Tapered Microstrip Balun

Tapered microstrip balun is used as an impedance transformer network in feeding sections of spiral antennas as it provides impedance transformation over a large range of frequency and also serves the purpose of conversion of single ended port to a symmetric port. The conversion from unbalanced to balanced line relies on a gradual change of cross section of the line.

A tapered microstrip balun consists of two tapered lines etched on either side of a substrate. One line is atleast three times wider than the other at the unbalanced end, and together the two form a microstrip line. At the balanced end, the lines are of equal width and appear as parallel strips. At the input, the cross section of the line resembles microstrip, while at the output the strips are of equal width, constituting a balanced line. Ideally the

balanced line should support only odd modes. In order to design a tapered microstrip balun we need the characteristic impedances of this line at the input and the output ports. The impedance at the balanced end is known beforehand (which is according to the requirement). The widths of the parallel strips at this end can be found iteratively using the equations of the selected taper. The length of the taper is also given by a criteria which is specific to each taper. The structure is then simulated in any EM modeling software like HFSS and the impedances at the two ports are observed. If they are offset from the designed values, the widths are adjusted and procedure is repeated till the values agree sufficiently with the desired ones.

Reference:

Inanc Yildiz, 'Design and Construction of reduced size planar spiral antenna in the 0.5-18 GHz frequency range', METU 2004