Two part spacer for a high-frequency coaxial cable

A spacer is indicated for a high-frequency coaxial cable with an inner conductor, a tube-shaped outer conductor and a dielectric cavity located between the two conductors which, through the use of different materials, has a higher thermal load capacity in the inner conductor area than in the outer conductor area. When installed, the parts, which are composed of different materials and are superimposed in the radial direction, are mechanically interconnected.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention concerns a spacer for a high-frequency coaxial cable with an 
inner conductor, a tube-shaped outer conductor and a dielectric cavity 
located between the two conductors which, through the use of different 
materials, has a higher thermal load capacity in the inner conductor area 
than in the outer conductor area. 
2. Description of the Prior Art 
High-frequency coaxial cables (HF cables) of different sizes are used 
mainly as antenna conductors for transporting HF-energy between an antenna 
and a transmitter-receiver station. A dielectric with a low dielectric 
constant and low dielectric loss is required between the two conductors to 
obtain the lowest loss of HF energy possible. This can be achieved with a 
dielectric cavity by selecting a suitable insulation material for the 
spacer. Such an insulation material is polyethylene. The spacer can be in 
the form of disks or individual supports which are attached to the inner 
conductor at a radial distance from each other and are used to support the 
outer conductor. In a preferred configuration, a strand of insulation 
material, which is helically wound around the inner conductor, is used as 
the spacer. 
The HF cable is heated by the transmission of HF energy. The highest 
temperature occurs at the inner conductor. The spacer must be configured 
so that it retains its shape when the inner conductor reaches its maximum 
temperature. DE-C-1 640 711 describes a spacer that withstands high 
temperatures. In that case, three individual supports made of a hard 
elastic material are held together by a spring-steel bow. This spacer 
proved itself in practice. However, it is altogether costly. 
DE-C-1 515 832 describes a spacer in which the spoke-shaped spacers are 
made of different materials. The part which rests against the inner 
conductor exclusively comprises a radially outward protruding crosspiece 
made of polyvinyl-carbazole, a brittle material which has a high thermal 
load capacity. The ends of the crosspieces which rest against the outer 
conductor are made of flexible insulation material. They are configured as 
expanded rockers. The publication does not specify how the two different 
materials are interconnected. In addition, this spacer is costly as well. 
SUMMARY OF THE INVENTION 
It is an object of the invention to configure a spacer in a way so that it 
is simple to construct and has a high thermal load capacity. 
This object is fulfilled by the invention in that the parts, which are made 
of different materials and are radially superimposed, are mechanically 
interconnected in the installed condition. 
When it is made of the proper material, this spacer has a high thermal load 
capacity, as well as being simple to construct and cost-effective. A 
high-temperature resistant material is used for the part of the spacer 
that rests against the inner conductor. This material can be 
polytetra-fluorethylene (PTFE) or fluoridated ethylene-propylene (FEP). 
This part of the spacer is kept as small as possible in the radial 
direction. Lower cost materials with high dielectric properties may be 
used for the part of the spacer that is located radially outward of the 
part of the spacer adjacent the inner conductor. The parts of the spacer 
(two or more) can be joined by interlocking protrusions and cut-outs, or 
also by cementing the unit when it is finished, or in the installed 
condition, into a solid single-piece structure. This can already take 
place during the preliminary mounting. However, the parts can also be 
joined during the production of the HF cable. Even though the farther 
outside lying material does not have such a high thermal load capacity, 
the result is a cost-effective, high grade dielectric spacer with a high 
thermal load capacity. 
The invention will be fully understood when reference is made to the 
following detailed description taken in conjunction with the accompanying 
drawing.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 schematically illustrate a coaxial HF cable. It comprises an 
inner conductor 1 and a tube-shaped outer conductor 2, between which the 
dielectric cavity 3 is located. Preferably, both conductors 1 and 2 are 
made of copper. 
The dielectric cavity 3 contains a spacer which coaxially connects the 
inner conductor 1 and the outer conductor 2 with each other. According to 
FIG. 1, the spacer comprises disks 4 arranged at an axial distance from 
each other on the inner conductor 1. The outer conductor 2 rests on the 
outside of the disks 4. According to FIG. 2, a strand 5 is used as the 
spacer which is helically wound around the inner conductor 1. The outer 
conductor 2 rests against the outside of the strand 5. Instead of the 
disks 4 and the strand 5, individual supports which are attached to the 
inner conductor 1 can also be used as a spacer. 
FIG. 3 illustrates a cross-section through a disk 4 of the spacer. 
Accordingly, it comprises two parts 6 and 7 which are interconnected in 
the finished spacer. The part 6 is designed to be applied to the inner 
conductor 1 of an HF cable. It is made of an insulation material with a 
high thermal load capacity, such as polytetra-fluorethylene or fluoridated 
ethylene-propylene. Its radial dimensions are kept as small as possible 
and result from the expected temperature range between inner and outer 
conductors at maximum power input to the HF cable. Part 7 is made of a 
high grade dielectric insulation material whose thermal load capacity is 
lower. For example, polyethylene can be used for the part 7. 
In the finished spacer, the parts 6 and 7 are interconnected. This can 
basically be achieved by cementing, i.e., by applying an adhesive to the 
surfaces to be connected. However, according to FIGS. 4 or 5, the part 6 
can have protrusions 8 which engage in the cut-outs 9 of part 7 by means 
of tongue and groove configurations when the two parts are joined. 
Protrusions 8 and cut-outs 9 can also be alternately located in the other 
part or in both parts. In FIGS. 4 and 5, part 6 can be adhesively 
connected to part 7. 
The described construction of a disk 4 applies equally to the helically 
wound strand 5, which is made up of two radially superimposed partial 
strands. In this case, the protrusions 8 and cut-outs 9 can extend without 
interruption over the entire length of the two partial strands. However, 
separate protrusions 8 with corresponding cut-outs 9 can also be used. The 
partial strands can also be continuously cemented to each other. 
The described construction of disks 4 and strand 5 also applies 
analogically if these elements of the spacer are, or will be, made of more 
than two radially superimposed parts. 
Both the disks 4 and the strand 5 can be produced by joining the parts 6 
and 7 during preproduction, i.e., before they are applied to the inner 
conductor 1. They are then applied in a continuous manner to the inner 
conductor 1, which moves lengthwise. Accordingly, the outer conductor 2 
can be formed around the spacer during the same operation. It is also 
possible to first apply the part 6 of the spacer to the inner conductor 1, 
and then to join part 7 to part 6. This solution is especially useful when 
the strand 5 is used as the spacer. 
The preferred embodiments described above admirably achieve the objects of 
the invention. However, it will be appreciated that departures can be made 
by those skilled in the art without departing from the spirit and scope of 
the invention which is limited only by the following claims.