AE6PM

1

• Introduction to Antenna Modeling
• Antenna analysis using the computer.
• Don Steinbach
• AE6PM
• Santa Clara County Amateur Radio Association
• November 8, 2010

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

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

4
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 cant measure anything about
antenna performance except the swr and feedpoint
impedance.
5
Software Available

• EZNEC-ARRL (Included with the ARRL Antenna Book _at_
  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.
6
EZNEC-ARRL

• A version of EZNEC 3.0 thats 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
• Cant save the input data (Description) file

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

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

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

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

11
Demonstration

• Launch EZNEC. The Control Center window appears.
  All I/O is accomplished from this screen.

12
Demonstration

• Click on the title bar and enter a new name.

13
Demonstration

• Change the frequency to 14 MHz.

Note that the wavelength changed as well. The
program did this.
14
Demonstration

• Change units to feet (was meters).

15
Demonstration

• Specify the wire x, y, z coordinates and size.

X is the direction Im 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.
16
Demonstration

• Define the source. Its in the middle of the
  wire.

17
Demonstration

• Select the ground type. Was Free Space.

18
Demonstration

• Select the Ground Characteristics.

19
Demonstration

• This completes the creation of the model.

Model inputs
20
Demonstration

• View the antenna. Rotate, zoom, etc.

21
Demonstration

• Select the Plot Type output.

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.
23
Demonstration

• But I want to know what the azimuth plot looks
  like at 15 degrees elevation

24
Demonstration

• Select the Source Data output.

Negative reactive part indicates that the antenna
is too short for 14 MHz (is operating below
resonance).
25
Demonstration
SWR plot shows that lowest SWR is at 14.45 MHz
Cursor
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
• Whats the effect of rotating the elements
  downward?
• How long do the elements need to be compared to a
  single horizontal dipole?
• Whats the effect of the spacing of the wire ends
  from its neighbor?
• How does 15 meters look?

27
Real-Life Application

• Note description of each of seven wires.

28
Real-Life Application

• View the antenna.

29
Real-Life Application

• Run SWR plot (7 to 35 mHz by 0.2 mHz).

Wire tips spaced 4
Wire tips spaced 18
30

• The End