The microstrip lines widely used in microwave PCBs are unbalanced lines, although a Vivaldi antenna requires a feed with a slotline transmission line, which is balanced. The balun required for the unbalanced-to-balanced transformation must operate over a frequency range of at least two octaves, and up to several octaves. Preferably, the balun would be frequency independent. 6 To demonstrate the effectiveness of TSA designs, a Vivaldi antenna was chosen from other possible designs, due to the wealth of existing research on this configuration. In designing any antenna, the choice of dielectric substrate is critical. The choice of substrates is large, with a wide range of characteristics and dielectric constants among those choices. For this experimental Vivaldi antenna, it was preferable to fabricate the transition and the Vivaldi antenna on a low dielectric constant substrate, and to avoid the use of a drilled short hole. The experimental antenna was fabricated on RO4003C substrate material from Rogers Corp. (www.rogerscorporation.com), which has a dielectric constant of 3.38. Agilent's ADS software was used to optimize the design for use from 8 to 12 GHz.
A microstrip-to-slotline transition was chosen for the Vivaldi antenna because of its many benefits compared to other approaches. One of the main advantages is that this type of transition can be easily fabricated by normal photo-etching processes, making it possible to realize two-sided PCBs with microstrip on one side and slotline on the other side.
Kayani et al. proposed a simple, compact Vivaldi antenna in 2005.11 Their single-sided design employed stripline-to slot-line coupling as shown in Fig. 4. The main advantage of this design is that the antenna can be made smaller compared to an antipodal Vivaldi antenna. In addition, because of the small size of the antenna, it features a relatively short simulation time when using computer- aided-design (CAE) software tools. Figure 4 shows the double-sided Vivaldi antenna for operation from 8 to 12 GHz.
The length is 7.48 cm and the width is 2.08 cm. The width of microstrip line is 0.29 cm. The diameter of the circular slot stub is 1 cm and the slotline gap is 0.08 cm.
The design and simulation process using the Momentum EM analysis tool from Agilent's ADS software suite consists of numerous steps:
1. Create the physical design for a high-frequency Vivaldi antenna.
2. Choose the desired Momentum operating mode for simulating the Vivaldi antenna.
3. Define the required substrate material and its characteristics.
4. Solve for the substrate parameters by precomputing the substrate used.
5. Assign antenna ports and define their properties.
6. Set up and generate a circuit mesh.
7. Set up and simulate the performance of the Vivaldi antenna.
8. View the S-parameters and radiation pattern results.
The actual steps applied to creating and simulating the Vivaldi antenna with Momentum13 software will be detailed in Part 2 of this article series, in next month's (August 2008) issue of Microwaves & RF. Part 2 will provide a comparison of measurements performed with commercial test equipment in the 9-GHz band and simulations on the Vivaldi antenna performed with the Agilent Momentum planar EM simulation software from Agilent Technologies.
This ends Part 1 of this article on Vivaldi antenna design, to be concluded in the August 2008 issue.
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11. J. K. Kayani, A. B. Rashid, and A. Naveed, "Design and Testing of Vivaldi Antenna for UWB Application," Fourth International Bhurban Conference on Applied Sciences and Technology, Bhurban, Pakistan, 2005.
12. J. D. S. Langley, P. S. Hall, and P. Newham, "Balanced antipodal Vivaldi antenna for wide bandwidth phased arrays," IEE Proceedings -- Microwaves, Antennas and Propagation, 1996, Vol. 143.
13. Agilent Technology (www.agilent.com), Momentum planar electromagnetic simulator, Advanced Design System, 2005.