Introduction to Antenna Modeling Antenna analysis using the computer. Don Steinbach 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 can’t measure anything about antenna performance except the swr and feedpoint impedance.

5. 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.”

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

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 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.

16. Demonstration Define the source. It’s 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: • 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?

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