Transcript Introduction to Antenna Modeling

Introduction to Antenna
Modeling
Antenna analysis using the computer.
Don Steinbach
AE6PM
Santa Clara County Amateur Radio Association
November 8, 2010
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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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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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Why Antenna Modeling?
• Low cost/instant answers
– 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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Software Available
• EZNEC-ARRL (Included with the ARRL Antenna
Book @ $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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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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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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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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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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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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Demonstration
Launch EZNEC. The Control Center window
appears. All I/O is accomplished from this
screen.
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Demonstration
Click on the title bar and enter a new name.
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Demonstration
Change the frequency to 14 MHz.
Note that the
wavelength
changed as
well. The
program did
this.
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Demonstration
Change units to feet (was meters).
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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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Demonstration
Define the source. It’s in the middle of the wire.
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Demonstration
Select the ground type. Was “Free Space”.
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Demonstration
Select the Ground Characteristics.
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Demonstration
This completes the creation of the model.
Model
inputs
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Demonstration
View the antenna. Rotate, zoom, etc.
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Demonstration
Select the Plot Type output.
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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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Demonstration
But I want to know what the azimuth plot looks
like at 15 degrees elevation:
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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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Demonstration
SWR plot shows that lowest SWR is at 14.45
MHz
Cursor
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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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Real-Life Application
Note description of each of seven wires.
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Real-Life Application
View the antenna.
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Real-Life Application
Run SWR plot (7 to 35 mHz by 0.2 mHz).
Wire tips spaced 4”
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Wire tips spaced 18”
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The End
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