Topband: Reality check request

[K9AY]

>That would suggest that 150 foot high
> towers could hold full sized 160 meter verticals and that shunt feeding
> towers would be unnecessary.
>
> Tod, K0TO
>

Tod,

Your modeling seems OK, but the above statement is not. Your 57-foot high 
scenario is too short to have much coupling to the inverted-L. The coupling 
will be mainly the capacitance between the wire and grounded tower. Coupling 
will increase as the tower gets taller, and as resonance is approached, 
there will be extreme variations in impedance. Your models should show this 
clearly as you try different configurations.

In practice, I've seen significant interaction on a couple of 80M 
inverted-Ls supported by 35 feet of aluminum mast. Things changed noticeably 
after tying the mast to the radial system, requiring a change in antenna 
length.

BTW, I use a 5" diameter 'wire' to represent 25G-size tower. I can't recall 
where I saw it 10 years ago or so, but this number was determined using a 
lattice structure in NEC, and confirmed (approximately) by a ham who built a 
new vertical of 25G.

73, Gary
K9AY