TBARC Programs Antenna Modeling with 4NEC2. By Randy Rogers AD7ZU 2010

Transcription

1 TBARC Programs Antenna Modeling with 4NEC2 By Randy Rogers AD7ZU 2010

2 Getting Started 4NEC2 is a completely free windows based tool suite to aid in the design and optimization of antenna systems 4NEC2 is an exceptional value for amateur antenna system designers! Download and install 4NEC2 Default and the 4NEC2 3D extension from: Bookmark the page to access additional documentation and the 4NEC2 forum The NEC2 Forum contains vast information on antenna modeling with 4NEC2 2

3 4NEC2 3

4 4NEC2 4

5 4NEC2 Settings 1. 4NEC2 Main Window is used to configure default settings 2. Launch 4NEC2x (4NEC2x is the 3D extension) 3. Select the Settings pull down and select the following options: Default Editor: NEC editor (new) Input Power: 100 Char-Impedance: 52 Phi / Azim Unit: Azim / Elev Length Unit: Feet Radius Unit: In / AWG 5

6 Modeling Basics Think in 3 Dimensions! 4NEC2 Models use a 3 dimensional coordinate system to define antenna structures The example models use the following coordinate system: X Axis (horizontal) positive = 0 deg Y Axis (horizontal) positive = 90 deg Z Axis (vertical) positive = height above ground Y axis The driven element structure lies along the Y axis in this presentation. 6

7 4NEC2 Coordinate System 7

8 Segmentation The NEC2 engine computes currents in a series of small segments then then integrates each to compute the total response Accurate models require a small segment size 51 segments / half wavelength is sufficient (~.02 wavelength / segment) Use an odd number of segments in each element if possible Driven element models DO NOT contain feed point gaps The feed point gap is assumed infinitely small in NEC2 An insulator is required when constructing the real element. All modeled elements require at least 1 segment Even if the dimensions are smaller than.02 wavelength Example: folded dipole end wires 8

9 Sources, Loads and Grounds Sources and Loads are placed along the driven element(s) Sources provide excitation to compute currents in the modeled elements Sources and Loads are positioned by segment number The element material (copper, aluminum, etc.) and dimensions are modeled as Loads Best results are obtained using Sommerfeld Ground Models 4NEC2 Real Ground is the most accurate ground model Arizona soil is not sea water! Your backyard is not free space! 9

10 Symbols and Expressions 4NEC2 Optimizer requires Symbols and Expressions 1. Start with the known dimensions: Height = 52 Remember we are working in units of feet Length = 135 First guess we ll let the optimizer do the real work later 2. Next we need the X, Y, and Z coordinates for end 1: wire_1_z1 = Height wire_1_x1 = 0 Remember the element is along the Y axis wire_1_y1 = Length / 2 Half of the length in the positive Y axis 3. Now for end 2 of the wire: wire_1_z2 = Height Height: are both ends at the same height? wire_1_x2 = 0 The element is along the Y axis so x doesn t change wire_1_y2 = -1* Length / 2 Half of the length in the negative Y axis 4. Now we have a wire defined in 3 dimensions: 6 coordinates are required to define a line in 3 dimensional space 10

11 Now A Real Example You have just completed your Ft. Tuthill80 QRP Transceiver and now you need a suitable antenna The Ft Tuthill80 as built by WA4MNT Order yours here: 11

12 Modeling an 80m Dipole using 4NEC2 Steps to Model and Evaluate the 80m Dipole 1. Model the Structure Using 4NEC2 Editor (new) 2. Model Source, Load, and Ground Data 3. Check Geometry and Segmentation using 4NEC2 4. Evaluate Model performance 5. Optimize the Performance Using the 4NEC2 Optimizer 12

13 Build A Model Using 4NEC2 Editor (new) 1. Select EDIT on the 4NEC2 Main window then select Input (.nec) file 2. The 4NEC2 Edit Window should now be displayed Note: the 4NEC2 (new) editor runs in a separate window we are now using the 4NEC2 Edit Window NOT the Main Window 3. Initialize a New.NEC file for this project: Select File then New to begin editing a new model In the Scaling checkbox at the bottom of the 4NEC2 Editor screen select Feet Select File then Save As TBARC_80m.NEC to name the new project 13

14 Enter the Model Symbols and Expressions 1. Select the Symbols tab in the 4NEC2 Edit Window In our example we have arbitrarily selected 52 feet as the height above ground of the TBARC 80m dipole But! Accurate Models require accurate measurements! So MEASURE before modeling! in this case height matters 2. In the symbols and equations column enter: height = 52 Now lets experiment a bit and later compare the 4NEC2 results with the text book formula The formula for the length in feet of a ½ wave dipole in free space = 468 / frequency in Mhz We wish to optimize the performance of the antenna at the 80m QRP CW calling frequency Mhz Our estimate is 468 / = ft. 3. Select the cell just below the height entry and enter length = Enter symbols for each of the wire end coordinates: wire_1_x1 = 0 wire_1_x2 = 0 wire_1_y1 = length / 2 wire_1_y2 = -1 * length / 2 wire_1_z1 = height wire_1_z2 = height 5. Save the.nec file by selecting File then Save as TBARC_80m.NEC Hint: The 4NEC2 editor rows may be copied to and from an Excel spreadsheet 14

15 Model the Geometry 1. Select the Geometry tab in the 4NEC2 Edit Window We are now defining the 6 coordinates required to define a line in 3 dimensions. The dipole will be modeled as a single wire but constructed with a center insulator 2. Select the cell in the first row under Type a pull down will appear select Wire We are constructing the dipole of #14 wire. 3. Select the cell in the first row under Tag enter 1 4NEC2 references wires by tag number each wire requires a unique tag number 4. Select the cell in the first row under Segs enter 51 We are using 51 segments per ½ wave. 5. Select and enter the coordinates of the wire ends x1, y1, z1 and x2, y2, z2 in the geometry cells Now we must enter all the coordinate symbols for each of the wire end coordinates Cell Symbol X1 wire_1_x1 Y1 wire_1_y1 Z1 wire_1_z1 X2 wire_1_x2 Y2 wire_1_y2 Z2 wire_1_z2 6. Select Radius then enter #14 #14 is a predefined wire gauge symbol in 4NEC2 designating #14 wire 15

16 Model Sources and Loads 1. Select the Source / Load tab in the 4NEC2 Edit Window 2. Verify both Show Source and Show Loads are checked if not check both entries 3. In the Source(s) section select Type then select Voltage-src 4. In the Source(s) section select Tag then enter 1 5. In the Source(s) section select Seg then enter 50% 6. In the Source(s) section select Real then enter 1 7. In the Loads(s) section select the first cell then select wire conductor 8. In the Loads(s) section select Tag-nr then enter 1 9. In the Loads(s) section select First-seg then enter In the Loads(s) section select Last-seg then enter In the Loads(s) section select Cond(S) then select Copper 12. Save the TBARC_80m.NEC file 16

17 Model Frequency and Ground 1. Select the Freq./Ground tab in the 4NEC2 Edit Window 2. Enter in the Frequency cell 3. Select Real Ground in the Environment pull down 4. Select Rocky, steep hills in the Main ground unless you have a flat irrigated yard 5. Save the TBARC_80M.NEC input file 6. Then close the 4nec2 Editor We are at last finished editing model input data! 17

18 Geometry and Segmentation Check 4NEC2 Geometry and Segmentation checking can identify many common modeling errors 1. Checking Geometry and Segmentation Select Window from the 4NEC2 Main Window then select Geometry The Geometry Window should be displayed Verify the model displayed in the Geometry window looks like what you intended 2. Check the Geometry Select Validate in the Geometry Window then select Run Geometry Check 3. Check the Segmentation Select Validate in the Geometry Window then select Run Segment Checks 4. If there are errors then Go Back to the Geometry Editor and make corrections! 18

19 Evaluate Model performance Calculate the NEC Output Data Lets see the far field pattern in the horizontal (X Y) plane: Select Calculate from the 4NEC2 Main Window then select NEC output-data The Generate dialog is now displayed Select Far Field pattern Select Hor to calculate the horizontal plane data Set the resol to 5 degrees and The to 0 degrees Select Generate The Pattern window should now be displayed The Horizontal Pattern is displayed by selecting Show then selecting Next pattern until the Hor-gain [dbi] is displayed in the pattern window It is helpful to display the structure in the pattern window to see the pattern in relation to the antenna. To display the structure: 19

20 TBARC 80m Dipole Horizontal Pattern As expected The figure 8 pattern broadside to the dipole structure 20

21 TBARC 80m Dipole Vertical Pattern Vertical Pattern The vertical pattern is calculated by selecting Ver on the generate dialog then generating the NEC output data Most of the energy is radiated at an angle higher than 45 degrees due to the relatively low height. This is ok for the near vertical propagation on 80m 21

22 Bandwidth and SWR Calculate the SWR over the full 80m band Remember we specified the length using the text book formula SWR and Reflected Coef. The resonant point is at 3.60 Mhz the design frequency is 3.56 The feed point impedance is 73.6 j28.7 (its too short!) The SWR is 1.77 at the design frequency. The 2:1 SWR bandwidth is Approx 130Khz This is close but we can do better 22

23 Optimizing the Performance Lets Use the 4NEC2 Optimizer to improve the performance at 3.56 Mhz Select Calculate then Start Optimizer from the 4NEC2 Main Window The 4NEC2 Optimizer Window should now be displayed We wish to optimize the antenna length to resonance at 3.56 Mhz -- In the variables window select length Note that the length variable is now in the selected window At resonance the reactance either capacitive or inductive will be minimum -- We also wish to minimize the SWR and reflected coefficient Click in the SWR box to set the optimization weighting to 100 for SWR Click in the X-in box to set the optimization weighting to 100 for X-in (input reactance) Set the Frequency to 3.56 Mhz Select Start to start the optimizer, note the optimizer outputs as it attempts to find the optimal length value minimizing BOTH the SWR and input reactance When the optimizer is finished select Update NEC file this updates the NEC input file with new values We can then run the calculations again to compare with the first attempt using the text book formula 23

24 The Results SWR = :1 bandwidth ~ 120khz Impedance 76.6 j1.19 The text book formula is close but this looks better! 24

25 What s Wrong with the TBARC 80m Dipole? A Dipole is a Balanced Antenna, Coax is NOT a balanced Line The usable bandwidth is limited to 130 Khz How can we fix it? Add a Balun to balance the line at the feedpoint We need a weather proof toroidal balun Or a lot of coax hanging to make a quarter wave section Or a lot of ferrite beads to choke off the common mode currents The usable bandwidth is limited to 130 Khz Hmmmm not much we can do about that! 25

26 The TBARC 80m Folded Dipole A Better Idea? Characteristic Impedance is 280 ohms 2:1 SWR bandwidth is ~ 300 Khz 2x that of a single wire dipole Feed with 280 ohm #18 window line which has far lower loss than even the best coax Impedance matching inside out of the weather using either a balanced line antenna tuner or a 4:1 balun 26

27 3.5 Mhz to 3.8 Mhz SWR 80m Folded Dipole 27

28 Useful Hints 4NEC2 Models are only models Model accuracy is very good, however other factors can affect your results: Nearby Utility Lines Nearby Rain Gutters Accurate Measurements Measure to establish all model reference points Wire Stretch Use quality copper clad low stretch wire Check the specifications to determine the stretch 4NEC2 models assume uninsulated conductors Wire insulation reduces the velocity factor resulting in an element being cut too long Avoid using conductive supports Half wave driven elements have the highest RF voltages at the ends of the element that couple into supporting conductive structures Low stretch dacron rope or cord works much better Measure SWR and adjust AFTER building the model for the best results A good approach is to initially cut 5% long, then trim 28

29 Useful Links and References The QST 4 part series on basic antennal modeling by L. B. Cebik, W4RNL (SK) nec_part1.pdf nec_part2.pdf nec_part3.pdf nec_part4.pdf EZNEC by W7EL Nittany Scientific Nec Win 4NEC2 Example Models The 4NEC2 software includes many antenna example models which may be adapted or used directly. The ARRL Antenna Book has extensive information on modeling using NEC based software 29

30 Thank You! 30