Leaky coaxial cable wherein aperture spacings decrease along the length of the cable

The outer conductor of a high frequency electric cable of the kind employed as a stationary elongate element for transmitting high frequency signals to, or receiving high frequency signals from a receiving or transmitting device carried by a mobile body, has extending along its length at least one row of mutually spaced apertures, the mutual spacing between adjacent apertures of the or each row decreasing along the length of the row, being a maximum value at one end of the row and a minimum value at the other end of the row. The or each row of apertures may be sub-divided into a plurality of sub-groups of apertures, the mutual spacing between adjacent apertures of each sub-group being constant, and the mutual spacing between adjacent sub-groups decreasing along the length of the cable.

This invention relates to high frequency electric cables of the kind 
employed as a stationary elongate element for transmitting high frequency 
signals to, or receiving high frequency signals from, a receiving or 
transmitting device carried by a mobile body, for instance a vehicle or 
person, which may be in the open air, in a building or underground, for 
instance in a tunnel, mine or similar enclosure where signals radiated 
from a point source are rapidly attentuated, or employed in a system for 
detecting the presence of a person, vehicle or other mobile body in an 
area which is in the open air or in a building and which is under 
surveillance. 
It is an object of the present invention to provide an improved high 
frequency coaxial cable of the aforesaid kind which, over a finite length 
of the cable, will transmit or receive high frequency signals of 
substantially more uniform signal strength than high frequency cables of 
the aforesaid kind hitherto proposed and used. 
According to the invention, in a high frequency coaxial cable comprising an 
inner conductor and, insulated from and surrounding the inner conductor 
throughout the length of the cable, an outer conductor of metal or metal 
alloy having extending longitudinally throughout at least a finite part of 
its length at least one row of apertures that are mutually spaced along 
the outer conductor, each aperture being of such a size and the mutual 
spacing between adjacent apertures being such that high frequency signals 
can be received by or transmitted from the cable, the mutual spacing 
between adjacent apertures of said row decreases along the length of the 
row, being a maximum value at one end of the row and a minimum value at 
the other end of the row. 
Preferably, the mutual spacing between each pair of adjacent apertures of 
the row or at least one of the rows of apertures, except the last pair of 
adjacent apertures whose mutual spacing is said minimum value, is greater 
than the mutual spacing between one of the neighbouring pairs of adjacent 
apertures so that the mutual spacing between adjacent apertures of the row 
decreases throughout the length of the row. 
In some instances, the row or at least one of the rows of apertures may be 
sub-divided along its length into a plurality of sub-groups of apertures, 
the mutual spacing between adjacent apertures in each sub-group of 
apertures being substantially constant, and the mutual spacing between 
each pair of adjacent sub-groups of apertures, except the last pair of 
adjacent sub-groups of apertures whose mutual spacing is a minimum value, 
being greater than the mutual spacing between one of the neighbouring 
pairs of adjacent sub-groups of apertures so that the mutual spacing 
between adjacent sub-groups of apertures of the row decreases at spaced 
positions along the length of the row. The substantially constant mutual 
spacing between adjacent apertures in all of the sub-groups may be the 
same or, in some cases, the substantially constant mutual spacing between 
adjacent apertures in each sub-group, except the last sub-group in which 
the substantially constant mutual spacing between adjacent apertures is a 
minimum value, may be greater than the substantially constant mutual 
spacing between adjacent apertures of one of the adjacent sub-groups. 
In all cases, preferably the apertures of the or each row are substantially 
the same shape and size and, in a preferred embodiment, the apertures are 
of substantially circular form. 
The or each longitudinally extending row of apertures is preferably 
substantially parallel to the longitudinal axis of the cable. 
Preferably, the outer conductor is formed of a longitudinally applied, 
transversely folded tape made wholly or partly of metal or metal alloy, 
the apertures of the or each row being punched, drilled or otherwise 
formed in the tape, before the tape is applied to the cable, in such a 
configuration that when the tape is applied to the insulated inner 
conductor a longitudinally extending row or longitudinally extending rows 
of apertures is or are provided in the outer conductor with the desired 
mutual spacing between the adjacent apertures. 
The present invention is especially, but not exclusively, applicable to use 
in the coaxial cable described and claimed in the Complete Specification 
of our British Patent No. 1,424,685. 
According to another aspect of the invention, the outer conductor of a 
preferred high frequency coaxial cable is formed by a method which 
comprises winding a tape made wholly or partly of metal or metal alloy 
into a convolute coil; at at least one position around the circumference 
of the coil drilling radially through the adjacent turns of the tape to 
form an aperture in each turn; and longitudinally applying the tape to, 
and transversely folding the tape around, the insulated inner conductor of 
the coaxial cable to form an outer conductor having at least one row of 
apertures mutually spaced along its length, the mutual spacing between 
adjacent apertures of said row decreasing along the length of the row and 
being a maximum value at one end of the row and a minimum value at the 
other end of the row. 
Where the convolute coil is radially drilled at one position only around 
its circumference, the mutual spacing between each pair of adjacent 
apertures, except the last pair of adjacent apertures whose mutual spacing 
is said minimum value, will be greater than the mutual spacing between one 
of the neighbouring pairs of adjacent apertures, so that the mutual 
spacing between adjacent apertures of the row decreases throughout the 
length of the row. 
Where the convolute coil is radially drilled in two or more 
circumferentially spaced positions around its circumference, the row of 
apertures in the outer conductor will be sub-divided along its length into 
a plurality of sub-groups of apertures, the mutual spacing between 
adjacent apertures in each sub-group of apertures being substantially 
constant and the mutual spacing between adjacent apertures of one 
sub-group of apertures, except the last sub-group of apertures in which 
the mutual spacing between adjacent apertures is said minimum value, being 
greater than the mutual spacing between adjacent apertures of one of the 
neighbouring sub-groups of apertures, so that the mutual spacing between 
adjacent apertures of the row changes at spaced positions along the length 
of the row, and the mutual spacing between adjacent sub-groups of 
apertures decreasing from one end of the outer conductor to the other. 
The improved high frequency coaxial cable of the present invention has the 
important advantage that the apertured outer conductor is graded to 
compensate for a gradual decrease in strength of a transmitted or received 
signal along the cable length due to attenuation.

The high frequency coaxial cable shown in FIG. 1 comprises an inner 
conductor 1, an extruded layer 2 of plastics insulation, an outer 
conductor 3 formed of a longitudinally applied, transversely folded metal 
tape, and an outer plastics sheath 6. The outer conductor 3 has extending 
throughout its length a single row of circular apertures 5, the mutual 
spacing between each pair of adjacent circular apertures, except the last 
pair of adjacent circular apertures whose mutual spacing is a minimum 
value, being greater than the mutual spacing between one of the 
neighbouring pairs of adjacent circular apertures so that the mutual 
spacing between adjacent circular apertures of the row decreases 
throughout the length of the row. 
The second form of high frequency coaxial cable shown in FIG. 2 comprises 
an inner conductor 11, an extruded layer 12 of plastics insulation, an 
outer conductor 13 formed of a longitudinally applied, transversely folded 
metal tape, and an outer plastics sheath 16. The outer conductor 13 has 
extending throughout its length a single row of circular apertures 15. The 
row of apertures 15 is sub-divided along its length into a plurality of 
sub-groups 14 of circular apertures, each sub-group consisting of four 
circular apertures. The mutual spacing between adjacent apertures in all 
of the sub-groups is substantially constant and is the same. The mutual 
spacing between each pair of adjacent sub-groups 14, except the last pair 
of adjacent sub-groups whose mutual spacing is a minimum value, is greater 
than the mutual spacing between one of the neighbouring pairs of adjacent 
sub-groups so that the mutual spacing between adjacent sub-groups of 
apertures decreases at spaced positions along the length of the row. 
FIG. 3 shows diagrammatically apparatus for use in a preferred method of 
making an apertured metal tape for use as the outer conductor of a third 
form of high frequency coaxial cable. The apparatus comprises a rotatably 
driven shaft 21 on which is mounted a pair of support plates 23 between 
which a convolute coil C of metal tape is clamped by means of a bolt 22. 
Associated with the shaft 21 is indexing mechanism 25 comprising a toothed 
wheel 26 coaxial with the shaft and a spring-loaded pawl 27 for engaging 
the toothed wheel 26 to limit the extent of rotation of the shaft 21. A 
rotatably driven drill 28 is mounted radially with respect to the 
convolute coil C. 
In use, with a convolute coil C of metal tape clamped between the support 
plates 23, at each of a plurality of uniformly spaced positions around the 
circumference of the coil, a hole is drilled radially through the adjacent 
turns of tape to form a circular aperture in each turn. The tape so formed 
will have a single row of circular apertures extending throughout its 
length, the row of apertures being sub-divided along its length into a 
plurality of sub-groups of apertures. The sub-groups of apertures will 
each consist of the same number of apertures and the mutual spacing 
between adjacent apertures in each sub-group will be substantially 
constant. The mutual spacing between adjacent circular apertures of one 
sub-group, except the last sub-group in which the mutual spacing between 
adjacent apertures will be of a minimum value, will be greater than the 
mutual spacing between adjacent apertures of one of the neighbouring 
sub-groups of apertures, so that the mutual spacing between adjacent 
apertures of the sub-groups changes at spaced positions along the length 
of the row. The mutual spacing between adjacent sub-groups of apertures 
decreases from one end of the metal tape to the other. 
To form the outer conductor of a high frequency coaxial cable, the 
apertured metal tape so formed is applied longitudinally to, and is 
transversely folded around, the insulated inner conductor of the coaxial 
cable.