April 1946 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
If anything qualifies for meeting the criteria of the old adage
that says "Necessity is the mother of invention," it is coaxial
transmission cable. Wireless communications during World War II
was the necessity that drove the rapid development and continuous
improvement of coax. Other than materials technology for wire, dielectric,
protective jacket, etc., the basics of coax cable have not changed.
It was during the war that polyethylene was developed and adopted
as a dielectric material much superior to previously used
copolene. Understanding of how electromagnetic fields propagate
within and, under non-ideal conditions - on the outside of the cable
has increased significantly thanks for refined theory and high speed
computer simulations. RG-58 were early 50 Ω coax types,
and RG-59, RG-11, and RG-6 were early 75 Ω coax types
that are all still in significant use today.
Flexible Coaxial Cable
New Developments in Solid-Dielectric Lines
By Ringland M. Krueger *
Many electrical components were improved considerably during
the accelerated development and production programs of the war,
and modern semi-flexible solid-dielectric coaxial cable is an outstanding
example. Expedited by the close joint cooperation of the various
manufacturers and the armed forces, under the leadership of the
Army-Navy R.F. Cable Coordinating Committee, a number of types were
developed and produced in large quantities, to the close mechanical
and electrical tolerances necessary for full interchangeability
of sections of like type designation. These cables had to withstand
the extreme cold encountered at high altitudes, the terrific heat
in mobile equipment stationed on a desert or on a South Pacific
island, the bending experienced in the turret of a tank or the scanning
radar antenna in a bomber, and the terrific shock when mounted on
a battleship firing a full salvo.
Many a ham was impressed during the war with the wonderful
advances that were made in flexible coaxial cable, and many
a ham now has an eye on some for his new or revamped rig. Here
is a story for the fellows who didn't have a chance to become
acquainted with it during the past few years.
The first flexible coaxial cable was made with bead spacers,
but there was always the definite danger of moisture collecting
within the cable and seriously reducing the breakdown voltage. For
example, when a fighter plane or bomber climbs rapidly from near
sea level to a high altitude, the temperature of the plane may drop
as much as 100 degrees F. in a matter of minutes, and this sudden
reduction in temperature causes condensation of any water vapor
present in the air within the cable. It was, therefore, imperative
that a solid-dielectric coaxial cable be developed, to eliminate
all air spaces between the center conductor and shield. The first
solid-dielectric semi-flexible cable, meeting the electrical and
mechanical requirements of the Services prior to Pearl Harbor, used
a dielectric called "copolene." Early in the war, through the joint
cooperation already mentioned, "polyethylene" was developed and
adopted as the dielectric material.
Polyethylene has a host of desirable characteristics as a dielectric
for solid coaxial cables. It is very flexible under extreme cold
and readily passes a cold bending test at temperatures as low as
-40 degrees C. It has a high softening point and will withstand
temperatures as high as 185 degrees F. Of paramount importance,
however, is its excellent low power factor of 0.0003 to 0.00045
and its dielectric constant of 2.29, the combination of which produces
a desirable low loss factor. Polyethylene is not affected by acids,
alkalis, aviation gasoline, oil, hydraulic brake fluid or sea water.
There is no known solvent for polyethylene at ordinary temperatures.
From an electrical standpoint, polyethylene is almost comparable
to polystyrene and it has the advantage of flexibility. Solid polyethylene
coaxial line has more constant impedance than washer-spaced lines,
because the dielectric material is homogeneous and of constant
diameter over the entire length of the cable.
The center conductor of solid coaxial cable is made of either
solid or stranded copper wire, and in some lossy applications a
nichrome-wire inner conductor is used. A polyethylene sheath is
extruded over the inner conductor, and several inspections are made
to insure constant diameter and permissible eccentricity. The eccentricity
is tested by X-ray methods and is held below 10 per cent. For the
smaller cables, this is only 0.0055 inches!
The outer conductor is then woven over the core. This conductor
is usually one or two layers of bare copper braid, but for some
special applications the braid is made of tinned-copper or of a
silver-coated braid covered with a plain copper braid. The braid
in turn is covered with a vinylite jacket. Vinylite - or "vinyl"
- is a plastic material that is unaffected by oils, gasoline, hydraulic
brake fluid, water or sunlight, and its weatherproof qualities have
proved themselves many times in the past few years. Some cables
for military applications are made with an additional steel-braid
armor jacket over the vinyl, but these are of limited interest to
One great advantage of solid coaxial cables like those described
above is that they are very easy to install, and can even be buried
in the ground without additional treatment. Extended tests on cables
buried underground for fifteen months, in a naturally low spot where
water or frost was a constant threat, showed no ill effects on the
cable either mechanically or electrically. Actually, no change in
the electrical characteristics could be detected with very precise
Table 1- List of Standard Coaxial Cables
Velocity factor is s term used to describe the decrease in velocity
of transmission in a coaxial cable and is expressed as a percentage
of the velocity in free air. In other words, if r.f. energy has
a wavelength of 100 cm. in air and only 66 cm. in coaxial cable,
the velocity factor is 0.66. A method of determining this factor
is to select a piece of cable and connect a small half loop between
the inner and outer conductors at one end of the cable, leaving
the other end of the cable open. The half loop is then coupled to
a grid-dip meter and the lowest frequency is found that will dip
the meter. The cable is an electrical quarter wavelength at this
frequency, and the velocity factor can be computed from
F = velocity factor
= frequency in Mc.
L = length in feet
If the far end is short-circuited, the length becomes a half
wave and the above formula requires a constant of 492 instead of
246. The coupling loop must be made as small as possible consistent
with sufficient coupling to the grid-dip meter, or else a small
error will be introduced. Incidentally, this grid-dip meter method
is an excellent one for checking the electrical length of a quarter-wave
piece of coaxial line used as a matching transformer between the
radiator in a close-spaced array and a higher-impedance transmission
Coaxial cables suitable for feeding antennas are available in
impedances of 50 and 70 ohms. Wide variations in physical size and
power capabilities are available, as can be seen from Table I, a
compilation of the standard cables now available and most likely
to be used by amateurs. The proper selection of cable types depends
upon the requirements, such as the operating frequency, desired
impedance, power level and the length of the cable run (attenuation).
A low-capacity line is available. It has a novel type of construction
that uses a small thread of polyethylene spiralled around the center
conductor to act as a supporting medium in a tube of polyethylene.
The conventional shield braid is placed over this, along with the
normal jacket. This particular type of line is used where low capacity
and not constant impedance is the important factor.
"Twin Lead" Parallel Line
A new development of great interest to the amateurs who have
tested it is Amphenol's polyethylene insulated "twin-lead" line,
which is manufactured in impedance values of 300, 150 and 75 ohms.
The 300-ohm line was brought out under recommendations of the R.M.A.
as a standard impedance for use with television and facsimile receivers.
Amateurs, always quick to adopt new ideas, have utilized "twin-lead"
for feeder applications in the bands now open. This line was not
brought out with the thought in mind of supplying the amateurs with
a transmission line, but reports have been received indicating successful
performance of the 300-ohm line with 500 watts of power at 30 Mc.,
where the standing wave ratio was less than 2 to 1. The attenuation
of the 300-ohm line is 0.88 db. per 100 feet at 30 Mc., and its
velocity factor is 0.82. This line is excellent in dry weather,
but being an open type of line some change in impedance is to be
expected when frost, condensation, or rain collects on the line,
or when it is run closer than several inches to metallic objects.
It is therefore recommended that, for the best all-around operation
in any type of weather, a coaxial line be used in place of any parallel-wire
type of line. However, many amateurs may feel that the lower-priced
"twin-lead" line will be satisfactory for his operations, and this
is quite true provided he is familiar with its limitations.
Coaxial cables have been used very extensively throughout the
entire war period and have been most satisfactory under all possible
conditions. It is felt that the amateur will use more coaxial cable
for links between the various stages of his transmitter and for
his antenna feed. He will find that he has not only an excellent
antenna set-up, but that regardless of weather conditions he will
not be bothered by flash-over of his feed line while "on the air."
* American Phenolic Company, Chicago, Ill.
1 - Mix, "A Low-Power 28-Mc. 'Phone - C.W. Transmitter," QST,
Posted April 4,2016