AE6PMIntro_to_Ant_Modeling_R1

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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.

One wire, 30’ high and 33.43’ long.

X is the direction I’m lookingY is to my left and rightZ is up and downXYZ are mutually orthogonal

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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” Wire tips spaced 18”

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The End