FM Transmitting Antenna

Problem 1

A five elements directional antenna to operate in the band from 87.5 MHz to 89.5 MHz will be designed. Suppose that the transmitter output impedance is 50 Ohms and that a minimum gain of 7.5 dBd is required. NEC Lab will be used to determine the best positions and lengths of the wires to meet the required goals. Assume that the five elements are initially anywhere in the plane XY.

Step 1. Create the project.

Create the project.
Select the first wire by making double click on the first wire on the left panel. The Wire dialog will be open as shown.

Step 2. Edit Wire 1.

The wire dialog allows editing both ends of the wire. For each end and for each 3D coordinate, it is possible to set:
- a numeric a value
- a variable, or
- a value that NEC Lab will optimize
For this example, Wire 1 will be placed at the origin, and only the values of Y1 and Y2 will be modified.
Click on the Drop Down box next to Y1 and set Y1 = A1.
Click on the Drop Down box next to Y2 and set Y2 = -A1.

Step 3. Set the range for A1.

NEC Lab will optimize the value of A1 when running the designer, it will however require a range to create an antenna draft. Click on the Y1 Setup button. A new dialog to edit the possible range for the variable A1 will be open. Verify that the range is as shown below. Press OK to close the A1 dialog.
Before closing the dialog, observe that there is voltage source connected to this wire, thus Wire 1 will be the active element of this antenna.
Press OK to close the Wire dialog.

Step 4. Add a second wire.

On the toolbar click: Add Wire
Set X1= -B2 and X2 = -B2
Set Y1 = A2 and Y2 = -A2

Step 5. Set the range for A2 and B2.

Press the Y1 Setup button and verify that the range of A2 is from 5.00 to 100.00 centimeters
Press the X1 Setup button and set the range of B2 from 3.00 to 100.00 centimeters

Step 6. Add wires: 3, 4 and 5.

Following the procedure previously described add three more wires as shown:

Wire 3	Range
X1= +B3	B3 from 3 to 100
X2= +B3
Y1= +A3	A3 from 5 to 100
Y2= -A3

Wire 4	Range
X1= +B4	B4 from 5 to 200
X2= +B4
Y1= +A4	A4 from 5 to 100
Y2= -A4

Wire 5	Range
X1= +B5	B5 from 5 to 200
X2= +B5
Y1= +A5	A5 from 5 to 100
Y2= -A5

Step 7. Frequency Setup.

On the toolbar click:

Set the frequency parameters as shown

Press OK to close the dialog

Step 8. Antenna Impedance Setup.

On the toolbar click: Designer Goal Impedance
Select Z = 50 Ohms
Press OK to close the dialog

Step 9. Designer Priority Setup.

On the toolbar click: Designer Priority
The VSWR is a key factor for a transmitting antenna, set the VSWR priority to its maximum value.
The antenna gain is also very important, set the gain priority to its maximum value.
Adjust the Radiation Pattern priority as shown.
Press OK to close the dialog

Step 10. Designer Goal Setup.

On the toolbar click: Designer Radiation Pattern Goal
NEC Lab defines the radiation pattern as a set of 3D beams. The radiation pattern of this antenna will be defined using only two beams: beam 1 and beam 2
On the radiation pattern goal dialog change the Antenna Gain to 7.5 dBd as shown
On the radiation pattern goal dialog change the Beam 1 Width to 10 as shown
On the same dialog, change the Normalized Gain of Beam 2: Norm Gain < -20 dB
Press OK to close the dialog

Step 11. Running the designer.

Once the antenna requirements have been defined, it is a good idea to save the project. On the toolbar click: NEC Save
Select an appropriate folder in your computer and set the filename. Press the Save button to close the dialog.
NEC Lab uses artificial intelligence and is capable of designing a great range of antennas. On the toolbar click: Run the Designer
Once the designer is running, NEC Lab will periodically report the mean-squared error measured between the actual antenna performance and the target requirements.
First, the designer will create a draft. Second, the designer will improve the design. Depending on your computer speed and the antenna complexity, the designer may run for one to several hours. NEC Lab will estimate the remaining running time. However, it may stop early, if not further improving is expected.
In some cases and depending on the antenna requirements, the designer will not be able to reach appropriately all the goals. In this case, the mean squared-error of the antenna will not decrease. Consider for example a two element antenna with a requirement of a minimum gain of 8 dBd; with only two elements the designer will not be able to attain the required gain no matter what the lengths or positions of the wires are.

Step 12. Analyzing the results.

Once the designer stops, the Optimization Results dialog will open. Review the performance antenna. When done press the Close button. You may open this dialog at any time by clicking:

If the optimizations results are not optimum, you may:

run the designer again
modify the designer goals, or
modify the antenna structure.

Note that it is normal to get different results every time the designer runs. You should try running the designer several times and select the antenna with the best results.

Step 13. Check the Radiation Pattern.

On the toolbar click: Top - Polar Radiation Pattern
Using the right button of the mouse you can export the plot or data to other applications.
You may use the rightmost scrollbar to see the radiation of pattern of this antenna at other frequencies.
Press OK to close the dialog

Step 14. Check the 3D radiation pattern.

On the toolbar click: 3D - Radiation Pattern
Using the right button of the mouse you can export the plot or data to other applications.
You may use the rightmost scrollbar to see the radiation of pattern of this antenna at other frequencies.
You may use the other scrollbars to turn and zoom the radiation pattern.
Press OK to close the dialog

FIGUFE rp_3d

Step 15. Check the antenna gain.

On the toolbar click: Antenna Gain
Using the right button of the mouse you can export the plot or data to other applications.
Press OK to close the dialog

Step 16. Check the antenna VSWR

On the toolbar click: Antenna VSWR
Using the right button of the mouse you can export the plot or data to other applications.
Press OK to close the dialog

Step 17. Check the antenna power distribution.

On the toolbar click: ICON3D - Polar Distribution
Using the right button of the mouse you can export the plot or data to other applications.
This plot allows you to compare the performance of this antenna with the performance of a simple dipole or other antennas. For example, this antenna has a gain of -20 dB or less in 70 % of the surface of an imaginary sphere around the antenna; meaning that the other 30 % of the antenna has a gain of -20 dB or more. On the other hand, a simple dipole has a gain of -20 dB or more in all its surface.
In the same manner, this antenna has a gain of -10 dB or more in 17 % of its surface, while the simple dipole has -10 dB or more in 96 % of its surface.
You may use the scrollbar to see the power distribution of this antenna at other frequencies.
Press OK to close the dialog

Step 18. Save the design.

Once the antenna has been designed and verified, it is a good idea to save the final antenna design. On the toolbar click:

FM Transmitting Antenna

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