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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