[BC] Long Wire

[Phil Alexander]

I would agree that the longer the skirt, the larger the drape
wire seems to be intuitive, however the reactive component
of drive impedance increases with frequency. This is the 
reason for my opinion about using more, larger, stranded
wires in the skirt.

Tension is clearly important for electrical stability in wind.
Extending the distance from the tower to **at least** five feet
reduces wind effects on electrical stability as it reduces the
drape to support structure capacity.

Another advantage of six wire drapes is that this balances the structure loads of the attachment/support arms which, in turn,
eliminates side load torque on the legs of the structure. In 
this configuration a very robust arm such as four or six inch 
channel, perforated to reduce wind loading, becomes possible.

When wind is a serious consideration, I would prefer adding
stay guys (broken with insulators in the usual way) at the 
drape mid-points, pulling the drape wires further from the
structure into a slim "egg beater" configuration. I have never modeled this but I expect an adjustment to the drape bottom 
height would be necessary to keep the total drape length at 90 degrees. While adding insulated drape guides is common I do not
care for the idea of creating future problems, especially when 
it increases project cost. I do not believe in field testing
the torsional moment rating of insulators, but perhaps that is
just me. <g>

The slightly greater loss of galvanized wire is offset by more
wires of larger size and stranding, thus the performance 
difference becomes truly insignificant. Guy cable, grips and
other accessories are readily available, so I see no reason to
rule out galvanized guy cable for drape wires in skirts.

If cost were no object I might be inclined to use stranded
#4 or #2 copper, preferably bare to minimize weight, but
I cannot see the justification for a fraction of a dB. 

Phil Alexander, CSRE, AMD

-----Original Message-----
>From: Mike McCarthy <towers at mre.com>
>
>I think the size of the drape wire is/should be more determined 
>by the length of the skirt, limits of the supporting structure,
>and need to sustain a stable physical relation than outright
>conductor resistance. For a lower frequency (say below 800khz), 
>a wire will require suitable tension approaching that needed 
>for 3/8" line to remain mechanically (read...electrically) 
>stable across the 350ft. and longer span. Even with including
>stabilizing arms spaced 75-100ft. to minimize deflection.
>
<Tail snipped>