1. Introduction to Antenna Modeling
Antenna analysis using the computer.
Don Steinbach
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2. What is Modeling?
Using a computer program specifically designed to predict performance
Not perfect
Cost effective
Much cheaper than building hardware
Instant answers
Can give misleading results
Garbage in, garage out
Tool widely used by engineers
Spacecraft thermal
Weather
Propagation
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3. Why Antenna Modeling?
Predict antenna performance and electromagnetic parameters
Gain
Far-field radiation pattern in azimuth and elevation
Impedance at the feed-point
Current distribution in the elements
Near-field E and H field intensity
Evaluate effect of changes in configuration
Antenna
Conductor size & type
Elevation (height above ground)
Physical environment
Local ground characteristics
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4. Why Antenna Modeling? Low cost/instant answers Repeatable results
No wire to buy, no beams to build
No towers to erect
All paper, no hardware
Repeatable results
Propagation not a factor
Independent of weather, sunspots, etc.
The average ham can’t measure anything about antenna performance except the swr and feedpoint impedance.
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5. Software Available
EZNEC-ARRL (Included with the ARRL Antenna $39.95)
Antenna Model ($85)*
EZNEC v5.0 ($89)*
NEC-Win Plus ($150)*
NEC-Win Pro ($425)*
EZNEC-M Pro ($450)*
EZNEC/4 ($600, must have license)*
GNEC ($795)*
*Ref: The ARRL Antenna Book, 20th Edition, page 4-2. “Commercial Implementations of MININEC and NEC-2 Programs.”
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6. EZNEC-ARRL
A version of EZNEC 3.0 that’s included on the Antenna Book CD-ROM
Provided by Roy Lewallen, W7EL
Works with the specific antenna models that are also bundled on the CD-ROM
There are about 400 of them, many based on antennas in the book
They can be modified by the user
Works with your user-specified inputs as well
Can’t save the input data (Description) file
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7. EZNEC-ARRL User input is limited to a maximum of 20 segments
Typically enough for a dipole or a two-element beam
EZNEC 5.0 allows 500 segments (1500 in the + version, 20,000 in the Pro version)
Antenna Book models are not constrained to 20 segments
High fidelity analysis even with user modifications
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8. EZNEC-ARRL
Great learning tool, especially when used with existing (provided) antenna models
Many to choose from
Easy to modify
Downside is not being able to save your input data (Description) files
Not a big deal for everyone, but was for me
Data entry is time consuming and error prone
Not convenient for “what-if” or parametric analyses
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9. Typical Program Inputs
Three dimensional (x, y, z) description of each “wire”
Number of segments in each wire
At least 10 per half-wavelength
Conductor type and size
Placement and type of the driving source
Frequency
Ground/soil characteristics
Loads/loading coils
Transmission lines, transformers, networks
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10. Typical Program Outputs
Source (driving point) impedance
Power gain
SWR graph
Far-field azimuth and elevation plane patterns
Polarization
RF current distribution
Rotatable, zoomable 3-D views of the model
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11. Demonstration
Launch EZNEC. The Control Center window appears. All I/O is accomplished from this screen.
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12. Demonstration
Click on the title bar and enter a new name.
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13. Demonstration Change the frequency to 14 MHz.
Note that the wavelength changed as well. The program did this.
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14. Demonstration
Change units to feet (was meters).
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15. Demonstration Specify the wire x, y, z coordinates and size.
X is the direction I’m looking
Y is to my left and right
Z is up and down
XYZ are mutually orthogonal
One wire, 30’ high and 33.43’ long.
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16. Demonstration
Define the source. It’s in the middle of the wire.
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17. Demonstration
Select the ground type. Was “Free Space”.
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18. Demonstration
Select the Ground Characteristics.
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19. Demonstration This completes the creation of the model. Model inputs
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20. Demonstration
View the antenna. Rotate, zoom, etc.
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21. Demonstration
Select the Plot Type output.
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22. Demonstration Select the Far-Field plot output. Cursor position
Note that the gain is in dBi. Subtract 2.15 dB to convert it to dBd.
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23. Demonstration
But I want to know what the azimuth plot looks like at 15 degrees elevation:
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24. Demonstration Select the Source Data output.
Negative reactive part indicates that the antenna is too short for 14 MHz (is operating below resonance).
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25. Demonstration SWR plot shows that lowest SWR is at 14.45 MHz Cursor
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26. Real-Life Application
Given:
Fan-dipole antenna for 40/20/10 meter bands.
Inverted V, center 30’ high. End attach points 8’-6” high, 28’-6” and 19’-6” from center support.
Wanted:
What’s the effect of rotating the elements downward?
How long do the elements need to be compared to a single horizontal dipole?
What’s the effect of the spacing of the wire ends from its neighbor?
How does 15 meters look?
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27. Real-Life Application
Note description of each of seven wires.
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28. Real-Life Application
View the antenna.
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29. Real-Life Application
Run SWR plot (7 to 35 mHz by 0.2 mHz).
Wire tips spaced 4”
Wire tips spaced 18”
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30. The End
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