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 Reading this article about the
G-line antenna lead-in in a 1955 issue of Radio & Television News magazine,
I wonder whether the
Broadband over Power Line (BPL) system designers
have considered its use. According to author Leonard Lieberman, the presence of
insulation over the conductor prevents or at least significantly reduces the amount
of radiated energy, thus lessening not just the signal loss but also the unintentional
interference with unrelated receiving equipment - such as the well-publicized
impact on amateur radio. The
David Bogen Company,
of New York City, (now in New Jersey) marketed G-Line under under license by Surface Conduction, Inc.
Current BPL systems are typically under 100 MHz, so the scheme should be
applicable. The logistics of implementation might be complicated too much
because of discontinuities at insulator attachment points and most particularly
passing through line transformers. One of the prime organizations researching
power line carrier (PLC) issues today is the
Open PLC European Research Alliance (OPERA). U.S. attempts
at implementation of BPL has been vehemently resisted by the ARRL because
most - if not all - installations to date have failed to meet unintentional
radiation standards agreed to per licensing contracts.
See also The
"G-Line" Community TV System in the November 1956 issue of Radio &
Television News.
Unique Transmission Line featuring Low Losses over Long Lengths at U.H.F. is
Excellent for Fringe Areas.
By Leonard Lieberman
 An unusual type of antenna lead-in which
may be found useful for difficult u.h.f. installations is called "G-Line," after
its inventor, Dr. G. Goubau. It is shown in Fig. 1. This new lead-in works on the
surface-wave transmission principle.
It is well known that a single wire in air which has a voltage impressed on it
will radiate energy. This, of course, is basic transmitting antenna theory. Dr.
Goubau, in an effort to develop a simple super-high-frequency wave guide, found
that when a single wire is coated with a dielectric material, some interesting things
occur. The first of these is that the wire becomes nonradiating. As a result, virtually
all the energy impressed at one end of the wire is recovered at the other end.
The second result noted is that as against the 72-ohm impedance of a coaxial
cable of the same size, the impedance of the "G-Line" is approximately 300 ohms.
This gives a better match to most antennas which are designed with a 300-ohm output
impedance.
Another result is that since there is only one wire in the system, as compared
to two in the twin-lead lead-in, the wire loss is half that of the twin-lead. A
further benefit comes from the fact that just as the wire does not radiate, conversely,
it cannot pick up external noises as might come from faulty electric motors, car
ignitions, etc.
Figure 1 give a view of the launchers used at the antenna and house end
of the "G-Line."The wire itself is strung directly from the antenna to the house
entrance of the lead-in wire; the launcher sends the received signal down the wire
in the method peculiar to this type transmission line. Figure 2 details the
launcher cone.
To understand why these conditions prevail, let us examine the structure of the
unit. If we consider the inner surface of the dielectric coating on the wire as
one side of a capacitor and the outer surface as the other side, the amount of radiation
would be a function of the leakage current through the dielectric. If the material
has a good dielectric constant, this leakage is small and the radiation field is
small. It has been found that this field can be kept within one-quarter wavelength
from the center conductor.
The construction of the line makes it virtually unaffected by rain or layers
of soot. In all two-wire systems, these installation bugaboos create low-impedance
paths between the two lines. This is true to a greater or lesser extent of all two-wire
systems. In the "G-Line," all that occurs is that the dielectric is increased with
a very small increase in the loss resulting from additional dielectric currents.
This effect is hardly measurable with laboratory-type equipment.
To develop the proper wave transmission along the line, special "launchers" are
required. These launchers are hollow cones. The large diameter of the cone is determined
by the desired size of the field around the wire and by the lowest operating frequency.
(It might be noted that these launchers, because of the last-named requirement,
also act as high-pass filters aiding the set in the rejection of interfering signals
below the u.h.f. band.)
The launchers (Fig. 2) are 17 inches long and 914 inches in diameter at the large
opening. A nonconducting cone is attached over this opening to keep the launcher
weatherproof. The dielectric-covered wire is connected to a "balun" (balance-to-unbalance
transformer). This balun has a flat standing-wave ratio over the entire u.h.f. band.
The other end of the balun is connected to the short 300-ohm lead from the antenna,
or the 300-ohm lead into the building.
The loss caused by the two launchers and the two baluns is approximately one
db total and is constant over the entire u.h.f. band. The loss in the wire is 1
db per hundred feet. The dry loss in the best of the two-line systems is 1.9 db
per hundred feet. Some elementary arithmetic shows that for runs over 125 feet with
dry lines, the total loss of the "G-Line" becomes progressively smaller than the
loss in the best two-wire system. When the comparison is made with new ribbon line
of the type usually used, the "G-Line" is more efficient even for runs as short
as 90 feet when dry and 30 feet when wet.
These facts indicate the potential of the "G-Line" in fringe reception areas.
At u.h.f., one of the limiting factors on the antenna tower height or distance from
the house is the loss in the lead-in wire. This loss makes it inadvisable to take
advantage of sturdy trees or a hill which might be more than a very short distance
away.
With the "G-Line," it is possible to run the line from a point 300 to 500 feet
from the house without any more loss than would be sustained with 100 feet of currently-used
wire. Since the line is made with #14 AWG wire covered with 0.060" of polyethylene,
it can be strung for as far as 500 feet without requiring any supports. If it is
desired, the line can be supported by nylon string (such as used in fishing line)
as long as the wire isn't bent more than 30° at any point. The launchers and
wire are connected as shown in Fig. 1. The wire, itself, should be kept at least
one foot from the house or the ground.
One particular application of this transmission line is in installations where
the house is "shadowed" by a hill or obstruction. This line allows the planting
of the antenna mast at any convenient high location.
"G-Line" is presently being manufactured and marketed by the David Bogen Company,
Inc., of New York City, under license from Surface Conduction Inc.
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