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