Antenna Theory and Design Take Home Midterm Project
The goal of this assignment is two-fold. The two parts are not related to one another. The first is the analysis of the performance of antenna arrays, and the second is to design a low-profile Planar Inverted F Antenna (PIFA) antenna using Ansys' High Frequency Structure Simulator, HFSS. You may team with another student on this project; however, teaming is not required. Only one person needs to upload the file on D2L, but make sure the names of both team members appear on the assignment.
1. Write your own linear array computer program that computes the antenna array pattern given an N element array with uniform spacing yet arbitrary amplitude and phase. Assume isotropic radiators and let N be even for this project. Write the array factor in terms of complex exponentials (do not use cosine functions.) It is quite important in Step 5 to write this as a sum of exponentials. From the antenna pattern you should be able to:
a) plot the pattern in dB vs. θ, (assume array is along the z-axis). You will normalize your pattern so that the peak is at 0dB.
b) compute the directivity (from direct pattern integration),
c) compute the 3dB beamwidth, and
d) compute all of the sidelobes levels in dB.
2. Check the accuracy of your linear array program. Consider a ULA of isotropic elements with λ/2 spacing. Use your program to compute the patterns for a) a 6-element array, and b) a 20-element array. Plot the antenna patterns in dB versus θ. For each case, make a chart that compares the directivity and beamwidth computed by your program to the approximate formula (for large arrays) given in the book for an equispaced broadside uniform array. What is the peak sidelobe for each case? Make sure to clearly indicate which formula you are using from the text when you create your comparison chart. What is the difference between the closed form equations in the book and the values computed from the program?
3. Now consider a tapered distribution that gives a Dolph-Chebyshev pattern using an array with 20 element equi-spaced (d = λ/2) elements, for a constant sidelobe level of -21.5dB. Plot the antenna pattern in dB versus θ. Also plot the array excitations (an). Make a chart that compares the directivity, beam width and sidelobe levels of the 20 element Dolph-Chebyshev and uniform arrays. Discuss any differences you observe in the distribution or patterns for the ULA and Dolph-Chebyshev arrays.
4. Now consider the effects of element spacing for both the ULA and the Dolph-Chebyshev array. Step 3 used half-wavelength spacing. Show and explain what happens to the pattern when the spacing is decreased (to d = λ/4) or increased (to d = λ)?
5. Investigate the effects of failures the Dolph-Chebyshev array you studied in Step 3. A failure in this case assumes that the element excitation is zero. This would occur by a break in a feed line or from an active module being shorted out. Modify your computer program to randomly incorporate antenna failures so that the element with the failure is randomly assigned. This works if you used the summation of complex exponentials in your array factor. If you assumed a symmetric array factor, then you need to recast your array factor, so that the failures do not happen symmetrically. Investigate the results of failures on the 20-element array, consider level-a) 1 element failed and level-b) 4 elements failed, For each level of failures, run 10 different cases with failures. Ten cases are needed, because each time the failures should be on different elements. (If this does not occur, make sure you provide a different seed to your random number generator for each case and check the results). For the 4-element failure level, print out the distribution with failures for four of your cases. For each degree of failures explain what happens to the sidelobe levels. The error free pattern has the ideal Dolph-Chebyshev pattern. Make a chart that compares the performance for each level of failures. For each degree of failures, plot one of the patterns with failures.
6. You were given the design of a PIFA antenna in the HFSS tutorial. This antenna works at 0.9 GHz. Scale the antenna so it will work at 1.5GHz BUT.... Keep the height of the antenna (i.e., the separation between the ground plane and the top plate fixed, it should stay at the value it is for the 0.9 GHz antenna). This means that you cannot just simply use the "SF" feature in HFSS to come up with the 1.5 GHz antenna. This also means that after you resize the antenna dimensions from the 1.5 GHz antenna, then final dimensions of the top plate may have to be slightly adjusted to get a good match at 1.5 GHz. (you can adjust the length and width of the top plate).
7. Create an HFSS tutorial that shows one possible modification to the PIFA to create a dual band element. The antenna should work at 1.5GHz and 2 GHz. Some ideas include adding a slot in the top plate for dual band performance, adding another element on the side of the PIFA, adding a stacked element. The tutorial should be in the form of a powerpoint file. The tutorial details are given below.
You should clearly label any comparisons with the number of the steps listed above. You should clearly describe your steps and label what you have done. Turn in your programs and comparison plots. Please make sure to summarize your findings and comment on the results. Turn in your computer printouts that display the different antenna parameters computed in each item.
The formal report for your special mid-term in Antennas should include sections of papers in the IEEE Transactions on Antennas and Propagation. Use this journal as a guide for the different sections report. Your report should only be in a single column (do not use the double column format in these papers, but DO organize your sections according to the format used in these papers.) The reports should include the following:
1. A cover page with the title, author, affiliation (Department of Electrical & Computer Engineering, University of Arizona, Tucson, AZ 85721), a short 50-250 word abstract, and an indication that the paper is a part of the requirement for ECE 484 or ECE 584.
2. Section I of the main body should be entitled "Introduction" and should describe the overall paper as well as give the motivation and significance of the work.
3. The following several sections, II, III, IV, ..., should contain the major portions of the information of the paper. They should be subdivided further if any one section become excessively long (> two pages). Each section and subsection should have an appropriate title.
4. The final section should be entitled "Conclusions" and should summarize the important points of the paper.
5. "References" should be listed in the order that they were mentioned in the main body of the paper.
Use the format of the IEEE Transactions for each reference.
Appendices should be used at the end of the entire paper to present any ancillary information. Figures and tables should be included in the main body of the paper as the page following its being mentioned in the text. Each figure and table should be numbered consecutively and have a caption (see IEEE Transactions for examples). Copies of figures from other sources can be used as long as they are referenced properly. Equations should also be numbered (in parentheses to the right of the equation).
It is expected that each student or team does his/her individual work on the projects; however, journal articles and reference texts may be used. The project grade will be determined by the presentation of the solution process, the validity of the engineering principles used to approach the project, and the thoroughness of the project results.
Reference: K.L. Virga and Y. Rahmat-Samii "Low Profile Enhanced Bandwidth PIFA Antenna for Wireless Communications Packaging," IEEE Transactions on Microwave Theory and Techniques, 45, no. 10 (1997): pp. 1879-1888, October 1997.