'Spirlling' was done to 'reduce' the effect the feed becoming part of the antenna.
Back in 300 ohm Ribbon tape days this was done as well as reducing the effect the metal mast had with up to 60 feet of tape running paralel with it.
According to our now long past local installer, he said the feed line became part of the antenna in areas where the signal was Vertical as it was here and in some cases, as well as 'sprilling' the tape, he put it 'behind' the mast to reduce its abilty to 'receive' a signal.
Wave lengths of feed line were also part of reducing interference too.
It was common here for masts to be from 30 to 60 feet high with the odd beauty pushing 80 feet, Huge Multi element single or stacked Yagis or High Gain single or Dual phased arrays and MHA's used.
I dont ever remember seeing Co-Ax used except near the coast, it was standard Black or a perforated 300 ohm ribbon or rarely that ladder line which was fairly expensive and harder to install.
I have even seen some put insulators on the guy wires to break up the 'wave length' effect.
Remember all Open Lines are antennas and it was that problem that lead to the discovery of Stellar radiation on long distance telephone lines that became the Radio Astronomy of Today.
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