[BC] Long Wire

[Phil Alexander]

Definitions:
-----------
Girdle - a large wire (or cab be a hoop of tubing)
         connecting ends of drape wires. Note:
         girdles appear preferable at top and bottom.

Spider - large wires (or tubes) radiating from a central 
         point so as to provide equal length paths to
         the lower end each drape in the skirt or to the
         equidistant points on the girdle between adjacent
         drapes. Note: Modeled performance is usually better
         when both a girdle and spider are used to balance 
         currents equally in all conductors.

Attachment/
Support: Conductive support arm of sufficient length that 
         the end away from the tower is not less than five
         feet from any part of the tower's horizontal
         cross-section at any point. Note: Depending on
         tower geometry it may be necessary to provide
         more clearance for three drapes in a six drape
         system so that the tower is centered in a hexagonal
         "cage." 

General Notes:
------- -----
Drape Wire Height: The lower girdle and spider must be some
                   distance above ground level. That may be 
                   3 or 4 feet in the case of a fenced area,
                   or, for safety, it may be perhaps ten feet
                   up in the air. What I discovered in the 
                   model is that an unterminated, upward
                   extension of the drape - call it a lightning
                   rod - above the attachment/support arm
                   improves performance slightly. (Somewhat
                   more at 10 or 12 feet than at 3 feet)

Radiation Efficiency:  Will be ~ 95~98% of the same tower 
                       when series fed.

Drive Point Impedance: Will normally be highly inductive and
                       is strongly influenced by steel height.
                       Resistance will be elevated as compared
                       with the series fed case and is also
                       influenced by steel height. Other factors
                       are drape conductor spacing because 5 ft.
                       is a minimum while the maximum may be a
                       few feet greater. Modeling before
                       construction, but the model can diverge
                       from actual. Note in this regard that
                       model impedance adjustment to the real 
                       world was one of the sticky points in 
                       the new DA modeling rules and resulted 
                       in incorporation of language regarding
                       corrections to conform models to the 
                       real world. As an aside, this is what
                       they called a pitfall for the unwary
                       when I went to school back when we hid
                       from the dinosaurs in caves. <G>

-----Original Message-----
>From: Steve Lewis <steve at theengineeringbureau.com>
>
>A couple of questions, Phil...
>
>Please explain a "girdle" you recommend for the top 
>and bottom of this configuration - I'm assuming it 
>is something more than simply a ring that connects 
>the drape wires.
>
>And the bottoms of the drape wires are connected 
>together in some sort of spider arrangement where 
>all drape wires are fed with equal length jumpers
>and no ring, correct?  Is that true of the top as 
>well?
>
>And if I understand you completely, the vertical 
>wires are always to be 90 degrees.  In the case of 
>a 90 degree tower, the feed point would be near the
>ground or at least the top of the pier?
>
>If this arrangement were perfect, what would you expect the impedance of the
>skirt to be?