Low-Loss Coaxial Cable
December 1966 QST
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.
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. As time permits, I will
be glad to scan articles for you. All copyrights (if any) are hereby acknowledged.
all available vintage QST articles.
Reduced Attenuation Through Improvements in Construction
F. Desmond and Richard Tuttle (W1QMR)
Times Wire & Cable, Wallingford, Conn.
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 of frequency.
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.
3)Flexible 50-ohm coax (T-4-50 or RG-8A/U).
N connectors or flexible-cable to solid-sheath splice.
cable (1/2-inch Alumifoam).
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.
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.
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
d = Center conductor
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
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
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.
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
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
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 Fig. 4.
Installation 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
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,