Dave Oldridge wrote:
Thomas anonymous wrote in
:


How critical is it to mount twinlead feedline (300 ohm TV ribbon type)
so that it doesn't come into contact with anything? Obviously, with


You don't want unbalanced coupling between one side of the feedline and
something else. That would unbalance the feedline. You MAY not want the
impedance bump that comes with equal coupling on both sides, though that
might be tolerable if you're tuning the line anyway. And you don't want
lossy coupling of any kind.

In general there is a magnetic/electric field between the conductors in
twinlead feed line. If this field is in contact with a metal or other
conductive surface it will distort the impedance of the wire. Twinlead
with the round surfaces between the conductors will keep the field out
of the way of any surface the twinlead is resting on. This helps to
maintain the constant 300 ohm impedance of the twinlead. That is why tv
leadin uses standoffs to restrain the wire from the antenna to the
receiver/transmitter.
There is nothing wrong with using twinlead. It's cheap, light weight and
easy to install.

Dave WD9BDZ