Analysis of Dual Frequency Inverted FL Antenna using XFdtd^®

Accurate results obtained after antenna is easily generated using XFdtd 2D editor.

The paper 'An Inverted FL Antenna for Dual-Frequency Operation' by Nakano, Sato, Mimaki, and Yamauchi appears in the August 2005 IEEE Transactions on Antennas and Propagation. The Inverted FL antenna is a planar antenna designed to operate at 2.45 and 5.2 GHz. This antenna is extremely easy to analyze using XFdtd. The geometry of the planar antenna is shown in Figure 1.

The paper includes complete dimensions for the antenna. It is easily drawn using the XFdtd 2D editor. To facilitate this the 2D editor grid is set to a convenient spacing and grid snap is used to position the polygons used to generate the various parts of the antenna. The result is shown in Figure 2 below.

The 2D antenna geometry is then meshed in XFdtd and a feed port is added with the result shown in Figure 3. This entire process of drawing the antenna and setting up the calculation takes just a few minutes.

The XFdtd calculation is performed with a transient excitation to determine the resonant frequencies of the antenna. The resulting VSWR vs Frequency plot from XFdtd is shown below. The inset shows the results from the paper. Clearly XFdtd produces a very accurate result. Calculation time was about 14 minutes on a dual Xeon work station using UPML absorbing boundaries and -40 dB convergence setting. To efficiently investigate preliminary design changes XFdtd can be used with relaxed convergence and outer boundary absorption to produce nearly identical results to the plot below in less than 3 minutes.

It is informative to used XFdtd to display the antenna currents at the two resonant frequencies. The following figure shows the antenna current distribution at 2.45 GHz. Note that the current on the long 'L' part of the antenna at the upper left of the antenna is relatively strong at this lower frequency.

The next figure shows the corresponding current density at 5.2 GHz. At this higher resonance the current is stronger on the shorter 'F' portion of the antenna and on the modified parasitic 'L' at the upper right. The dual frequency nature of this antenna is clear from considering these two current displays.