Antenna Question.

**porkop** · 27-02-11, 01:06 PM

I have a HF dipole feed with 450 ohm ladder line. This is then coupled to a remote autotuner. Bit like a G5RV but not.

Works on all bands 160m thru to 6m with the tuner.

My question. I noticed today that the ladder line twists from the bottom (tuner) to the top, Dipole apex. So I stupidly wondered if the spiralling of the ladder line would have an effect on the propergation charastics of the whole system given that the ladder line probaly acts as a vertical radiator on some bands?

Penny for your thoughts??

**gordon_s1942** · 27-02-11, 02:59 PM

'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.

**tytower** · 23-03-11, 08:18 AM

I probably differ a little here from the previos poster.

An open feeder like you have should have a consistent air gap from one wire to another.that gap provides the reactance of the feeder at various frequencies .That reactance is different at different frequencies. On a dipole an air gap of 1 inch provides roughly a 50 ohm reactance value which matches most trancievers.

While the spacing is accurate there will not be any signal received on or emmitted by the feeder. A coaxial cable is the same and the insulated center core is surrounded by a consistent gap in this case plastic filled , from the shield . Surprisingly the feeder cone on a satellite receiver operates on the same principle.

Twisting of that feeder will not affect this in any way. Placing the feeder close to any other metal object will however interfere with the reactance of the feeder at those points with unpredictable results. So hiding it behind a pipe would not be doing much even if it might seem to work.

Those old high TV antennas have to run the coax or feeder somewhere so it seems sensible to run it parallel to the pipe with a consistent gap and have it attached rather than loose and changing reactance. . The guy wires can be insulated but then they form radiators of whatever length they are cut to and will absorb signal on their tuned frequencies and on harmonics thereof and re-radiate that signal at those frequencies with a time separation or phase difference that may enhance or degrade the main antennas signal.

The yagis on the pole are all earthed. Even the driven element. So the guy wires being attached without insulators become part of the whole apparatus. It makes for interesting fun to get your yagi working at its potential best and remember put a little power out watch the SWR meter and swing the VFO . where power out reaches its highest is where the antenna system as a whole is suited to and that is how you adjust the length of the driven element . The other dimensions are all kept consistent as percentages of the driven element. Eventually you will get it pretty close to where you want it.

**gordon_s1942** · 23-03-11, 10:53 AM

tytower, your correct that 'twisting' or 'spilling' didnt affect the gap either between the conductors nor the mast rather this was done to 'break up' the long length of the tape.
Around here some ran some very long (500 metre plus) open copper lines to antennas located on the surrounding hills and I was told the optimum gap for this single strand hard drawn copper wire was 4 inches.
On a mast the tape was run through 'Stand offs' about 4 inches from the mast, co-ax was just taped to the mast as it is today.
If you tried that with 300 ohm, you would have lost most of the signal.
No sharp bends in the tape and kept away from all gutter edges.
I found a similarity in building of transmission lines for both High Voltage and RF.
The way I read the information was that although RF has High Frequency/low 'current, the HV line with the reverse performs in very similar ways regarding arc over,line loss and terminations.
And line loss is what its all about.
I once ran some 60 metres of 300 ohm perforated tape to a neighbors TV and it worked untill it rained.
Another experiment was with some damaged Co-Ax, ie water had entered the case and the braid was 'rotting' and went high resistance so I cut it in half, laid the 2 pieces side by side, connected up the centre wire and it worked.
To me transmission lines are a fascinating and a vital part of any antenna system but are too often igored (I plead guilty) .
I currently have 115 metres of RG6 quad sheild enclosed in black plastic garden pipe suspended from a wire running through a three trees and along the boundry fence from my antenna to the house.

**mtv** · 23-03-11, 01:33 PM

Balanced, open feeder does not radiate or act as part of the receiving antenna, when it's correctly matched to the antenna and transceiver.

Many AM broadcast stations used open-wire feeder, run horizontally to the vertical mast. There are some AM transmitters still using that type of feeder system.

Open feeders are usually required to be run at right angles away from the antenna feedpoint.

The length of the feeder can be important, so as it matches the antenna impedance. This requirement will depend largely upon the antenna design.

The twisting/spiralling of balanced feedlines is to reduce interference, typically with TV ribbon cable, as it usually runs parallel to the mast, not at right angles as it would with a horizontal long-wire dipole.

The signal behaviour with open wire feeders varies too, between MW/HF and VHF applications, with VHF being less forgiving with impedance matching, multipath signals, etc.

**gordon_s1942** · 23-03-11, 05:03 PM

I remember 2KA Katoomba had a pair of 'Open' wires from the Transmitter Building to the 'Antenna'.
In those days it was 2 masts with the antenna strung between like you see on a ship.
2LT Lithow was a single lattice type mast which was the antenna but I cant remember seeing the 'Feed'.
The 'New' one relocated to Springvale lane uses 2 masts fed by a cable underground to 'Beam' the signal because of propogation problems.
I wouldnt necassarily understand the technicalities but I have always wanted to be given a tour of a broadcasting site to see how its all set up.
Those two words, 'Balanced and Correctly Matched' is what its all about.