Because this project required the use of an ultrahigh broadband, a regular dipole antenna is not possible. This is because a dipole antenna cannot be matched at all frequencies. This results in many internal reflections throughout the antenna.
These unwanted reflections can be reduced by including resistive materials in the antenna. The resistive loading profile that was used is called the Wu-King resistive loading profile, named after the man who developed it. This resistive loading profile allows a broadband wavelength to be emitted with minimal reflectance. However, the Wu-King profile needed to be modified to accommodate a broad band in the 8-12Ghz ranges. The resulting modified formula is shown below:
Where Rval is the resulting resistance for each resistor of length x2-x1, l is the length of the antenna (cm), and x1 and x2 are the increments of length (in mm) progressing up the antenna. K is a coefficient which when increased, increased the resistor values used.
In order to determine the proper value of K and the resulting resistor values, simulations were done with LC, an electromagnetic computer simulation program. It was found that as the resistor values increased, the signal became cleaner as unwanted reflections were minimized. However, because of the increased resistance values, the power emitted was also decreased from the already inefficient antenna. Finally a value of 1100 was chose for K, and the resulting waveforms can be seen below in Figures 2 and 3 below.
Figure 2 shows the differentiated gaussian pulse that is emitted from the antenna at two distances. As can be seen, the differentiated gaussian pulse, although not perfect looks as expected. It also gets smaller as it dissipates in the air. Figure 3 shows the gaussian voltage at the top and the bottom of the resistively loaded antenna. As can be seen, the resistors in the antenna have mostly dissipated the signal at the top of the antenna.
After the resistive loading profile was chosen, the resistors had to be ordered and soldered together. LC was also used to compare the simulated values with the actual measured values. It was found they only differ by about 10%, which was considered a success.