The continuously tapered coax to parallel wire transition acts both as an impedance transformer between the typically lower impedance of coax to the typically higher impedance of a parallel wire transmission line and as a balun [Duncan]. By using a continuous taper, performance can be obtained over a very wide bandwidth.
Two arbitrary real impedances may be matched over a desired bandwidth by using a multi-section matching transformer. As the number of discrete sections increases, the step change in characteristic impedance between adjacent sections becomes smaller; this, in the limit of an infinite number of sections, approaches a continuous taper. The type of continuous taper determines the characteristics of the passband.
The types of tapers implemented here are Klopfenstein and linear. For a given taper length, the Klopfenstein taper has been shown to achieve the lowest reflection coefficient, while for a maximum reflection coefficient specification, it has the shortest length [Klopfenstein]. The linear taper has less optimal performance, but is simpler to construct [Baker]. ...
The tapered coaxial to parallel wire transition is recommended when impedance transformation and balun action is needed over very wide bandwidths. For bandwidths of less than 13:1 a Marchand balund may be superior [Cloete][Baker], but for ultra wide bandwidths the tapered coax transition is indicated. Tapered coax transitions are often used for spiral antennas [Baker] and other ultra-wideband antenna types such tapered slot antennas [Knott].