[TowerTalk] Double Coax Runs to Reduce Loss

[Patrick Greenlee]

The skin effect will not force all the current to flow in a corroded 
surface layer with good copper below it.  Current density will be 
distributed proportional to the impedance presented by the different 
depths.  This includes the skin effect's redistribution of impedance.

If a simple application of irrefutable skin effect phenomena were 
applied, the insulation over copper wire would be the current carrying 
medium at RF freqs rendering insulated wire unusable in most applications.

Except as specifically noted above I mostly agree with the post 
reproduced below.

It might be interesting to put a watt meter first at one end of the DIY 
twisted pair and then at the other end and see what the efficiency is.  
Why debate the number of angels dancing on the head of a pin when we can 
use a loupe and count the little buggers?

Patrick

On 12/22/2016 9:08 PM, Guy Olinger wrote:
> Be careful to never, ever use THHN (with insulation NOT stripped off)
> outdoors.
>
> Also personally, I would never use it for RF transmission line. Do not
> confuse Z0 with efficiency. What part of the Z0 is actually loss? Do not
> assume that the characteristics of THHN have been engineered in any way
> whatsoever for RF.
>
> THHN is manufactured for 60 Hz indoor power uses to comply with the US
> National Electrical Code. All the manufacturer's efforts are designed to be
> compliant with the code with the least possible cost of manufacture.
>
> There is ZERO testing of THHN for RF usage. Why should there be? So why
> should we expect different batches and different manufacturers to all the
> the same at RF. "Well, I tested mine and it was fine". Except you can't
> extrapolate it. If yours was fine, you were lucky. Get in a supply of #12
> teflon sleeve and #14 double polyimide wire. Use teflon sleeved #14 double
> polyimide for RF and winding on cores. Wind it, install it, forget it.
>
> Also, using THHN to keep dipole ends from shorting to trees did NOT work. A
> length of #11 teflon sleeve over the otherwise bare solid #12 did work, has
> worked, and will continue to work.
>
> What does seem to be a very practical *outdoor* use of THHN for ham use, is
> solid #14 or #12 THHN bought by the 500 foot spool, and **then stripped**
> for bare #14 and bare #12. This results in bare copper wire at last check
> something like half the cost of already bare copper bought from the online
> "copper stores". 500 foot spools of THHN are manufactured and shipped in
> extreme bulk, economies of scale in full operation, all to our advantage.
>
> **Measured and verified,** THHN used for elevated 65' radials became
> severely deteriorated from approximately 3 years of UV exposure. Apparently
> corrosive artifacts of the deterioration etched the copper surface into a
> weird "crumbly" surface, rendering the copper resistive at RF, roughly an
> ohm per foot at 1.83 MHz, while DC testing with an ohmmeter showed the
> usual low R for copper wire.
>
> An interesting demonstration of skin effect. What happens when skin effect
> forces RF current into a deteriorated area of poorer conductivity? What if
> the conductor surface gradually blends into an insulator?
>
> RF measurement designed for measuring radial ground induction was 85 ohms
> per 65' deteriorated THHN radial. Same measurement after replacing with NEW
> THHN with insulation STRIPPED was 18 ohms. 15-20 ohms was expected and
> normal for that length with the area's typical ground characteristics. 67
> of the original 85 ohms was from deterioration. 85/4 = 21.25. 18/4 = 4.5.
> The antenna DID have to be retuned afterward.
>
> A duplicate experiment with an identical second set of radials on the same
> site produced values within 5% of the first set.
>
> The business of twisted pair THHN in the shack for anti-RFI 12 VDC high
> current transmission is splendiferous. Yes, that is a real word you can
> look up :>)
>
> 73, Guy K2AV
>
> On Thu, Dec 22, 2016 at 2:52 PM, Jim Brown <jim at audiosystemsgroup.com>
> wrote:
>
>> Hi Ed,
>>
>> No, not twisted pair for chokes. But I have recommended parallel enameled
>> wires and THHN (ordinary house wire) wound on #31 toroids connected as
>> two-wire transmission line. You're correct that closely spaced enameled
>> wire yields Zo about 50 ohms; closely spaced THHN yields Zo in the range of
>> 85 ohms - 100 ohms. Jerry Sevick, W2FMI, noted this result in late editions
>> of his book, and my measurements confirm that within measurement accuracy.
>>
>> 73, Jim K9YC
>>
>> On Thu,12/22/2016 11:22 AM, Edward Mccann wrote:
>>
>>> Re twisted pair, I seem to remember you suggesting at one time that using
>>> twisted pair enameled wire on FT-240 #31 mix provided about 50 ohms on the
>>> CMC Choke.
>>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2016.0.7924 / Virus Database: 4739/13640 - Release Date: 12/23/16