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This antenna project  consists of designing antennas using 4nec2 antenna simulator as a design aid and the construction/testing of a 10m dipole, a 50 Ohm 1:1 Balun and a 50 Ohm Line Isolator. The antenna design phase of the project is based on using 4nec2 antenna simulator using Microsoft Windows. There are many simulators that are free and most of them are based on a very old program that uses Fortran (NEC from the Lawrence Livermore Lab). Download the file from the 4nec2 link above and install it on your laptop computer. If you do not have a reasonable laptop computer, don’t worry as you will paired with a group member who can help you.  The process that we use is to start with a simple example dipole from the 4nec2 library. This example is very simple but does not contain many of the features that we will need. So, we edit the file and save it in folder that you probably name as your call sign where you will store all of your work.  The editing will consist of : 1. Changing the frequency, antenna length, and wire radius 2. Add a dielectric coating for the antenna wire and wire loss 3. Increase the antenna height 4. Add a ground and finally invoke the optimizer to resonate the antenna at your chosen frequency (CW or phone band)               Let’s start by creating a very simple 28 MHz dipole: lossless, free space, no droop, 50 Ohm environment. To do this, download, install and open 4nec2 if you have not already done this. You will see some open windows but at this stage focus on the main window. The first time you open 4nec2, a sample 300 MHz file is open. This can be used  as a basis for all of the antennas you will need.  In the main window click on settings and select  the nec editor (new) that we will be using to design a suite of dipoles.  Set the characteristic impedance to 50 Ohm and the input power to 100 W. The characteristic impedance tells the simulator that the antenna is connected to a transmission line of characteristic impedance Zo: in this case Zo = 50 Ohm. Also, set the NEC  engine to Nec2dxs*.exe You should see the following Main window on your screen. Notice that the antenna efficiency is 100% as expected from a lossless antenna. This will change as we add copper loss in the wires.
Now let’s use this file to see what this antenna does.  In the Main Window, click the Window button and select Geometry.  A window pops up which shows the geometry of the antenna. In this case there is a single wire located along the horizontal Y axis which be thought of a North- South orientation with the X axis being East-West. The vertical axis is the Z axis. In this example, the antenna is located with Z = 0 and there is no ground (free space).  This window is always used to check for errors in your file.
Now, calculate the far field gain of the antenna using the Calculate button in the main window. Follow the instructions buy using the NEC-output button. The F9 key will show you a 3D plot of the antenna pattern which is extremely useful for visualizing the directivity of the antenna.   A 3D plot of the sample dipole is shown below.  Since the dipole is oriented is along the Y axis , the antenna has its main radiation pattern oriented along the X axis.  This makes sense, so it looks like the files are working OK. NOW notice that maximum  gain of this dipole is equal to 2.12 dBi and that all calculations used in 4NEC2  are relative to a fictitious isotropic antenna that radiates equally in all directions. The subscript “i”  in dBi means isotropic.  
Calculating the frequency response of the antenna is similar to all other calculations using the Main window Calculate button and the Frequency Sweep  button. Once the frequency sweep is completed, you can view the SWR/Reflection Coefficient or the Impedance.  The graphs below show both. Notice that the antenna is tuned quite well with  low SWR of 1.44 at 299 MHz. The SWR is not 1.0 because the antenna impedance is 70 Ohm at 300 MHz as expected for an ideal dipole in free space.
Now, let’s edit the antenna file using the NEC Editor (New) so it is tuned to 28 MHz and includes wires with dielectric coating, losses and a ground plane.   To do this we will use symbols such as L for the antenna length to make the simulation much easier and to enable the optimizer which will speed up the antenna design.  Before attempting to edit the antenna data, check to see if you have the correct editor chosen from the settings in the Main window. Use NEC Editor (new). Now select Edit from the Main window and select edit input file.  This is where all of the antenna data is stored. The editor window appears (see below) with the antenna geometry highlighted.  The geometry tab describes the location of the wire ends , the number of segments, the wire radius and a field for comments.  Notice that in this example all of the data is given as numbers (no symbols used).  Imagine if you wanted to model a 20 element Yagi with many wires and wire spacings and you wanted to make a change!  The numbers would have to be inserted manually into the editor  which would quickly escalate into a major problem.  If you wanted to make the antenna work on another band, all of the numbers must be changed.  So, we will use SYMBOLS to overcome this problem and let the program do all of the calculations.
Now open up the Symbols tab  and assign the symbols. Set Freq = 28, L=5, Rad = 0.645 mm (#16 gage) and Segs = 21 where Segs defines how many antenna segments to use for the analysis.  I usually use 21 segments for each halfwave length of wire. Also, place the antenna 5.355 metres above ground as shown.  Set the radius of the wire coating 2.25/2 mm. (The wire outer diameter including the insulation is 2.25 mm so the wire radius including the coating in metres is 2.25/2/1000.) We have not yet told the simulator where to add the transmission line. This is done with a voltage or current variable. We will use a voltage source and place it in the center of the antenna at the center segment.  For an antenna using 21 segments, the center is located at segment 11.  Center segment in general = (nr. of segments +1)/2 for an odd number of segments. We let the simulator do this calculation for us.  Also, we have not added copper loss to the wires yet. Open the Source/Load window to do this.  In the Source line set the source type as a Voltage of value 1.0 placed at         Seg = (1+segs+1)/2 as shown. Also, set the wire conductivity to copper for all segments. Use the drop down list to select Wire-Conductivity
Using the Optimizer to tune the antenna to 28 MHz. Here we will tune the length of the antenna to resonate exactly at 28 MHz. Resonance means the impedance will be purely resistive with no capacitive or inductive component.  To do this, we use the built-in optimizer in 4nec2.  Optimizers must be used with a great deal of care and can sometimes diverge from the expected results like tuning the antenna to a harmonic of the operating frequency where it is still resonant. To do this, save and close the editor, then open the optimizer from the  Main window using the calculate tab. The optimizer is shown below after I have used it to adjust the antenna length.  In this case the starting length was 5.0 metres and the final length was 5.04 metres.  So the antenna was tuned quite well before using the optimizer.  The parameter to be optimized is L, the antenna length and the function to be tuned is Xin, the reactance which we want to be zero (resonance) at 28 MHz.  The optimizer output shown below after several iterations shows that the input impedance is 65.7 -j0.038 Ohm as desired.
Now open the Geometry tab in the editor and insert the symbols into the boxes instead of numbers.  “Tag” refers to a wire, only one in this example.  All of the other variables are defined in the Symbols dialog window.  The Geometry window defines the antenna wire placement. X1,Y1 and Z1 define the placement of one end of the wire while X2,Y2,Z2 define the other end.  This part is now complete.
Now set the frequency and the type of ground using the Frequency/Ground tab in editor window. The frequency will be 28 MHz which is set by the Freq parameter in the Symbols list. For this antenna set the ground type to average loss and dielectric type. Set the Frequency to Freq, the Environment to Real Ground and the Main ground using drop down lists.
Now it is time to test the antenna to see if you have any errors or omissions.  Compute the far field from the Main window and display it in three dimension (F9) hot key.  You should see the Window below. This window indicates that the frequency is 28 MHz and that the antenna is mounted above ground. It also shows the 21 segment antenna mounted well above ground with 21 segments loaded with copper loss. If you look closely (use the down arrow on your keyboard to enlarge the picture) there is a voltage source connected to the center of the antenna as required.  The maximum gain is 7.17 dBi.