[TowerTalk] Open-wire transmission line

[Joe Subich, W4TV]

On 1/2/2024 6:12 AM, Rob Atkinson wrote:

 > Then this is a question about how to detune a tower.  He needs a
 > detuning kit.

A tuned skirt is a detuning kit.  In essence, the skirt is a coaxial
section while a single wire is an open wire line.  They work the
same way short one end, tune the open end with the appropriate
reactance (capacitor).  If one knows the Zo of the line section,
determining the starting capacitance is straightforward.

I would prefer a three or six wire skirt for detuning simply because
it is easier to calculate the effective diameter(s) of the triangular
tower section and the skirt.  Any errors should be systematic and
the *ratio* of the two diameters will be consistent.  However, either
the single wire (loop) or skirt will work to detune a tower.

73,

    ... Joe, W4TV


On 1/2/2024 6:12 AM, Rob Atkinson wrote:
>> In private correspondence, K1XX told me he was trying to "de-tune" a
>> tower by dropping down a wire from the top and then connecting it
>> to ground through an adjustable reactance.
> 
> Then this is a question about how to detune a tower.  He needs a
> detuning kit.  I faintly recall that there was an article in QST a few
> years ago about how to detune a tower.  BTW if he had asked how to
> detune a tower a lot of time would have been saved.    Thank you for
> making an inquiry.
> 
>> Which brings up an interesting question..
>> What would the best impedance be for open wire line, assuming you have some
>> sort of tuner at the feeding end,
> 
> It's not clear to me where the matching network is.  Is it at the
> antenna feed point?  Or is the "feeding end" the end of the feedline
> at the transmitter?  If the former, then the line is a flat line
> assuming the line's impedance is the same as the impedance looking
> into the matching network at the antenna.  Then for your particular
> circumstances (power level, and line length) you have to work out a
> balance between conductor surface area and spacing for the lowest
> resistance while still achieving a useable impedance.  For ham power
> and typical run lengths it's not as critical as it may seem, i.e. you
> don't have to run rigid copper tubing.   :  )
> 
> Rob
> K5UJ
>