G-Line
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The G−Line (aka G−String) RF transmission system is a rather amazing invention contrived through out-of-the-box thinking by its inventor, Dr. George Goubau (from whence the "G" in the name derives). He determined that a sort of waveguide could be made with a single conductor surrounded by insulation with a specific dielectric constant that would cause the dielectric-air interface to reflect the wave in a manner similar to atmospheric channels that facilitate long distance communications. The G-Line is designed to efficiently transmit UHF television signals (470 - 806 MHz), and like a waveguide exhibits a lower cutoff frequency (~300 MHz), thus acting like a highpass filter. G−Line has its weak points, like that it must not come near to obstacles that will affect the dielectric-air interface or the line will radiate like an antenna. Also, over time the insulation cracks and/or absorbs moisture and changes the impedance parameters, thus affecting the transmission characteristics. History has shown that the G−Line never became a practical means for widespread commercial use even though the article states it would be good for hostile environments. G-Line
The launcher (from the antenna) and the catcher (at the window) match the G-line to the 300-ohm lead. Photos courtesy of David Bogen Co., Inc.  One of those fantastic scientific conceptions that seem fitted only for expression in long strings of mathematical calculations has come down to (or near) Earth, and may soon be seen on rooftops carrying u.h.f. TV antennas. The new science-fiction-like apparatus is the G-line being made and sold as u.h.f. lead-in by Bogen. To all appearances, it is a single wire ending in a horn-like device at each end. (See photos and Fig. 1.) But radiomen know that u.h.f. does not travel readily on a single piece of wire. It tends to radiate off the wire into space, so little gets to a point any distance along the wire. For u.h.f., very special twin-line, coaxial or other types of transmission lines are needed, and even their losses go up rapidly with frequency. Is this new line some special kind of wire? The G-line, so called after its inventor, Dr. George Goubau (Radio-Electronics, May, 1950, and June, 1951) is a very special piece of wire. The ultra-high-frequency currents travel, not in the wire itself, but in the insulation around it! (To be more exact, they are confined to the area around the wire by the difference between the dielectric value of the insulation and that of the air around it.) This is not as hard to understand as it may seem. We are all familiar with coaxial cable. At higher frequencies, the center conductor of the cable may be removed; and we have a waveguide. We know that u.h.f. current can be piped down waveguides without trouble. The G-line; in effect, removes the outer conductor, using only the inner one. The only difficulty is that we might expect the u.h.f. to radiate out in all directions from the wire, and that little would reach the end. This is just what does happen on an ordinary piece of wire. As the waves spread outward from the wire of the G-line, they are reflected back toward it again by the boundary between the insulation and air. The short waves (3-30 mc) are similarly confined to the area near the surface of the Earth by bending due to the thinning out of the atmosphere. Waves which are not too near vertical are reflected back toward the Earth, instead of going on into space.
Fig. 1 - Diagram of launcher and balun with lead-in and G-line connections.
Fig. 2 - Exaggerated illustration showing r.f. traveling between wire and surface of the insulation on G-line. Thus, in the G-line, signals travel much as in the exaggerated drawing of Fig. 2. The signals are picked up by an ordinary antenna with a balanced impedance of about 300 ohms. Therefore they must be launched onto the single (unbalanced) line. The launcher includes - at the narrow end, a balun (balance-unbalance transformer) and a gradually widening horn. The signals which may at first be inclined to treat the wire-horn combination as a new kind of coaxial finds the impedance rapidly going up as the horn widens, so that more and more of it follows the center conductor. A similar unit at the end where the lead enters the house transforms the signal back to a, balanced one and puts it on a standard 2-conductor 300-ohm line. The two matching units contribute a loss of only 1/2 db each, and the line itself has a loss of 1 db per hundred feet. Radiation and noise pickup are very low because of the self-contained nature of the line, and it has an almost complete cutoff below 300 mc. Insulation presents a problem. While the field falls off rapidly with distance from the surface of the wire, anything approaching it closely would cause severe losses. Therefore the line is supported wherever necessary by loops of nylon cord stretched between the ends of a small bracket, so that a minimum of solid material is brought near the line. Similar brackets are used wherever a bend is made, as the line must not have sharp turns, and 3 supports are needed for a 90-degree bend. The new line will be especially useful wherever long runs have to be made. It will also be valuable in bad-weather and industrial areas and salt-air installations, since moisture, soot, or salt do not increase its losses.
Posted May 18, 2022 |
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