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Velocity Factor and resonant frequency
K7ITM wrote:
Some more info he http://www.answers.com/topic/goubou-line If you search for Goubou line AND Goubau line, you can find lots more. Older editions of "Reference Data for Radio Engineers" had design info on them. The losses, as with any good line, are mainly due to I^2*R loss in the wire. The current is lower than it would be for the same wire in coax, for a given power, and thus the loss is lower. "YMMV" when it rains, or when the line gets coated with soot and grime. I believe I've seen it described as "quasi-TEM". Clearly if you look immediately next to the wire, you'll find magnetic field symmetrically encircling the wire, and electric field is always perpendicular to good conductors so the electric field is radial. That's the same as in coax, but if you look at the article Tom posted a reference to, you'll see that the field lines do not remain perpendicular to the wire further out. According to Goubau's original paper [1] the mode is a surface wave which is attached to the wire, but decays over a distance of a few wavelengths sideways from the wire. Unlike a normal TEM wave, the energy in this surface wave remains confined to its cylindrical near field, and does not radiate into the far field. Its only direction of long-distance propagation is along the wire, which is what makes it usable for transmission-line purposes. The surface wave also has the rather odd property that on a bare wire of infinite conductivity, it will not propagate at all! However, it will propagate successfully if the wire is coated with a magnetic or a dielectric material, and for practical applications Goubau favoured various forms of insulated wire. The practical problem is that the surface wave requires a feedhorn of several wavelengths in diameter, to selectively excite this particular mode without also exciting the radiating TEM mode. Out along the wire, any disturbance to the propagating fields tends to cause mode conversion back into TEM, which makes the G-line revert to radiating like any normal wire antenna. When the wire is viewed as a transmission line, any far-field radiation represents a loss. To sum up, the G-line surface wave is very different from the normal TEM waves around an isolated wire. Unless you take specific steps to excite this particular mode, it won't occur at all, so it isn't relevant to the main topic under discussion. [1] Georg Goubau, 'Single-conductor Surface-Wave Transmission Lines'. Proc IRE, June 1951, pp 619-624. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
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Velocity Factor and resonant frequency
Ian White GM3SEK wrote:
snip To sum up, the G-line surface wave is very different from the normal TEM waves around an isolated wire. Unless you take specific steps to excite this particular mode, it won't occur at all, so it isn't relevant to the main topic under discussion. Ian, Thanks for the explanation. It was very helpful as well as concise. And I didn't think it had much to do with the main topic when I diverted to this one, except for the phrase "single wire transmission line", but that never stopped anyone else here before! tom K0TAR |
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