Fundamental considerations in the design and application of high precision coaxial connectors

All sexless precision coaxial connectors existing today are comprised of two principal coaxial line sections. The sections are, an air line section adjacent to the coplanar, butt joing mating plane and a dielectric line section which supports and aligns the inner conductors. A review of lossless and low loss coaxial air-filled line theory shows that the best attainable impedance accuracy of air-filled coaxial lines is limited by our knowledge of the velocity of light and the relative permittivity of air. The physical properties of air-filled coaxial lines can have a significant affect on the impedance accuracy. Diametrical tolerances and eccentricity are most important and must be maintained within very narrow limits. Surface roughness, conductor ellipticity, and thermal expansion errors are shown to be negligible when good machining and laboratory practices are employed. The maximum useful frequency range of a precision coaxial line is determined by the TE11mode cutoff frequency at the upper end and the tolerable impedance error due to finite conductor conductivity at the lower end. The characteristic impedance considerations for air-filled coaxial lines are also applicable to the dielectric line section; however, in addition, the possible uncertainties in the effective relative permittivity of the bead support can be the cause of considerable impedance errors if material control and tight dielectric-conductor fits are not implemented. Connector bead supports are either solid or semi-solid (air-plastic composites). It is shown that air-plastic composites are readily calculable with good accuracy by using static capacitance relationships. These structures have comparatively small step discontinuities at the bead-air line interfaces. Solid support beads are more easily fabricated; but the step discontinuity capacitances at each interface are somewhat greater. Since in both cases the reflection due to the steps can be greatly reduced by carefully lowering the relative permittivity of the bead for a short distance adjacent to the steps, solid beads are usually preferable because of fabrication simplicity and superior strength.