Dielectric Support For High Frequency Coaxial Lines

Abstract

A coaxial structure employing a dielectric support positioned and secured within an outer conductor and having a central opening for receiving a short conductive member for joining two sections of coaxially aligned inner conductors whose end faces engage the end faces of the dielectric support member adjacent a central opening provided therein for receiving the short conductive member. The diameter of the short conductive member is less than the diameter of the central opening in the dielectric support to form a hollow, annular-shaped air gap for eliminating the occurrence of damaging peak field strength within the coaxial structure. The side faces of the dielectric support are each provided with annular-shaped compensation grooves wherein the axial thickness of the dielectric support in the region of the compensation grooves is less than the axial length of the annular gap. The size of the annular gap is selected to prevent the occurrence of inadmissable peak field strength.

Description

The invention relates to a dielectric support for coaxial lines which is supported on a portion of reduced diameter of the inner conductor and is provided with end-face grooves for compensation purposes, the transition to said inner conductor portion from both sides being via a conical surface and an end ring surface bearing on the dielectric support at the end face.

The fixing of the dielectric support to the inner conductor presents considerable difficulties because when air gaps are present between the conducting surface of the inner conductor portion and the dielectric support high field intensities arise in the air spaces present due to the fact that air has a low dielectric constant compared with the dielectric constant of the support. Since however it is in practise extremely difficult or impossible to avoid small air spaces between the inner conductor portion and the dielectric support, the solution adopted has been to provide the internal bore of the dielectric support, consisting for example of ceramic material, with a conductive coating by for example electroplating methods, said coating conducting the current over the axial length of the dielectric support. This conductive coating is led outwardly on both sides to an end ring surface at which the support engages axially the end ring surface of the continuing inner conductor.

Admittedly, in this manner air inclusions and the occurrence of high field intensities therein are avoided. However, it can happen that under high current load this coating splits. Moreover, the contacting at the end ring surfaces with the galvanic coating is extremely difficult to effect.

The problem underlying the invention is to avoid the occurrence of damaging peak field strengths without conductive coatings of the dielectric support.

According to the invention this problem is solved in a dielectric support of the type explained at the beginning in that an annular air gap is left at least over the axial extent of the supporting wall of the support between the latter and the inner conductor portion and the mechanical fitting support is effected at the two ends of the dielectric disc in the region of the compensation grooves.

The invention is based on the knowledge that in the outer region small air inclusions between the supporting parts of the inner conductor and the dielectric support cannot have a disadvantageous effect because in this cross-sectional plane due to the compensation grooves of the support between the outer and inner conductors there is a long air gap and only relatively short gaps in the dielectric of the support, i.e., in the outer portion and at the inner collar. Due to this long air gap even small air inclusions in this region cannot produce high peak field intensities. In the region of the supporting wall of the support an air gap is deliberately provided and has such a length that the field intensities cannot assume inadmissibly high values. Between the supporting portions and the air gap calculable transitions may be provided, but it suffices for practical conditions to provide a stepped transition. The latter may be provided both in the dielectric support and in the inner conductor portion. For practical reasons the latter alternative is chosen and the inner conductor portion is provided within the axial length of the support with two supporting collars on both sides and a portion of further reduced diameter therebetween. For safety reasons it is advisable for the annular air gap to extend over a greater axial length than the supporting wall of the support.

An example of embodiment of the invention will be described hereinafter with reference to the drawing. The single FIGURE of the drawing shows a section of a support constructed according to the invention and inserted in a coaxial conductor.

Inserted between the outer conductor 10 and inner conductor 12 is a dielectric support 13. The latter lies in a groove 14 of the outer conductor 10 and is supported on a reduced-diameter portion of the inner conductor. In the region between the inner and outer conductors the dielectric support comprises for compensation purposes on both sides grooves 15 between which the supporting wall 16 is left with an axial length 1. The inner conductor 12 merges via a conical surface 18 into an end ring surface 19 which bears against the dielectric support at the end face and then into a collar 22 whose diameter is matched as accurately as possible to the internal diameter of the inner bore 20 of the dielectric support. In the intermediate portion 24 the inner conductor is further reduced in diameter, resulting in an annular air gap 26 between the inner conductor and the dielectric support.

The axial length of the supporting collars 22 is less than the axial depth of the grooves 15, i.e., the axial length of the annular gap 26 is greater than the axial thickness 1 of the supporting wall 16. This construction results in an acceptable field strength distribution in every cross-sectional plane; any air gaps present in the outer portions do not have a detrimental influence due to the large air gap (small dielectric constant) formed by the grooves 15. In the centre planes, i.e., the cross-sectional planes passing through the wall 16, an air gap is connected in series with the dielectric of the dielectric support, said gap having radial dimensions large eough to prevent the occurrence of inadmissible peak field strengths.

Claims

What we claim is:

1. A coaxial structure comprising a tubular outer conductor;

a disc-shaped dielectric support member having an annular periphery

the interior surface of said outer conductor having an annular groove for receiving the periphery of said dielectric support member;

said dielectric support member having a central opening axially aligned with the longitudinal axis of said outer conductor;

at least first and second annular shaped inner conductor sections having their longitudinal axes coaxial with the longitudinal axis of said outer conductor;

adjacent ends of said inner conductor sections being positioned on opposite sides of said dielectric support, said inner conductors having conical shaped transitions joining the end surfaces of said adjacent ends to the annular surfaces of said inner conductor sections;

each of said end surfaces engaging an adjacent side of said dielectric support member;

a third inner conductor member having its longitudinal axis coaxial with the longitudinal axis of said outer conductor; said third inner conductor member being positioned within the central opening in said dielectric member;

the diameter of said central opening being less than the diameter of said first and second inner conductor sections;

the diameter of said third inner conductor member being less than the diameter of said central opening whereby the outer surface of said third inner conductor member lies a spaced distance from the surface of said central opening to form a hollow annular shaped air gap;

means for mechanically joining and electrically connecting the ends of said third inner conductor member to the adjacent end surfaces of said first and second inner conductor sections;

the faces of said dielectric member each having an annular shaped compensation groove;

said joining means comprising a supporting collar joined to each end surface and extending a small distance into said central opening;

the axial length of said annular air gap being greater than the axial thickness of the supporting wall of the dielectric support member in the region of said compensation grooves to provide an acceptable field strength distribution, the grooves in said dielectric support member preventing detrimental influences in the surface of the dielectric support member due to the large air gap formed by the annular grooves and the air gap between the dielectric support member and the third inner conductor member being in series with the dielectric support member to prevent the occurrence of inadmissible peak field strengths.