[Boatanchors] Fw: Slightly OT: What is the optimum "twistratio" inwindow-line

[Brian Clarke]

Hello Jim,

Sail manufacturers look at the effect of wind on furled sails. An 
uni-directional helically twisted piece of window line is exactly like a 
furled sail. We were discussing twist in window line, not in open wire 
feedlines.

Twisting of window line is done in an attempt to equalise any unbalancing 
possibility. This effect is enshrined in telegraphy wire stringing - that's 
where the theory was developed just over 100 years ago. The ARRL Antenna 
Handbook covers the installation of balanced feed lines, such as window or 
open wire, and makes no mention of windloading whatever. The only reference 
to the elements is in terms of ice loading. In Australia, we add concern for 
the effect of the sun in degrading the window material.

You can still see the twisting in overhead telephone wire stringing between 
poles, and in the latest Cat 5 and Cat 6 IT cabling - nothing to do with 
wind loading, I'm afraid, Jim.

73 de Brian, VK2GCE.

On Sunday, November 20, 2011 11:04 PM, Jim said:


> Proximity to other objects, particularly metal, will unbalance any
> window or open wire line no matter the orientation Brian.  Twist will
> not mitigate that unbalance, but it might minimize the exposure at a
> given point as you suggest.  Never the less the exposure is there and
> will cause an unbalance.
>
> Twist in the line was something my mentor and all his friends did back
> in the 40s and 50s to help minimize the exposure to the wind and
> equalize the stress on the attach points at the antenna.  The practice
> was suggested in discussions and literature on the antennas at the time.
> It may not have worked but that was the rational, and I wonder why wind
> sail manufacturers would test open wire line.  I know nothing about
> sailing so don't explore those sites.
>
> Jim/W5JO
>
>
>
>
>
> ----- Original Message ----- 
>
>> Not so, Jim.
>>
>> The purpose of the twist in window feedline is to balance the line
>> with respect to all possible unbalancing materials, such as metal
>> masts, metal downpipes, metal rooves and gutters.
>> Go to any wind sail manufacturer's wind tunnel results to see the
>> effect of uni-directional spiral twisted lines.
>>
>> 73 de Brian, VK2GCE.
>>
>> ----- Original Message ----- 
>> From: "Jim Wilhite" <w5jo at brightok.net>
>>>
>>> Mac you hit the nail on the head.  The purpose of the twist is
>>> resistance to air flow not radiation nor pickup of extraneous
>>> signals.
>>> If you have an interfering signal, it won't care what orientation the
>>> feedline is.  One, obviously, does not want to twist the line
>>> excessively, that can cause problems.  The amount you suggest is
>>> good.
>>>
>>> Jim/W5JO