[BC] Long Wire

[Phil Alexander]

The big problem with 3-12's is lead X(L) from the small dia.
The other thing I'm guessing is that these drapes are only
about 24~30" off the tower where the minimum distance SHOULD
BE 60". From a lot of model experimentation I have concluded
that 6-3/8" drapes give superior radiation performance. The
3/8" cable can be standard galvanized stranded guy cable.
Obviously, this will require strong supports probably made
of 4 or 6 inch channel with circular perforations to cut
wind load, or a triangular fabricated support design of 3"
3/16 or 1/4" angle. 

It was Ron Rackley who pointed me in the right direction for
making skirts perform and when I started doing modeling,
surprise, surprise the model showed his ideas hit the nail
squarely on the head, namely: minimum 5 foot clearance; top
attachment at **90** degrees; use an ATU to match the skirt
to coax - never try to do it with jumpers between skirt and
tower.

Some other things I've found while modeling: use girdles at
top and bottom; use a balanced drive spider at the bottom;
use top lightning rods extending above the attach/support
point equal in length to the height of the bottom girdle thus
making the extended drapes exactly 90 deg total height, 
attached exactly 90 deg above the base of the steel.

In the model this configuration always yields < 0.5 dB loss
(usually quite a bit less) as compared with the same tower
driven with a series fed base and base insulator.

The curious part is that dropping the height from 90 to 80~
85 degrees may introduce a loss of 1.5 to 3 dB which tends
to confirm Ron's point that skirts should never be applied
to towers < 90 degrees (because the correct attach point 
is non-existent). 

Phil Alexander, CSRE, AMD

-----Original Message-----
>From: Mike McCarthy <towers at mre.com>
>
>I have a 160ft. skirt using the 3-#12 wire supplied by Nott.
>I'm thinking the cable is not heavy enough. Then again, my
>members are 3" x 3" x 3/16" angle with diagonal supports top
>and bottom. It's not going anywhere.  If you don't have 
>2" x 2" x 3/16" minimum angle, it's likely too light a member.
>
>If the angles are bending/flexing/deflecting, they're too
>light a member and not properly supported. Period. The load
>presented between the 3-#12 Nott supplies and even a full 
>1/4" wire or even 5/16 is fairly insignificant. We're talking
>about less than 1000 Lbs. tension overall and not very much
>relatively speaking.
>
>So...it sounds like the skirt was poorly designed.  
>Especially for only being 85ft.