A quick suggestion, why insulate at all. You can shunt feed each tower and have
the benefits of a DC grounded antenna. Adjusting the height and the spacing of
the shunt arm along with a series capacitor will give you a perfect match at
your favorite frequency. Multiple shunt arms will give you several bands too.
This also gets around the problem of calculating the top loading the antennas
on top present. The top loading and your 25 mtr. towers will give you somewhere
around a 3/8 electrical wave length radiator for each of your four square. This
would make a great system where all elements are phase fed. If you chose to use
the parasitic method it will require a bit more work to get reflectors to play
right but still doable.
If you are still set on insulated, the two methods I have seen are to put a
taper to point section on bottom with a commercial "keg" type insulator or two
base plates.one on stubs out of base slab and one on bottom of tower assembly,
with three or six smaller insulators between plates.
If you want to use a fix it and forget it type of series capacitor, I have
several methods of doing this without a commercial assembly being required.
Whatever you do, you will need insulated guys(i.e. philstrand or equal) or
carefully broken up with insulators if conductive.
Good luck, you have quite a project going there !
73
NIck
WB7PEK
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