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
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are 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 antenna.
2)Transmitter output connector.
50-ohm coax (T-4-50 or RG-8A/U).
4)Type N connectors or flexible-cable
to solid-sheath splice.
5)Solid-sheath foamed-dielectric cable
6)Type N connectors or flexible-cable
to solid-sheath splice.
7)Flexible coax (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 Fig. 1.
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
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
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 center conductors.
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
High-molecular-weight, 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.
Attenuation is 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. is achieved.
In 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.
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 Fig. 4.
Using Semiflexible Coaxial Cable
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 also available.
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.
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 your antenna.
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 run.
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.