Low-Loss Coaxial Cable
it comes to low loss transmission media, it's hard to beat waveguide
and open wire. Open wire can exhibit less a couple tenths of a decibel
per hundred feet at low frequencies, but it is very susceptible
to perturbations from nearby objects, wind and moisture. Waveguide
exhibits a few tenths of a decibel per 100 feet at very high frequencies,
but it is expensive and difficult to work with. In the middle is
coaxial cable, which for a good quality product of appropriate size,
you can get very low attenuation. As with most things, you get what
you pay for in coax cable. I once used really expensive Andrew (now
Commscope) Heliax coax cable on an S-band radar (2.8 GHz) system
that had only a little more than 1 dB/100 ft, which was
necessary from a receiver noise figure requirement rather than for
transmitter power efficiency. This article from QST covers some
of the basics of low loss cable.
December 1966 QST
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.
Reduced Attenuation Through Improvements in Construction
By Freeman F. Desmond and Richard Tuttle
c/o Times Wire & Cable,
We have all used RG-8/U or similar cables for radio-frequency transmission.
Very often we have wished for a cable with lower losses and improved
power-handling capability - one that is also relatively easy to
install and reasonable in cost. Such a cable, with compatible connectors,
has been designed and is now available.
Fig. 1 - Relative loss distribution in coaxial cable as a function
Fig. 2 - Attenuation in decibels per 100 feet of cable vs. frequency.
Fig. 3 - Power-handling capacity as a function of frequency.
RG-8A/U coaxial cross-section.
T-4-50 coaxial cross-section.
1/2-inch Alumifoam (AM5012P) coaxial cross-section.
RG-17A/U coaxial cross-section.
Fig. 4 - Cross sections
showing construction of various types of cable.
Fig. 5 - Representative cable installation for a rotary beam
2)Transmitter output connector.
3)Flexible 50-ohm coax (T-4-50 or RG-8A/U).
4)Type N connectors
or flexible-cable to solid-sheath splice.
foamed-dielectric cable (1/2-inch Alumifoam).
6)Type N connectors
or flexible-cable to solid-sheath splice.
(T-4-50 or RG-8A/U).
To see how this improvement
is accomplished, we should first look closely at various coaxial
constructions and examine the factors which cause losses. The ideal
coaxial cable - an inner conductor suspended, by air alone, concentrically
inside an outer conductor - is for all practical purposes impossible;
actual coaxial cable must be manufactured with supporting material
or dielectric between the two conductors. The loss in percent of
total contributed by each of the cable components may be seen in
At 100 Mc. approximately 80 percent of the total
loss is copper loss in the center conductor. At lower frequencies
this percentage contribution to loss is even greater. Therefore,
the center conductor is the most important factor to consider in
reducing cable losses in a given frequency band.
The Center Conductor
It would be most desirable,
to achieve lower attenuation, to increase the size of the center
conductor. We do not, however, wish to change the characteristic
impedance nor contribute substantially to size or weight of the
cable. Since impedance is dependent upon the geometry of the cable,
where Zo = Characteristic impedance
= Dielectric constant
D = Dielectric diameter
= Center conductor diameter
merely increasing the size of
the center conductor would change either the characteristic impedance
or add substantially to the overall size and weight of the cable,
neither of which is desirable. The dielectric constant is the area
that we can change, if a material of lower dielectric constant can
be used practically.
Solid polyethylene (dielectric constant
K = 2.3) is the dielectric material of most coaxial cables. By changing
to foamed polyethylene (K = 1.5) we may increase the center conductor
size and lower the attenuation without changing the overall diameter
or the impedance. In this dielectric minute air bubbles are encapsulated
in the polyethylene during manufacture. Incorporating air bubbles
brings the finished product closer to the dielectric constant of
air (1.0), which is the goal, and achieves the lowest attenuation
characteristic. With cellular polyethylene, costly pressurization
of the cable is not necessary, as it is in the case of disk-supported,
helical-supported, or spline-supported semiflexible coaxial cables.
Figs. 2 and 3 show the attenuation and power capacity vs. frequency
of RG-8A/U, Times T-4-50 and Times 1/2-inch Alumifoam. The lower
attenuation is evident in the latter two because of increased size
of center conductor and changed dielectric constant.1
Fig. 2 also illustrates another point: When a stranded center
conductor is used instead of a solid, smooth, center conductor the
spiraling of the stranding results in a spiraling of the r.f. current
along the conductor, creating a longer r.f. path length. Coupled
with the higher resistivity of the center conductor because of the
contact resistance between the strands, this contributes to higher
attenuation in the finished cable. RG-8A/U has a stranded copper
center conductor, while T-4-50 and 1/2-inch Alumifoam have solid-copper
Another factor which may affect attenuation is the jacketing
material of the cable. Most flexible coaxial cables use polyvinylchloride
(p.v.c.) as the jacketing compound. However, to use p.v.c. - a relatively
hard, brittle substance - it is necessary to add plasticizers to
make the compound pliable and flexible. The nonresinous plasticizers
compounded with p.v.c. have a tendency, with sunlight and summer
temperatures, to leach out of the p.v.c. and migrate into the polyethylene
of the dielectric. The migration of the plasticizer through the
braid into the dielectric causes the dielectric constant and power
factor to rise, with a resulting rise in the v.s.w.r. and an increase
in attenuation. A rise in attenuation of 1 or 2 db. per 100 feet
is not uncommon, once contamination has begun. Also, with the migration
of the plasticizer the p.v.c. becomes brittle and nonpliable, resulting
in cracking and breaking of the jacket. RG-8/U, RG-11/U and RG-17/U
are examples of coax cables with contaminating p.v.c. jackets.
The path between transmitter and antenna can be a lossy one,
especially at v.h.f. and u.h.f. Every decibel lost subtracts
from antenna gain or transmitter output, so why lose any more
than is absolutely necessary? Here's a look at the characteristics
and application of some of the newer cables.
The dangers of this condition have been recognized, and in many
of the military cables, identical in every respect except for jacket
material, the older styles have been replaced by new ones. Cables
such as RG-8A/U, RG-11A/U, and RG-17 A/U use p.v.c. jackets with
a resinous plasticizer which does not leach out or migrate, and
thus does not contaminate the dielectric. Life expectancy of this
type jacket is in excess of fifteen years.
carbon-black-loaded, polyethylene jackets such as Xelon contain
no plasticizers of any kind, consequently a useful life of 25 years
or more can be expected. Because of this, polyethylene jackets permit
direct burial and are usually specified for submersible applications.
also increased by substantial v.s.w.r. Since v.s.w.r. is a function
of the impedance of a cable, it follows that the more uniform the
impedance the lower will be the v.s.w.r. (for a given termination).
Because coaxial cable is manufactured of plastic materials by means
of bulky extruders, it cannot be held to the tolerances of machined
parts, especially in lengths of many hundreds of feet. Each individual
extruder has its own peculiar eccentricities that cause variations
in the cable during manufacture.
These variations in dimensions
are very small but, unfortunately, sum up electrically along a length
of cable and, at specific frequencies, may result in a v.s.w.r.
as high as 4:1 even though the cable is properly terminated. In
cable constructions where impedance uniformity and low v.s.w.r.
are critical, the impedance can be held to tight tolerances by close
control of the extrusion processes.
Taking the foregoing into account, let us look at RG-8A/U, shown
in cross-section in Fig. 4. The center conductor is stranded copper
and the dielectric is solid polyethylene. The attenuation of RG-8A/U
could be improved by 25 percent if we could increase the center
conductor size and change to foamed polyethylene. This has been
done in cable such as Times T-4-50, now available at about the same
cost as RG-8A/U. Note that the overall diameter is the same, Fig.
4, but the attenuation is substantially improved (Fig. 2) and the
cable weight is improved (99 lbs./1000 ft. for RG-8A/U, 94 lbs./1000
ft. for T-4-50).
However, for longest life and most carefree
installation, even further improvements have been made. The largest
factor contributing to degradation of attenuation in foamed polyethylene
flexible coaxial cables, especially above 100 Mc., is moisture.
Since moisture affects the power factor, the effect of moisture
in the cable becomes significant as we increase frequency. This
can be seen from the formula for attenuation:
As frequency is increased, the power factor becomes a more significant
figure. Moisture has been known to degrade the power factor by as
much as ten times.
But how does moisture get into a cable?
It enters flexible cables as water vapor, which is a very penetrating
gas. This vapor condenses to water or moisture, changes the power
factor and consequently raises the attenuation. For this reason,
a solid, seamless, pinhole-free, metallic barrier or shield which
positively excludes water vapor gives the longest-lived cable. In
addition, with a solid metallic sheath the radiation into and out
from the cable is eliminated, and isolation the order of 100 db.
cables such as the Times Alumifoam series moisture is precluded
during manufacture by a completely dry core, and with the addition
of the aluminum tube the foamed polyethylene is under constant pressure.
Moisture traps and vapor paths are designed out, and the user has
a self-sealing cable.
For above-ground applications, the
seamless shield serves the dual function of electrical shield and
protective cover. It eliminates the necessity for an outer jacket
and thus represents the most economical use of weight and space
to achieve desired electrical characteristics. To approximate the
electrical characteristics of 1/2-inch Alumifoam in an RG cable,
it would be necessary to use RG-17A/U (attenuation, 0.85 db. at
100 Mc.; power handling, 3.6 kw. at 100 Mc.: cost, approximately
30 percent higher). Cross sections of the two types are shown in
System Installation Using Semiflexible Coaxial
Fig. 5 illustrates a typical system installation
employing 1/2-inch Alumifoam. The cable is simple to install, and
connectors are readily available for it.
It is generally
most convenient to run from the transmitter to the wall of the shack
with flexible coax (RG-8A/U or T-4-50), although to eliminate losses,
this run should be kept as short as possible. One end of this short
run should be terminated in a connector that will mate with the
transmitter, and the other end may terminate either in a type N
or go directly into a splice connector. Splice connectors to accept
flexible coax in one side and solid-sheath coax in the other are
The main feeder run should be cable similar
to 1/2-inch Alumifoam because of its low-loss characteristics. Since
the cable is designed to be bent upon installation, there is no
electrical or physical damage in bending it, even on a radius as
small as ten times the o.d. of the cable. The cable should be terminated
to match with the transmitter cable connector (type N or splice).
The type N connector is better than the PL-259 because of its lower
v.s.w.r., greater power-handling capability and improved radiation
characteristics. For still lower system v.s.w.r., the conversion
splice is the wiser choice.
The antenna end of the main feed
line should be terminated by the same procedure as the transmitter
end. Jumping from the solid-sheath cable to the antenna is accomplished
by means of a short length of RG-8A/U or T-4-50. The cable loops
once around the rotator and is terminated as you now terminate in
In conclusion, solid-aluminum-sheath cable
can be reasonably expected to deliver more power to the antenna
because of its low v.s.w.r. and lower attenuation characteristics.
Combining solid-sheathed, foamed-polyethylene dielectric, aluminum-shielded
coaxial cable and low-v.s.w.r. connectors gives a feeder connecting
system which, at v.h.f. and u.h.f., may deliver as much as 70 per
cent more power from transmitter to antenna in a comparatively short
1 T-4-50 is
a flexible cable with foamed dielectric and braid outer conductor;
Alumifoam is similar but uses seamless aluminum tubing as the outer
conductor. Both are made by Times Wire & Cable Div., International
Silver Co., 358 Hall Ave., Wallingford, Conn. 06492.--Editor.
Copyright: 1996 - 2024
BSEE - KB3UON
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