[TowerTalk] Need some actual Inv V lengths using 14ga THHN

[David Gilbert]

You're thinking in terms of the wrong "return path".  The current is 
only indirectly affected by the dielectric.  The current creates a 
magnetic field around the wire, but the difference in voltage along the 
wire is an electric field (obviously created by the current distribution 
along the wire) that is affected by the dielectric. The "return path" of 
interest here is simply the effect on the current caused by the 
difference in the electric field.  It's all intertwined, is affected by 
the dielectric, and has almost nothing to do with ground as far as the 
dielectric goes.  That description may not be 100% rigorous, but it's 
close and it's how I picture it all in my mind.

EZNEC is now a free download and is trivially easy to learn for what you 
want to know.

I don't intend to model it myself since it would be so easy for you to 
do it yourself, but I'd bet that you'll find that ground proximity has a 
greater effect on the resonant length of the wire than the insulation 
does ... and as you implied the actual distance to the effective ground 
is typically unknown.

Unless you get really lucky you're going to be doing some trimming anyway.

73,
Dave   AB7E


On 9/20/2022 10:46 PM, Edward McCann wrote:
> 
> Thanks for the observation.
>
> I’ve not yet heard from anyone whether and how EZNEC incorporates its modification mathematically to account for dielectric insulation, although I am quite familiar with the math suggested by Richmond, Lee and Popovic. But that was only through 1984 or so. The line has been dead since.
>
> Maybe someone will run a couple EZENECs and determine Vf of insulated wire in a meaningful manner.
>
> The obvious return path is through the ground, not the dielectric, just as it is the vertical, hat or no hat, with the theoretical ideal case being the hard-to-find perfect ground into which the image can be realized. Of course, as the bumper sticker says, stuff happens, and that perfect ground is unobtanium.
>
> Thanks again for your interest and reply.
>
> 73
> Ed McCann
> AG6CX
>
>
>> On Sep 20, 2022, at 7:06 PM, David Gilbert <ab7echo at gmail.com> wrote:
>> 
>>
>> There is indeed a "return path" for a single wire.  The wire capacitively couples back upon itself, and the dielectric affects that capacitance.  That may sound strange, but think of a capacitive hat loading at the top of a vertical or at the end of a dipole.  The wire itself is just a different geometry and therefore has less effect, but it's the same principle.
>>
>> EZNEC+ is based upon NEC2 and simply allows more wire segments than some other versions.  I can't say whether or not NEC2 handles dielectrics, or whether W7EL added a calculation on his own, but if you define a dielectric in EZNEC+ it will result in a shorter wire for the same effect as a bare wire.  That at least fits the physics of the situation.
>>
>> Dave  AB7E
>>
>>
>>
>>
>>
>> On 9/20/2022 6:36 PM, Edward McCann wrote:
>>> I’ve been looking for several decades for a technical analysis of the Vf of a single bare and insulated wire, hearing nothing but lore from just about everywhere, unbelievable proposals and formulas from some, exhortations from academics that such a search is pissin’ in the wind, since there is no return path, as there is in twin lead or in coax.
>>>
>>> 73
>>> Ed McCann
>>> AG6CX
>>> Sausalito
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