[Elecraft] OT; Feedline question

[Matt Zilmer]

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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-- 
"A delay is better than a disaster."
-- unknonwn

Matt Zilmer, W6NIA
[Shiraz]