An optimal design for a sleeve balun with maximum choking on a coaxial cable is determined using a full-wave body of revolution finite difference time domain method with perfectly matched layer boundary conditions. An analysis of the sensitivity of choke length L and outer diameter R2 on choking effectiveness was carried out. A balun with L=77.5 mm (0.232λ0) and R2=8 mm on a cable with R1=2 mm (R2/R1=4) results in an S21 of −20 dB at 900 MHz and −15.5 dB at 2730 MHz. The isolation of the balun at 900 MHz is quickly degraded as the R2/R1 ratio is reduced below 2. Increasing R2/R1 to 8, results in a reduction of optimum balun length L to approximately 0.215λ0 , approximately 14% shorter than the typical recommended length for an ‘ideal’ quarter-wave balun.
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Saario et al. (2002) studied this question.
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