[Towertalk] Ladder Line Twist

[Tom Rauch]

> Back in the time of outside TV antennas and 300 ohm twin lead, we
> always put some twist in the twin lead.  Forgotten the exact reason
> but that is what we did.

There are two electrical reasons unshielded lines should be 
twisted. Both are caused by the fact induction and radiation fields 
extend outside the line.

One requirement for twist is so each conductor is exposed to the 
environment around the line the same. Twisting the line averages 
out external unbalance over the length of the line, and helps the 
line stay balanced. In this case the twist-to-twist distance should 
be more frequent in distance than the spacing of the line from 
things that might unbalance it. 

That's why a small RF transformer with twisted two-wire line laying 
next to other windings must have many twists-per-inch, and the 
same wire in open air will remain balanced with very few twists per 
inch.

The second requirement is to prevent radiation. Even if the wire is 
held symmetrical to other objects, or is a large distance from them, 
the line still has a field outside of the line. The field maximum 
extends in a plane directly in a line through the two conductors, 
and is minimum at right angles to the wide dimension of the line.

If you do not twist the line at many fractions of a wavelength, that 
radiation will not cancel no matter how carefully you balance the 
line.

(One way to look at this problem is the line acts like a very close-
spaced two-element antenna, with each element 180 degrees out-
of-phase.)

Even the old open-wire telephone lines were transposed 
periodically to minimize these effects.  
 
> Would that have any value if running say 450 ohm ladder line alongside
> a tower.  I know that proper spacing should be maintained but would
> the additional twist be of any benefit?

Yes, and it also helps mechanically.
73, Tom W8JI
W8JI@contesting.com