AbstractsEngineering

In antenna measurements, it is not recommended to connect a balanced antenna directly to the coaxial cable feed. A balun is required to prevent surface current from flowing onto the outer shield of the feed cable. Balun chokes like the sleeve balun, also known as bazooka balun or quarter-wavelength balun, are commonly used in antenna measurements. In general, baluns do perform well, but their performance is band-limited. Hence, for wider frequency bands, multiple baluns with different operating frequencies have to be used. This presents a major problem, especially in broadband antenna measurements. To this end, a proposed design based on the quarter-wave bazooka (sleeve) balun has been investigated. The design consists of resonators arranged in a log-periodic manner. The main objective of this design is to have high impedance and therefore high common mode rejection ratios at different frequencies. To do so, resonant series LC circuits were placed between the outer shield of the coaxial cable and the inner wall of the balun. These are intended to create short circuit terminations within the balun at different design frequencies. Two models have been investigated: one with two LC circuits and the other with four LC circuits. The variables considered in the study included circuit placement and circuit quality factor Q. Simulations in Agilent ADS$^{\circledR}$ and HFSS$^{\circledR}$ 14.0 were performed to study the common mode rejection ratio (CMRR) parameter of the different proposed designs. In addition, three baluns were built and measured in order to compare with simulations. Results demonstrated that the Q of the inductor significantly affects the response over a frequency range. Comparisons of the performance of the different designs are presented in detail in this work. All designs achieved a common mode rejection ratio above 30 dB over a wideband frequency range.