[Antennas] windowed ladder line vs. true open wire line

[gareth]

Steve L. wrote:
> 
> On the subject of balanced transmission line, I have
> used 1000's of feet of commercial 450 Ohm windowed
> ladder line over the past ten years on 160-10m. The
> lines I've used have 14AWG stranded copper clad steel
> wire - they are very strong, very lightweight and
> withstand the weather extremely well. They don't age,
> crack, get brittle, etc.
> 
> HOWEVER, at the transmitter end the Z changes quite a
> bit from dry to wet (much more noticeable on 10m and
> 15m) so if the antenna you are using has a sharp tune
> (narrow bandwidth) you will be tuning and re-tuning as
> it starts to rain, rains heavier or starts drying.
> I've tried cutting every-other spacer out with a pair
> of scissors and I've rubbed pure carnuba wax on it to
> reduce the effect, but it's still fairly pronounced.
> 
> I've found this to be a pain and I'm in the slow
> process of replacing all my windowed ladder line with
> homebrew true open wire line. It's a LOT of line. I
> use 14AWG 7-strand hard drawn copper wire with 1/2" OD
> CPVC for spreaders. I space mine 2.25" and use a
> spreader every 20" for vertically hanging line and
> every 16-18" for line that has to bend or run long
> distances horizontally. The spacing is whatever you
> need to keep the wires from crossing, twisting, etc.
> It doesn't change at all with wet/dry condx unless the
> wires ice up and it handles legal limit on RTTY (100%
> duty cycle) without a wimper.
> 
> So, keep this in mind. The windowed ladder line is
> very good otherwise and I use it on my Field Day
> antennas and temporary antennas for ease of
> installation and to save time.
> 
> If you are putting up a 160-40m antenna, you will
> hardly notice this effect but on 15 and especially 10m
> it's a bugger.
> 
> 73, Steve N4SL


What is the formula for controlling impedance on home-brew ladder?