 |
| |
|
The configuration in this case is a uni-polar triangular bow-tie antenna with included angle of 90 degrees. The objective of this case is to calculate the input impedance of the bow-tie. The computed values are validated against the experimental measurements of Brown and Woodward[1].
This calculation uses the transmission line feed to calculate the input impedance of the bow-tie. This is one of many antennas that were constructed and tested by Brown and Woodward. In these experiments the frequency of the feed was maintained at a constant value and the size of the antenna was changed to cover a range of antenna size to wavelength ratios. In the present calculation only a single antenna is considered and the range of values of antenna size to wavelength is achieved by considering a range of frequencies. FDTD, being a time domain technique, can be do this in a single calculation with the response in the frequency domain being derived using a Fourier Transform of the time domain results. In the calculations the antenna is of height 50cm and cubic cells of size 1cm were used. The boundary was located 5 cells away from the antenna with a PML layer of 12 cells.
In this case the range of measurements carried out extends to low frequencies where the wavelength of the radiation is 10 times the size of the antenna. At this low frequency the boundary is located 100th of a wavelength from the antenna and so is very much a near field location. This is a stringent test of the boundary condition.
This case examines the drive point reflection from a fan top bow-tie. The configuration is similar to the case above except for the shape of the antenna. This calculation is validated against the work of Shlager et. al.[2]
In the study that this case is extracted from the aim is the optimisation of the bow-tie for pulse transmission. The case presented here shows the basic perfectly conducting antenna and examines the reflection on a 50W feed. The antenna has an included angle of 156.2 degrees and this gives an impedance of 50W for an antenna of infinite extent. For a finite sized antenna the reflected waveform within the feed line has two components, the reflection from the drive point and the reflection from the outer extent of the bow-tie. These components can be seen clearly in the results.
The figure shows the calculated drive point reflection in the time domain verses measurements from Shlager. The time is normalised against the transit time of the antenna, which in this case is 2.31ns.
1 Brown G H, Woodward O M
'Experimentally determined radiation characteristics of conical and triangular
antennas'
RCA Rev, Vol 13, Dec 52, pp 425-452
2 Shlager K L, Smith G S, Maloney J G
'Optimisation of Bow-Tie Antennas for Pulse Radiation'
IEEE Transactions on Antennas and Propagation
Vol 42, Nov 7, July 1994, pp 975-982