Crazy George wrote:
"Hal Rosser" wrote in message
news
I recall reading about an impedence matching technique where the conductors
of, say a coax segment, would be spaced slowly more and more apart (like the
braid becoming larger like the shape of a funnel) and extending until only
one conductor (the center conductor) is required for transmission. A similar
setup at the other end of the run would transform the impedence back to
usable levels.
The drawings in the article portrayed the transformation as appearing
like a funnel flaring out slowly with the center conductor eventually the
only conductor - then the same setup at the other end.
The idea of a single-conductor transmission line makes it an inviting
idea, but physically building the impedence matching sections on the ends
look like a real challenge.

Google "G-Line".

That's the name. It was once promoted as an alternative to large
waveguide for UHF transmission, until low-loss coax came on the scene.

Unfortunately G-line has a number of practical problems, all involved
with keeping the propagating EM field attached to the single wire, and
preventing it from radiating like a long-wire antenna. If the line
radiates, that energy fails to reach its destination at the other end -
which of course means loss.

I don't understand the detailed EM physics, but G-line does not use the
same TEM mode as coax or parallel line. Basically, Nature never intended
an EM field to propagate along a single wire, so it's always trying to
fall off and radiate. To keep the field attached, the wire needs a
fairly thick, low-loss dielectric covering (bare wire won't work). The
line also needs to be straight and uninterrupted, so practical
installation is extremely critical. G-line is always trying to become a
long-wire antenna. Make any mistakes, and that's exactly what you'll
get.

Coax of course is the complete opposite, so Heliax and other forms of
low-loss hardline have wiped G-line off the map. For many years it has
been nothing more than a technological curiosity.

But now it seems to have made a reappearance as a carrier for BPL. Yet
the same basic problem is still the in any practical installation,
G-line is *always* trying to become a radiating long-wire antenna.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek