On Tue, 05 Apr 2005 18:07:57 -0700, Roy Lewallen
wrote:

K7ITM wrote:

. . .
But beware that you are more
likely to have dielectric loss in open-wire line for a variety of
reasons...
. . .

Yes, this is something I didn't mention and should have. My statement
about the negligibility of dielectric loss below 1 - 10 GHz is strictly
true only for coax with decent (common) dielectric material (e.g., PE or
PTFE). When the impedance is higher, as it is with ladder line, the
effect of the dielectric loss is proportionally higher. On the other
hand, a good part of the ladder line field is in the air (although it's
most intense directly between conductors, where any insulation typically
is), which reduces the effect of loss in the dielectric.

Many years ago I measured the attenuation of some common 300 ohm TV
twinlead, and found that in some cases when wet its attenuation could
exceed that of RG-58 coax. The extra loss is intirely due to degradation
of the quality of the dielectric between conductors. See
http://eznec.com/Amateur/Articles/Po...Feed_Lines.pdf. I know Wes
has done similar measurements on window line and has posted the results
at his web site; perhaps he'll remind us again of the URL.

Su

http://users.triconet.org/wesandlinda/ladder.htm

Wes