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Antenna Modelling for Amateurs
• Walt Fair Jr, W5ALT
• Oct 14, 2013
University of Texas Amateur Radio Club
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Outline
• Why model antennas?
• Software and more information
• What is an antenna model?
- Wires
- Sources and loads
- Grounds
- Testing for adequacy
• Examples
- Shortened radials for vertical
- Meander dipole
- Spiral radials
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Why Model Antennas?
• Opportunities to “homebrew” are decreasing
- Complexity of equipment
- Miniaturization
• Homebrewing antennas are still possible
- Homebrew antennas can be excellent
- Portable, hidden, compromise, etc.
• One must understand how and why they work
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Why Model Antennas?
• Try antenna designs before building
• Modify existing antenna designs
• Scaling antennas more accurately
• Evaluate performance of existing
antennas
• Better understanding of antenna
performance
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Software
• Most are based on NEC-2 core
- Public domain (NEC-4 is proprietary)
- MiniNEC (trimmed down version)
• Numerical Electromagnetics Code
- Uses “card based” input
- Not user friendly at all!
• Variety of packages that add a user interface
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NEC-2 Input
CM 3-el 52-MHz 0.5" el Yagi
CE
GW 1 31 0 0 181.5914 0 0 298.4086 0.25
GW 2 31 40.692 0 185.9483 40.692 0 294.0517 0.25
GW 3 31 73.4506 0 191.9431 73.4506 0 288.0569 0.25
GS 0 0 .02540
GE 1
GN 2 0 0 0 13.0 0.005
EX 0 2 16 0 1 0
LD 5 1 1 31 3.0769E7
LD 5 2 1 31 3.0769E7
LD 5 3 1 31 3.0769E7
FR 0 1 0 0 52 1
RP 0 1 360 1000 81 1 1 1
RP 0 181 1 1000 -90 0 1 1
EN
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Software
• EZNEC $99, 20 segment free demo version (eznec.com)
• 4nec2 Free program for NEC-2 or NEC-4 (license!) (http://www.qsl.net/4nec2/)
• MMANA (based on MININEC) (http://www.smeter.net/antennas/mmana.php)
• NEC2GO $39.85, Free demo available (http://www.nec2go.com/)
• NEC-Win Plus, apparently discontinued
• MultiNEC Excel spreadsheet based, discontinued
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More Information
• ARRL information, links and articles (http://www.arrl.org/antenna-modeling)
• Unofficial NEC Archives, info, downloads, etc. (http://www.si-list.net/swindex.html)
• L. B. Cebik, W4RNL (SK) Website, lots of information (http://www.cebik.com/)
• Dan Maguire, AC6LA, Website, software, utility programs (http://www.ac6la.com/)
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What is an Antenna Model?
• Antenna Geometry
- Wires
- Oriented in space, divided into segments
- Diameter, material properties
• Environment
- Source, loads
- Frequency
- Ground
• Output
- Patterns
- Currents, Power
- Impedance, SWR
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Wires
• Includes tubing or any other conductors
• Need coordinates of all the wires
• Be accurate! NEC2 says wires are joined if ends are within
~0.001”
From ARRL Antenna Modeling Course, 2002
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Wire Guidelines
• Conventions
1. Keep elements equal on a centerline if
possible
2. Use Y as horizontal linear axis (elements
parallel to Y)
3. Use X for front to back dimensions
4. Use Z for height above base of antenna.
Adjust for ground later, if needed.
5. Number wires in a loop continuously
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Wire Guidelines
• Segmentation
1. Always join wires at ends – don’t cross
2. Use at least 10 segments per half wave at
highest frequency
3. Use segment length-to-diameter ratio of at
least 4:1
4. Make segment lengths as equal as possible
5. Align segments as close as possible for
parallel wires
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Conductivity and Resistivity of Common
Materials Used in Antenna Construction
Material ResistivityOhms/meter
ConductivitySiemens/meter
Pure Silver 1.59E-08 6.2893E7
Copper 1.7241E-08 5.8001E7
Pure aluminum 2.655E-08 3.7665E7
6063-T832 Aluminum alloy 3.25E-08 3.0769E7
6061-T6 Aluminum alloy 4.099E-08 2.4938E7
Yellow brass (35% zinc) 6.4E-08 1.5625E7
Phosphor bronze (5% tin) 1.1E-07 9.0909E6
Stainless steel type 302 7.1999E-07 1.3889E6
From ARRL Antenna Modeling Course, 2002
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Common AWG Wire Gauges and Associated
Diameters in Inches and in MillimetersAWG
Gauge #
Diameter
Inches
Diameter
Millimeters
AWG
Gauge #
Diameter
Inches
Diameter
Millimeters
1 0.2893 7.348 21 0.0285 0.723
2 0.2576 6.544 22 0.0253 0.644
3 0.2294 5.827 23 0.0226 0.573
4 0.2043 5.189 24 0.0201 0.511
5 0.1819 4.621 25 0.0179 0.455
6 0.1620 4.115 26 0.0159 0.405
7 0.1443 3.665 27 0.0142 0.361
8 0.1285 3.264 28 0.0126 0.321
9 0.1144 2.906 29 0.0113 0.286
10 0.1019 2.588 30 0.0100 0.255
11 0.0907 2.305 31 0.0089 0.227
12 0.0808 2.053 32 0.0080 0.202
13 0.0720 1.828 33 0.0071 0.180
14 0.0641 1.628 34 0.0063 0.160
15 0.0571 1.450 35 0.0056 0.143
16 0.0508 1.291 36 0.0050 0.127
17 0.0453 1.150 37 0.0045 0.113
18 0.0403 1.024 38 0.0040 0.101
19 0.0359 0.912 39 0.0035 0.090
20 0.0320 0.812 40 0.0031 0.080
Firom ARRL Antenna Modeling Course, 2002
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Example Wires
From ARRL Antenna Modeling Course, 2002
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Sources
• NEC-2 uses only voltage sources
• Software implements current sources, as well
• Usually current sources are required to
characterize driven arrays and phased element
systems
From ARRL Antenna Modeling Course, 2002
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Sources
• Can use split sources to feed elements in a corner
• Can use parallel sources on parallel elements
From ARRL Antenna Modeling Course, 2002
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Loads
• Represent lumped components
- Loading coils, capacitors
- Traps
• Usually can specify R, L and C values
• Idealized
- No radiation!
• Similar placements considerations as with sources
From ARRL Antenna Modeling Course, 2002
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Transmission Lines
• Transmission lines can dramatically affect losses and impedances
seen at the transmitter/receiver
• Can also use transmission lines for matching and filtering
• Could actually model wires, but normally use a special TL utility in
NEC2
From ARRL Antenna Modeling Course, 2002
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Grounds
• Cause reflection and attenuation of signals
- Performance over a ground may be much different
than in free space
- Performance may vary depending on ground quality
- Performance may vary depending on height above
ground
• Wires in NEC2 cannot penetrate the ground
• Keep wires several radii above the ground!
• For wires close to ground, the Sommerfeld-Norton
system should be used in software
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Conductivity and Permittivity of Common
Ground Conditions
Soil Description Conductivity
S/m
Permittivity
(Dielectric
Constant)
Relative
Quality
Fresh water 0.001 80
Salt water 5.0 81
Pastoral, low hills, rich soil, typical from Dallas, TX, to Lincoln, NE 0.0303 20 Very Good
Pastoral, low hills, rich soil, typical of OH and IL 0.01 14 Good
Flat country, marshy, densely wooded, typical of LA near the Mississippi River 0.0075 12
Pastoral, medium hills, and forestation, typical of MD, PA, NY (exclusive of mountains
and coastline)
0.006 13
Pastoral, medium hills, and forestation, heavy clay soils, typical of central VA 0.005 13 Average
Rocky soil, steep hills, typically mountainous 0.002 12-14 Poor
Sandy, dry, flat, coastal 0.002 10
Cities, industrial areas 0.001 5 Very Poor
Cities, heavy industrial areas, high buildings 0.001 3 Extremely Poor
From ARRL Antenna Modeling Course, 2002
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Grounds
• Dipole performance over 2
types of ground
From ARRL Antenna Modeling Course, 2002
• Vertical performance over
2 types of ground
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Output
• Far field radiation patterns
From ARRL Antenna Modeling Course, 2002
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Output
• Far field, ground wave patterns
From ARRL Antenna Modeling Course, 2002
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Output
• Near field analysis
- Required for radiation safety determination
From ARRL Antenna Modeling Course, 2002
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Output
• Tabulated data
- Current in wire segments
- Source data, power budget
From ARRL Antenna Modeling Course, 2002
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Output
• Frequency sweeps
• Current visualization
From ARRL Antenna Modeling Course, 2002
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Adequacy Testing
• We built a model, how do we know if it is correct?
• Most software has built-in guidelines and warnings
- First review and understand all the warnings!
• Convergence testing
- Mathematical convergence as model becomes finer scaled
- Increase segments in model and see if answers are same
• Average gain testing
- Based on conservation of energy
- Set elements to lossless, average gain should be 1.0
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Effect of Shortened Radials on Vertical
• What happens if you don’t have room for full 80m vertical? Can the
radial lengths be changed?
0
1
2
3
4
5
0
5
10
15
20
25
30
35
40
0 20 40 60 80
Reacta
nce (
oh
ms) T
ho
usan
ds
Resis
tan
ce (
oh
m)
Element Length (ft)
Effect of Element Lengths
(Vertical & Radials Equal)
Resistance
0
5
10
15
20
25
30
35
40
45
-20
-15
-10
-5
0
0 20 40 60 80
Take o
ff A
ng
le (
deg
)
Gain
(d
Bi)
Element Length (ft)
Effect of Element Length
(Vertical & Radials Equal)
Gain
0
1
2
3
4
5
0
5
10
15
20
25
30
35
40
0 20 40 60 80
Reacta
nce (
oh
ms) T
ho
usan
ds
Resis
tan
ce (
oh
m)
Radial Length (ft)
Effect of Radial Length
Resistance
0
5
10
15
20
25
30
35
40
45
-20
-15
-10
-5
0
0 20 40 60 80
Take o
ff A
ng
le (
deg
)
Gain
(d
Bi)
Radial Length (ft)
Effect of Radial Length
Gain
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Meander Dipole
• Why does it appear to work?
- To shorten a dipole, meander the wires along a balcony
- Published in RSGB, studied by US military, cell phones
- Hint: Experiments showed resonant wire length increased with
more meanders
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Meander Dipole
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Spiral Radials
• Can we make a small footprint antenna with a vertical
element by spiraling the radials in a small space?
• Built model for 2m antenna. Checked scaling to HF.
Top View
-6
-4
-2
0
2
4
6
-6 -4 -2 0 2 4 6
Front View
-5
0
5
10
15
20
25
30
-15 -10 -5 0 5 10 15
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Spiral Radials
• Resonant length and impedance varies, but not too bad
• Most radiation is from the vertical element
- Acts like a normal ground plane vertical
• High currents in radials, use larger wires
- Due to spiraling, induced currents, radiation cancels
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My Maracaibo Indoor Vertical
• When operating as YV1/W5ALT I was limited to indoor
antenna. I built a vertical and needed to determine how
it was working and where it could be improved.
Freq (MHz) H R X SWR(50) Max Gain @ El °
50.100 100 330.20 390.60 15.934 7.02 177
28.100 100 1459.00 -978.20 42.307 7.40 52
24.900 100 94.80 241.70 14.680 7.44 136
21.100 100 23.86 -145.20 20.196 7.03 127
18.100 100 34.84 -394.90 91.642 6.31 110
14.100 100 1489.00 -1290.00 52.146 6.22 118
10.100 100 43.55 -220.90 24.388 4.93 130
7.100 100 10.66 -658.10 817.465 7.67 91
3.600 100 2.30 -1596.00 999.000 4.79 120
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My Maracaibo Indoor Vertical
• Added capacitance hat
- Much better to tune
- Placed auto-tuner at base of antenna
Freq (MHz) H R X SWR(50) Max Gain @ El °
50.100 100 100.22 184.62 9.197 6.76 3
28.100 100 425.34 348.52 14.266 8.47 128
24.900 100 50.71 18.26 1.435 7.69 136
21.100 100 23.56 -206.02 38.594 7.42 127
18.100 100 43.66 -151.53 12.455 7.82 8
14.100 100 9.65 -379.78 304.435 8.03 170
10.100 100 208.52 -907.36 83.364 4.76 48
7.100 100 9.07 -774.77 999.000 7.35 91
3.600 100 1.19 -1723.81 999.000 4.42 121
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Thank You