Topband: Top Loaded Vertical

[Charlie & Vivian Vaughan]

I have an aluminum vertical element that is 67' tall.  I want to top-load it
on 160 meters and would like to use a three wire approach for physical
symmetry in support of the vertical element.  Does anyone have an idea of
about how long the three wire elements would need to be to top-load it for
160?  I was guessing about 40' but that was a wild guess.  It would be nice
to know approximately where to start if anyone has had similar experience.
Thanks  K4UWH