Topband: [Bulk] Top Loading wires

[Paul Christensen]

> On 2/4/2015 7:27 AM, Larry - K1UO wrote:
>Could someone tell me the length of top loading wires (4 wires planned)
>needed to add to the top of a base insulated 90 foot Rohn 25g tower to
>maximize the radiation resistance on 160 meters.  I understand there is a
>point of diminishing returns on the top loading lengths.  The 4 wire
>angles will be around 45 degrees or as close as possible.    If I knew
>how to model ,or even had a modeling program, I would attempt this
>already relatively simple task to many.
>  From 50+ years of practical experience, I am Œguessing¹ maybe 28 feet
>long each?"



I just modeled a 90 ft tower with a flat T top of #12 that varies from 28
ft to 60 ft from center. A total of 30, 1/4 wave radials are buried six
inches.  I¹m using 4Nec2 with NEC v4.2.  This is not your planned set-up,
but it serves as an illustration.

With 28 ft. flat-top spokes, the current distribution is still highest at
the ground.  By the time you get to 50 ft. spokes, current maximum is just
under the center of the vertical (~ 30 ft.).  With 60 ft. spokes, current
peaks in the middle of the tower.  Maybe as a compromise, 40 - 50ft would
be reasonable.  Current is still strong over the entire length of the
vertical radiator but maximum is lifted up away from the ground.

With a 60+60 =120 ft. total length flat hat, you¹re going to find
resonance is way down in the AM broadcast band (about 1410 kHz in this
example).   But so what?  Forget the frequency of resonance and pay more
attention to current distribution.  On 160m, a simple L network network at
the base will get us 50+j0.


Paul, W9AC