transport and storage
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Make it myself, because I am cheap thrifty!
Learn something about antenna design and analysis software
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Many free antenna design software tools are available
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NEC2 is the antenna analysis software tool of choice
Yagi Antenna Design Tools
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Yagi-Uda Antennas by VE3SQB - What I used
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MS Windows software, but runs on Linux using Windows
emulation (Wine)
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Free software at http://www.ve3sqb.com/yagi.exe
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A lot of other antenna design tools on his website
VHF/UHF Yagi Antenna Design by Martin E. Meserve (K7MEM)
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Part of his Javascript Electronic Notebook website
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http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf.html
Yagi Uda Antenna Calculator by Ajarn Changpuak
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Part of his ELECTRONICS-LAB website
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http://www.changpuak.ch/electronics/yagi_uda_antenna.php
This is just a small sample of the many design tools available
Three-Element Yagi Design
APPROXIMATE TAPER OF THE COLLAPSIBLE WHIP
CENTER OF 2-METER BAND
6 mm
2 mm
NEC2 Antenna Analysis Tools
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NEC2++ 1.3.1 (Currently at version 1.7.0) – Used for my original analysis
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C++ implementation of NEC2
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Runs on Linux or on MS Windows
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Free software at (via link to a Github Project Page)
http://elec.otago.ac.nz/w/index.php/Necpp
Qantenna 0.2.1 (Currently at version 0.3.0) – Used for my original analysis
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Process and visualize NEC2 files
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Runs on Linux only and is also a bit unstable
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Free software at http://qantenna.sourceforge.net/
4nec2 by Arie Voors (Currently at version 5.8.16) – Used for this presentation
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FORTRAN implementation of NEC2 with a lot of supporting tools
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Runs on MS Windows and Linux (via Wine)
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Free software at http://www.qsl.net/4nec2/
What is NEC2?
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Numerical Electromagnetics Code (version) 2
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Comprehensive package for the analysis of the electromagnetic
properties of structures
Can analyze radiating properties i.e. antenna gain, as well as
scattering properties of structures
Based on the method of moments solution of the electric field
integral equation (EFIE) for thin wires and the magnetic field
integral equation (MFIE) for closed, conducting surfaces
Uses an iterative method to calculate the currents in a set of wires
and the fields that result
NEC2 was originally written in FORTRAN (also C, C++)
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Does not model tapered elements such as those made of
telescoping aluminum (not directly, anyway)
Does not model buried radials or ground stakes
Using NEC2
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Due to its FORTRAN heritage, NEC2 utilizes
'cards' – a.k.a. formatted lines of ASCII text
Each 'card' specifies either an antenna
element, a stimulating source characteristic, or
an analysis command
If the antenna being analyzed is simply a
collection of 'wires', generating a NEC2 'card
deck' is a fairly simple process
Simplified Three-Element
Yagi NEC2 Input
Comments
End Comments
Reflector Element
Driven Element
Director Element
End Geometry
Frequency of 146 MHz
Extended Thin-Wire Kernel
Voltage on 13th Segment
Request Radiation Pattern
End Card
CM NEC Input File for three-element, 2-meter collapsible Yagi
CM PT control card suppresses printing of element currents
CM TL control card specs transmission line in terms of Z,length,and shunt Y
CM Whip antenna specifications (sections fully extended):
CM Segment 1 is 0.122 meters long and 0.00300 meters in radius
CM Segment 2 is 0.101 meters long and 0.00250 meters in radius
CM Segment 3 is 0.099 meters long and 0.00200 meters in radius
CM Segment 4 is 0.097 meters long and 0.00150 meters in radius
CM Segment 5 is 0.102 meters long and 0.00100 meters in radius
CM Reflector center hex stand-off specifications:
CM 0.02540 meters long and approximately 0.00367 meters in radius
CM Driven element hex stand-off specifications:
CM 0.01111 meters long and approximately 0.00367 meters in radius
CM Simplifying assumption is non-tapered elements 0.00200 meters in radius
CM The reflector should be 1.01080 meters long
CM The driven element should be 1.00030 meters long
CM The director should be 0.91113 meters long
CM The reflector should be 0.56281 behind the driven element
CM The director should be 0.30746 meters in front of the driven element
CM The yagi is suspended in free space
X1
Y1
Z1
X2
Y2
Z2
RADIUS
CE TAG# #SEG
GW 1
11 -0.56281
0.50540
2.00000 -0.56281 -0.50540
2.00000
0.00200
GW 21
25
0.00000
0.50015
2.00000
0.00000 -0.50015
2.00000
0.00200
GW 41
11
0.30746
0.45557
2.00000
0.30746 -0.45557
2.00000
0.00200
GE 0
FREQUENCY
FR 0
1
0
0
146
EK 1 TAG# SEG#
VOLTS
EX 0
21
13
0
10
RP 0
90
1
0
90
1
0
EN
4nec2 Geometry (F3) Display
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The upper-left element is
the reflector
The driven element is in
the center
The lower-right element
is the director
The tag numbers and
wire segments are
displayed
The coordinate axes
indicate that the Yagi is in
a horizontal orientation
Calculating a Field Pattern (F7)
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The Generate Window
(F7) enables the user to
determine what function
will be calculated
What is present in the
original file can be used
The Far Field pattern is
for a specific frequency
Models the antenna's
radiation pattern
4nec2 Pattern (F4) Display
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The top window shows the
horizontal radiation pattern
The bottom window shows the
vertical radiation pattern
About 7.9 dBi (decibels relative
to isotropic) gain obtained
along the forward direction,
below -10 dBi in the back end
An isotropic antenna has equal
gain in three dimensions and is
an idealized abstraction!
4nec2 3D Viewer (F9) Display
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The 3D view provides
another means of
visualizing the antenna
radiation pattern
The vertical, colored
scale indicates the gain
in dBi
Currents present at
different points on the
antenna are shown on
top of the antenna
Calculating SWR Over Frequency
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The Generate Window (F7)
also allows estimation of
antenna performance over a
range of frequencies
In this example, a sweep
from 144 MHz to 148 MHz in
1 MHz steps is performed
SWR/Gain/Impedance (F5) Display
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The SWR at 144 MHz is
about 1.1, which is good
The SWR at 146 MHz is
about 1.5, which is acceptable
The SWR at 148 Mhz is about
2.2, which marginal
The reflection coefficent gets
progressively worse (higher)
with increasing frequency
(from below -25 dB to about
-8 dB)
Gain and Impedance
Smith Chart (F11) Display
148 MHz
144 MHz
Review of Simplified Yagi Model
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Reasonably good radiation pattern over frequency range
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7.6 dBi gain, 18.6 dB front to back @ 144 MHz
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7.9 dBi gain, 22.1 dB front to back @ 146 MHz
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8.2 dBi gain, 18.3 dB front to back @ 148 MHz
Variant SWR over frequency range
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1.1 @ 144 MHz
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1.5 @ 146 MHz
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2.2 @ 148 MHz
Variant Impedance over frequency range
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46+j3 @ 144 MHz
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50+j21 @ 146 MHz
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32+j36 @ 148 MHz
Remember, the antenna geometry has been approximated
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Let's get more accurate...
Detailed Three-Element
Yagi NEC2 Input (Part 1)
Comments
End Comments
Reflector Element
Driven Element
CM NEC Input File for three-element, 2-meter collapsible Yagi
CM PT control card suppresses printing of element currents
CM TL control card specs transmission line in terms of Z,length,and shunt Y
.
.
. TAG# #SEG
X1
Y1
Z1
X2
Y2
Z2
RADIUS
CE
GW 2 4 -0.56281
0.50140
2.00000 -0.56281
0.43170
2.00000 0.00100
GW 3 4 -0.56281
0.43170
2.00000 -0.56281
0.33470
2.00000 0.00150
GW 4 4 -0.56281
0.33470
2.00000 -0.56281
0.23570
2.00000 0.00200
GW 5 4 -0.56281
0.23570
2.00000 -0.56281
0.13470
2.00000 0.00250
GW 6 4 -0.56281
0.13470
2.00000 -0.56281
0.01270
2.00000 0.00300
GW 7 1 -0.56281
0.01270
2.00000 -0.56281 -0.01270
2.00000 0.00367
GW 8 4 -0.56281
-0.01270
2.00000 -0.56281 -0.13470
2.00000 0.00300
GW 9 4 -0.56281
-0.13470
2.00000 -0.56281 -0.23570
2.00000 0.00250
GW 10 4 -0.56281
-0.23570
2.00000 -0.56281 -0.33470
2.00000 0.00200
GW 11 4 -0.56281
-0.33470
2.00000 -0.56281 -0.43170
2.00000 0.00150
GW 12 4 -0.56281
-0.43170
2.00000 -0.56281 -0.50140
2.00000 0.00100
GW 22 4 0.00000
0.49615
2.00000
0.00000
0.43170
2.00000 0.00100
GW 23 4 0.00000
0.43170
2.00000
0.00000
0.33470
2.00000 0.00150
GW 24 4 0.00000
0.33470
2.00000
0.00000
0.23570
2.00000 0.00200
GW 25 4 0.00000
0.23570
2.00000
0.00000
0.13470
2.00000 0.00250
GW 26 4 0.00000
0.13470
2.00000
0.00000
0.01270
2.00000 0.00300
GW 27 1 0.00000
0.01270
2.00000
0.00000 -0.01270
2.00000 0.00367
GW 28 4 0.00000
-0.01270
2.00000
0.00000 -0.13470
2.00000 0.00300
GW 29 4 0.00000
-0.13470
2.00000
0.00000 -0.23570
2.00000 0.00250
GW 30 4 0.00000
-0.23570
2.00000
0.00000 -0.33470
2.00000 0.00200
GW 31 4 0.00000
-0.33470
2.00000
0.00000 -0.43170
2.00000 0.00150
GW 32 4 0.00000
-0.43170
2.00000
0.00000 -0.49615
2.00000 0.00100
Detailed Three-Element
Yagi NEC2 Input (Part 2)
Director Element
End Geometry
Frequency of 146 MHz
Extended Thin-Wire Kernel
Voltage on 13th Segment
Request Radiation Pattern
End Card
TAG# #SEG
X1
GW 42 4 0.30746
GW 43 4 0.30746
GW 44 4 0.30746
GW 45 4 0.30746
GW 46 4 0.30746
GW 47 1 0.30746
GW 48 4 0.30746
GW 49 4 0.30746
GW 50 4 0.30746
GW 51 4 0.30746
GW 52 4 0.30746
GE 0
FR 0 1
0
0
EK 1 TAG# SEG#
EX 0 27
1
0
RP 0 90
1
0
EN
Y1
0.45157
0.43170
0.33470
0.23570
0.13470
0.01270
-0.01270
-0.13470
-0.23570
-0.33470
-0.43170
Z1
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
146
10
90
1
0
X2
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
0.30746
Y2
0.43170
0.33470
0.23570
0.13470
0.01270
-0.01270
-0.13470
-0.23570
-0.33470
-0.43170
-0.45157
Z2
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
2.00000
RADIUS
0.00100
0.00150
0.00200
0.00250
0.00300
0.00367
0.00300
0.00250
0.00200
0.00150
0.00100
4nec2 Geometry (F3) Display
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The positions of the
reflector, driven element,
and director are as
before
Note the individual wires
used to form each
element (solid dots)
Note the greater number
of modeling segments
A larger number of
segments yields a more
accurate model, but
takes longer to calculate
4nec2 Pattern (F4) Display
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Again, the top window is the
horizontal radiation pattern and
the bottom window is the
vertical radiation pattern
Now about 8.4 dBi gain in the
forward direction, but only -7.9
dBi gain in the back end
This antenna provides better
forward gain, but doesn't
suppress the rear lobe as well
4nec2 3D Viewer (F9) Display
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The 3D view again
provides the antenna
radiation pattern
The vertical, colored
scale indicates the
improved gain
Note that the sections
of the whip antennas
have been accentuated
SWR/Gain/Impedance (F5) Display
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The SWR at 144 MHz is
about 1.5, which is acceptable
The SWR at 146 MHz is
about 1.3, which is acceptable
The SWR at 148 MHz is
about 1.4, which is acceptable
The reflectance coefficient is
comparatively consistent over
the frequency range and not
too high (less than 3 dB
variation)
Gain and Impedance
Smith Chart (F11) Display
148 MHz
144 MHz
Review of Collapsible Yagi Model
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Reasonably good radiation pattern over frequency range
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8.2 dBi gain, 12.6 dB front to back @ 144 MHz
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8.4 dBi gain, 16.3 dB front to back @ 146 MHz
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8.6 dBi gain, 20.2 dB front to back @ 148 MHz
Relatively constant SWR over frequency range
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1.5 @ 144 MHz
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1.3 @ 146 MHz
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1.4 @ 148 MHz
Variant Impedance over frequency range
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40-j14 @ 144 MHz
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38-j5 @ 146 MHz
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35+j5 @ 148 MHz
How well did the actual antenna work?
Yagi Construction Details
Yagi Testing – Horizontal Polarization
Yagi Testing – Vertical Polarization
29.078 Miles from QTH to W5WB/W5AUY
Repeater (SW Dallas ARC)
Now you can get started creating
the next great antenna design....
Thanks for your time!
Questions?