[TowerTalk] Balanced Transmission Line

Rob Atkinson ranchorobbo at gmail.com
Sat Feb 24 07:29:16 EST 2018


>I've only been able to find 450 ohm line, commonly called "ladder line" that
>has a significant amount of dielectric material between the conductors.


The term for that is "window line."  The line consisting of parallel
conductors with spacers is "ladder line."   The other design that
sounds like what N6RK uses, in which the line is spaced apart by
tension on each conductor is simple "open wire line."  But the terms
are frequency used incorrectly for balanced line.  The dinky TV 300
ohm line is usually called "twin lead."

By the way, open wire line can be both balanced and unbalanced.  You
can achieve the geometry of a coaxial unbalanced line with a center
conductor and four outer conductors equally distant from each other
and the center.   This was used by broadcast stations when the load
was unbalanced before pressurized coax became common.   Open
unbalanced line works but only when the frequency is low.  I don't
know exactly at what frequency its loss goes up.  It is probably a
function of wavelength v. space between line conductors.

If you need the open line for 160 m., open unbalanced may be an
option, but constructing the supports with the insulated cable holders
spaced appropriately may be difficult.

If you want to get balanced line to work with unbalanced loads and
want to avoid balun loss, you can consider a remote controlled
balanced to unbalanced network at the end of the line.  The trick is
transforming the line's characteristic Z to the impedance of the load
across a range of frequencies but it can be done.

By the way, hamfests are a source for old ceramic balanced line
spacers and there is an on-line impedance calculator that tells you
what your line Z will be based on spacing between conductors and their
diameter.
http://www.smrcc.org.uk/tools/OpenWire.htm

Ceramic spacers are heavy but you don't need as many.  The conductors
don't go through the holes on each end but are wrapped against the
ends of the spacer with a short piece of wire that passes through the
hole.

73

Rob
K5UJ


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