On 4/3/2011 12:29 PM, Jerry Keller (K3BZ) wrote:
> 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?
>
Jerry the more top loading the better for overall efficiency and with a
60 foot tower you will need all you can get. Also rather than an omega
match I would put a bridge on the bottom of the cage and measure what
you have. You might have to move your tap point down to 43-47 feet to
get closer to the sweet point. Then all you might need is a simple L
network with much more reasonable voltages to deal with. Or even a CLC
with the L taped down to ground will match a wider ranger of
impedences. if you don't want to go through the guesswork with trial
and error then the bridge it first and then match is an easier approach.
Herb, KV4FZ
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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