thank you for all!
for example i want to design a balun with the bandwidth 2-18 G!
may be it is a Marchard Balun,but I have found the dielectric constant
is high,but I only have a dielectric constant equate 2.2 ,thinkness is 0.254mm!for the elaboration,the smallest width for the CPW is 0.1 mm
!how can I do it?
Here is an idea that I have never tried to simulate. It might make a good school project.
Split the band in two parts with diplexers. Use two baluns with equal geometric frequency bandwith ratios. Then combine the outputs per phase with diplexers. Your design then would use 2-6 and 6-18 baluns which will be more realizable.
Use single L and single C type diplexers similar to those used in speaker crossovers.
The problem will be the phase and impedance matching of the whole circuit near the crossover frequency.
If this is not very good it might make a masters thesis topic to find out the fundamental limitations and optimum (least defective) design.