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 since this article
appeared in a 1946 issue of QST magazine. 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
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.
a = outside radius of inner conductor
b = inside radius of outer conductor
c = speed of light in
a vacuum = 299,792 km/s = 186,282 mi/s
ε = dielectric constant = ε0
= permittivity of free space = 8.85419x10-12 F/m
= relative dielectric constant
μr = relative permeability
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.
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 homoge-neous and of constant diameter over the entire length of the
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 amateurs.
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 measuring equipment.
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
f = 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 line.
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 spiraled 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, March, 1946.
Posted September 6, 2022
(updated from original post