Topband: Tower Shunt Feed
Jerry Keller (K3BZ)
k3bz at verizon.net
Sun Apr 3 09:29:28 PDT 2011
I want to try a shunt feed on my 60' aluminum freestanding tower topped with
a 4 element SteppIR. The tower is a Heights and it tilts over on a 4' high
top-hinged base. I've come up with the following specs I dug out of K1ZM's
book "DXing on the Edge":
(1) Three #14 stranded insulated wires spaced 15" from tower leg and shorted
top and bottom as an 8"-on-a-side triangular cage, running from top of tower
down to where the tower joins the hinged base (4' above earth level because
of the hinge). That would make the shunt 56' high.
(2) I'd like to use an omega feed with air variable caps I liberated from an
old tuner. I notice that most often vacuum variables are used for this
rather than air variables. The plates in the air variables I have are spaced
about 3/16".... will they handle a KW without arcing?
(3) I want to put the feed caps in a plastic box at the 4' level so the
shunt won't interfere with tilting the tower over. The tower grounds and
the radial system would connect at ground level to a flexible copper line
running from ground level up to the feed box. Would that work OK at 160M?
If it's important, I could run the shunt the full 60' down to ground level,
but it's mechanically more difficult.
(4) With this arrangement, how important is the SteppIR? Would I need it for
top-loading? The SteppIRs elements "float" above the grounded boom...
should I plan to ground them to the boom?
Advice, alternatives, comments? Any input appreciated.
73, Jerry K3BZ
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