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

[Jim Brown]

On 9/20/2022 6:36 PM, Edward McCann via TowerTalk wrote:
> Not to beat a dead horse, but I recall a presentation by K6OIK created for Pacificom October 2017 in which he addresses insulated wire in modeling.
> 
> He states: NEC2 has no capabilities to address dielectrics; NEC3 and NEC4 handle dielectrics by accurate methods; Cebik in note 83 attempted to reverse engineer insulated wire corrections from NEC4 but did not discover the full answer; and
> 
> EZNEC v.4 and up claim to go insulated wires but use NEC2, which has no capability to model dielectric coated wires.
> 
> EZNEC Pro/4 uses NEC4 which has accurate dielectric caiability.
> 
> I’ve not used whatever AB7E calls EZNEC +, so I’m not in a position to challenge the position, but I am really interested in what EZNEC+ claims is the Vf of simple insulated wire, for  8, 10,12,14,16 gauge.

Steve Stearns, K6OIK, is retired from a high power engineering gig with 
a major aircraft mfr, for which he needed to model antennas around very 
complex shapes and environments surrounding objects. To accomplish that, 
he seriously looked under the hoods of a lot of modeling software. At 
least once, maybe twice, his talks at the Pacificon Antenna Forum were a 
survey of a half dozen or so he considered most useful, their 
capabilities, and their limitations.

I've used EZNEC Pro/2, which uses the NEC2 engine, extensively. It DOES 
allow the entry of the dielectric constant, thickness, and loss tangent 
of the insulation on a conductor. There are (at least) two catches -- 
first, you have to know what those constants are for the wire you have, 
and second, the NEC2 engine has to use those constants correctly. Seven 
years later, I don't recall what he said, but his talks are usually 
online, so we could look it up. :)

BTW -- the different versions of EZNEC, which is one of several UIs for 
NEC, is the number of data points that can be used to make the model, 
and features that the UI provides.

When we build an antenna and rig it, several factors will affect its 
apparent resonance if all we have to measure it is an SWR bridge. The 
feedpoint impedance of antennas is strongly affected by its 
surroundings, and the mutual coupling of a horizontally polarized 
antenna's reflection from the soil below it, which is, in turn, affected 
by soil characteristics and the effective height of the antenna.

With a well calibrated VNA, we can make a swept measurement from the 
shack, export the data to good Smith Chart software, and if we know the 
complex electrical parameters of the feedline, subtract out the feedline 
to see the resonance on the Smith Chart display, AND it's feedpoint Z at 
resonance. That's how I know that the feedpoint Z of my 125 ft high 80M 
dipoles at resonance is in the range of 85 ohms. The correlation with an 
NEC model will depend on how close actual soil parameters are to what I 
entered. But the ground beneath these antennas is pretty irregular, 
varying by 20-40 ft. So that's another variable. EZNEC can't handle it, 
but some of the other programs that Steve discussed CAN.

BTW -- I use SimSmith, Freeware by Ward, AE6TY, which has morphed to 
become SimNEC, using the NEC engine!

73, Jim K9YC