Tapered coax to parallel wire transition with 100:1 performance bandwidth

Tapered coax to parallel wire transition

Tapered coax to parallel wire transition

While designing an antenna, one of our engineers needed a transition from a coaxial transmission line to a parallel wire transmission line. Because he needed a balun with wideband impedance transformation he had a look at the baluns described in the balun article in Antenna Magus. (You can read more about this article in Newsletter 2.1). The Marchant balun would have been an overkill so he chose the tapered coaxial balun and looked at various tapers to find a good impedance transformation across the band.

The tapered coax to parallel wire transition transforms a lower coax impedance to a higher impedance of a parallel wire transmission line. It also operates as a balun while obtaining ultra wide bandwidths depending on which type of continuous taper is used.

The outer of the coaxial cable is gradually removed along the length of the transition, until a parallel wire transmission line is realised. The exact taper to which the outer is cut, will determine its performance across the band.

The graph below shows comparisons of normalized reflection coefficients between different Klopfenstein tapers and (an easier to manufacture) linear taper. The low, medium and high Klopfenstein tapers refer to the matching level designed for and the higher (or tighter) the spec, the harder it becomes to manufacture the taper. With the right equipment, it is possible to achieve maximum performance bandwidths of 100:1!

Comparing S11 for various tapers

Comparing S11 for various tapers

Author: Robert Kellerman

One Response to “Tapered coax to parallel wire transition with 100:1 performance bandwidth”

  1. kabukicho2001 says:

    HI, In the image shown, the transition is in the same coax cylinder or it protrudes the cylindrical coax shape?

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