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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 |
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