I was trying to find the dielectric constant (Epsilon sub r) for this
poly jacket that's used on the standard HRO-available line. The
calculations I did were for open wire line with air being the dialectric
(something like 1.0054). The first ladder line that I used was made
from BIC pens, cut in half for about 3" center to center. After that
much hassle to put together and raise, when it got wind-blown and torn
up, I just started using the HRO stuff.
Have only seen really heavy feeds in use at SW BC stations, and only
twice at that. Looked like 4 AWG, but might've been heavier. Some of
WWVH's curtain feeds lookmuch the same.
73,
matt W6NIA
On 4/15/2017 3:56 PM, Clay Autery wrote:
Thanks....
So, these numbers are for BARE copper, right?
Using insulated wire changes things a bunch....
For instance 18 GA THHN/THWN has an approx. dc of 2.35, XLPE has a dc of
5 or so.... As the dielectric constant gets larger, things get real....
fast.
18 GA THHN/THWN... (dc = 2.35 approx.)
To get the same 464 Ohms, the Center to center distance must increase to
7.52 inches or so...
The impedance with the quoted 1, 2, and 3 inch "D":
1" = 305-306 Ohms
2" = 359-360 Ohms
3" = 391-392 Ohms
I use 300 Ohm, 18 GA copper twin-lead with foam over wires and some hard
plastic over that.... It's close to true 300 Ohm...
I'll have to go calculate the min size safe against various power
levels... The separation distances get big fast with increase in the
conductor diameter, too.
73,
PS - there is an argument to be made for NOT using bare wire.... if
anything MIGHT touch the feedline... and wet wire.... wet wire
changes performance a lot... built with insulated wire, the changes are
MUCH smaller with wet wire...
______________________
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 4/15/2017 4:56 PM, Matt Zilmer wrote:
Very true!
Got back in front of my desktop 'puter and made an Excel worksheet to
calculate the characteristic impedance at various spacings and wire
sizes. The real variable is the ratio of spacing divided by wire
diameter (only the ratio matters). Everything else in the formula is a
constant for our purposes.
16 AWG, 1 inchspacing: Zc = 436 ohms
16 AWG, 2 inchspacing: Zc= 519 ohms
16 AWG, 3 inch spacing: Zc= 567 ohms
18 AWG, 1 inch sp: Zc = 464 ohms
18 AWG, 2 inch sp: Zc = 547 ohms
18 AWG, 3 inch sp: Zc = 595 ohms
Or for you QRO ops: 3/0 AWG, 9 inch spacing: Zc = 450 ohms. 3/0
(three-aught) is quite hefty, being 0.4" in diameter. Probably
overkill for the average Joe Ham.
The commercial ladder line I use is 18 AWG with about 0.9" spacing
(hard to measure), giving 451 ohms. I think there is a lot of this
type in use.
Anyway, for the wire sizes and spacings we would normally use, the Zc
is nearly the same value, which is all I was saying before.
The formulas for all this are pretty well documented at
http://hamwaves.com/zc.circular/en/index.html.
73,
matt W6NIA
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Matt Zilmer, W6NIA
[Shiraz]
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