Design of an Ultra-Wideband Antenna With AntSyn

Transcription

1 Application Example Design of an Ultra-Wideband Antenna With AntSyn Introduction Growing demand for wireless connectivity relies on integrated antenna solutions customized for optimal system performance, cost, and size. Achieving multiple performance metrics such as impedance matching, gain, radiation efficiency, and operating bandwidth is a time consuming process involving numerous iterative simulations and a significant amount of design knowledge. Fortunately, research into the use of evolutionary algorithms (EAs), a programmatic method for exploring the design space and automatically locating novel antenna designs, has matured into a viable technology. EA is proving to be more effective at generating antenna structures with higher performance than would otherwise be developed by traditional methods. This application note demonstrates the use of AntSyn software for novel antenna design using antenna synthesis and optimization technology. Design Requirements The requirement for this design was a wideband planar antenna fitting within a 7 x 10 form factor. The antenna s operating band needed to start as low in frequency as possible given the size constraint, ending at or above 1 GHz. Frequencies higher than 2.5 GHz were not of interest. It was preferable not to extend the size unless there was dramatic improvement in frequency coverage. A series of AntSyn evaluations were conducted to explore this design space. The final design was optimized to cover 325 MHz to 2.5 GHz and was 7.5 wide x 10 tall. Initial Evaluation The initial evaluation attempted to explore various antenna types and determine if it would be possible to get all the way to 300 MHz with the specified maximum form factor. Medium quality of results(qor) was used since the primary purpose was to become familiar with the problem, not to immediately find the final design. Given the AntSyn user s prior experience with antenna design, a select set of antennas of interest were pre-selected, rather than allowing AntSyn to explore the entire design space. The specification sheet for the initial evaluation is shown in Figure 1. The results of this initial evaluation are shown in Figure 2: The best of these results returned by AntSyn did not achieve 300 MHz with good voltage standing wave ratio (VSWR). Figure 1: Spec sheet for initial AntSyn design run. Figure 2: Results of this initial design run within AntSyn. ni.com/awr

2 Second Iteration Next, the user specified the objective (spec sheet) to try for a 300 MHz-1 GHz UWB dipole or monopole, but using a 7 wide and 10 tall form factor rather than the 10 wide x 7 tall one specified in the initial run. To summarize the results, the 10 tall form factor worked much better than the 10 wide one, but still did not meet the desired outcome. Third Iteration and More The next objective was to try for 200 MHz with a 7 wide x 10 tall form factor, to see if it would be possible to cover the 200 MHz single frequency in this form factor. User experience knows that if the frequency is not able to be met with a narrowband antenna of this size, it won t be able to be met with a wideband antenna. Long story short, it was becoming highly unlikely that a broadband antenna in this form factor with a start frequency in 200 MHz could be achieved; however, it seemed that an asymmetric design might be able to get lower in frequency and thus was explored further (Figures 3 and 4). Figure 3: Results for asymmetric design. Figure 4: Antenna performance metrics for asymmetric design.

3 Continuing on, it appeared difficult to get full performance all the way to 300 MHz in this form factor, and a VSWR of 4 was too high to be considered good performance. The user loosened the form factor to 12 tall to see if that might make a difference, and voila, the extra two inches in height did help. Two antenna types (Figures 5, 6, and 7) did well with the extra two inches, however, the 12 form factor was much less desirable, so further runs were done to explore the limits of both the 10 and 12 sizes. Figure 5: Results using 12 tall form factor. Figure 6: Performance metrics for the first antenna type with 12 tall form factor. Figure 7: Performance metrics for the second antenna type with 12 tall form factor.

4 Next, the upper frequency range was expanded to 2.5 GHz and the form factor width was expanded to 7.5. While performance was good, it looked like it would not be improved enough to be worth the extra size. The frequency was then extended down to 250 MHz. If the antenna could achieve coverage down to 250 MHz, it might be worth the extra height, yet it did not appear that this size would be able to work well at 250 MHz. After having shown that the 12 height was not sufficiently better, it was decided to go back to the 10 form factor and start to explore the low frequency limit. The designer also used high quality to try to get the best performance. Figure 8 is the spec sheet for this trial, Figure 9 shows a partial listing of antenna type results, and Figure 10 shows the performance metrics for the best (highest star rating) design. While the results were reasonable and the VSWR at 3.0 acceptable too, perhaps the start frequency could be lowered. Figure 8: Spec sheet for the 10 form factor at low frequency. Figure 9: Results for the 10 form factor at low frequency. Figure 10: Performance metrics for the best (highest star rating) design of the 10 form factor at low frequency.

5 The designer adjusted the lowest frequency to 325 MHz. The result was about right, however, the monopole style was not ideal for the application, so the following explorations looked at the dipole style to see if it might perform similarly. This time, the designer explored the UWB dipole type antenna with spec sheet settings as shown in Figure 11, the results of which are shown in Figure 12. Figure 11: Spec sheet settings for UWB dipole. Figure 12: Dipole results. Trying to utilize a symmetric dipole as it is easier to fabricate, the genetic-dipole was explored further. Figure 13 is the spec sheet for this trial and Figure 14 the best result. Figure 13: Spec sheet for genetic-dipole. Figure 14: Best result for genetic-dipole. Conclusion This dipole antenna was selected to be built because the VSWR was lower for GHz and the areas of higher VSWR were determined not to be an issue. In addition, the form Special thanks to Derek Linden, factor was better for the fabrication stage. AntSyn enables human judgement like this to be a AWR Group, NI, for his contributions factor in design selection by providing many different solutions and insight with little effort. to this application example National Instruments. All rights reserved. AWR, National Instruments, NI, and ni.com are trademarks of National Instruments. Other product and company names listed are trademarks or trade names of their respective companies. AN-AS-UWB