[TowerTalk] Open wire

[Martin Ewing]

On Mon, Sep 28, 2009 at 2:42 PM, w4lde <w4lde at numail.org> wrote:

> This is off topic but here goes.
>
> I am feeding a 80 meter center feed with open wire, about 100ft feed-line.
>
> Due to some new construction work that is going to start on the back of
> our home this week I am forced to move the feed-line.
>
> The original installation had about 50 ft vertical from the center of
> the dipole and then across and under a raised wood porch and then up to
> a second floor shack.
>
> The porch is going and a new covered screened porch is replacing it.
>
> I have three choices during construction, they are:
>
> 1.  Leave the 100' feed-line coiled out in the yard and not operate.
> (Not a great idea)
>
> 2.  Cut the feed line down to around 50' which would be temporary but
> then I would have to splice it later after construction.
>
> 3.  Coil about 40' near the 4:1 balun, however I am concerned that the
> coiled feed line will act a one big inductor and change and or
> significantly reduce the antenna efficiency.
>
> #3 is the easiest but I am worried about open wire feed-line coiled,
> thoughts please!
>
> 73 de
> Ron W4LDE
>
>
My vote would be #2.  Coiling transmission line is fine as long as there is
no coupling from turn to turn, ie. fine for coax.  For open wire, if you
coil tightly, each wire will "see" the wires in adjacent turns, leading to
various unpredictable resonances, unbalance, etc.  I suppose you could
devise a coiling (large diameter, widely spaced) so that the OW line is
several wire spacings separated from turn to turn, but it would be kind of
unwieldy.  Cutting the line should work - although your antenna tuner will
need different settings.

Of course, there is the "ham" way to do this - try #3 and see how it works.
Your "efficiency" may not change much, but your impedance match probably
will.

73 Martin AA6E