Introduction to Antenna Modeling Antenna analysis using the
- Slides: 30
Introduction to Antenna Modeling Antenna analysis using the computer. Don Steinbach AE 6 PM Santa Clara County Amateur Radio Association November 8, 2010 AE 6 PM Intro_to_Ant_Modeling_R 1 1
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 AE 6 PM 2
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 AE 6 PM 3
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. AE 6 PM 4
Software Available • EZNEC-ARRL (Included with the ARRL Antenna Book @ $39. 95) • Antenna Model ($85)* • EZNEC v 5. 0 ($89)* • NEC-Win Plus ($150)* • NEC-Win Pro ($425)* • EZNEC-M Pro ($450)* • EZNEC/4 ($600, must have license)* • GNEC ($795)* AE 6 PM *Ref: The ARRL Antenna Book, 20 th Edition, page 4 -2. “Commercial Implementations of MININEC and NEC-2 Programs. ” 5
EZNEC-ARRL • A version of EZNEC 3. 0 that’s included on the Antenna Book CD-ROM – Provided by Roy Lewallen, W 7 EL • Works with the specific antenna models that are also bundled on the CD-ROM – There 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 AE 6 PM 6
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 AE 6 PM 7
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 AE 6 PM 8
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 AE 6 PM 9
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 AE 6 PM 10
Demonstration Launch EZNEC. The Control Center window appears. All I/O is accomplished from this screen. AE 6 PM 11
Demonstration Click on the title bar and enter a new name. AE 6 PM 12
Demonstration Change the frequency to 14 MHz. Note that the wavelength changed as well. The program did this. AE 6 PM 13
Demonstration Change units to feet (was meters). AE 6 PM 14
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. AE 6 PM 15
Demonstration Define the source. It’s in the middle of the wire. AE 6 PM 16
Demonstration Select the ground type. Was “Free Space”. AE 6 PM 17
Demonstration Select the Ground Characteristics. AE 6 PM 18
Demonstration This completes the creation of the model. Model inputs AE 6 PM 19
Demonstration View the antenna. Rotate, zoom, etc. AE 6 PM 20
Demonstration Select the Plot Type output. AE 6 PM 21
Demonstration Select the Far-Field plot output. Cursor position Note that the gain is in d. Bi. Subtract 2. 15 d. B to convert it to d. Bd. 22 AE 6 PM
Demonstration But I want to know what the azimuth plot looks like at 15 degrees elevation: AE 6 PM 23
Demonstration Select the Source Data output. Negative reactive part indicates that the antenna is too short for 14 MHz (is operating below resonance). AE 6 PM 24
Demonstration SWR plot shows that lowest SWR is at 14. 45 MHz Cursor AE 6 PM 25
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? AE 6 PM 26
Real-Life Application Note description of each of seven wires. AE 6 PM 27
Real-Life Application View the antenna. AE 6 PM 28
Real-Life Application Run SWR plot (7 to 35 m. Hz by 0. 2 m. Hz). Wire tips spaced 4” AE 6 PM Wire tips spaced 18” 29
The End AE 6 PM 30
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