Coax to parallel wire transition

Image of the Coax to parallel wire transition.

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].