Re: [TenTec] SLIGHTLY OT

["Dr. Gerald N. Johnson" <geraldj@weather.net>]

Careful about the EZNEC model for impedance. None of the NEC models do 
coupling to the earth. Their earth model is only used for reflections 
for the elevation pattern.
On 4/8/2011 2:35 PM, Steve Hunt wrote:
> On 08/04/2011 19:29, Dr. Gerald N. Johnson wrote:
> > A quarter wave center fed wire has low radiation resistance and is very
> > capacitive. Bad phase angle.
> Correct - an EZNEC model of his antenna puts the feedpoint impedance at
> around 11-j930 Ohms
>
> > You run that through a 600 ohm line about a
> > quarter wave long, converts low R to high R and C to L.
> No, that's wrong. A quarter wavelength of 600 Ohm line transforms
> 11-j930 to 4.5+j386.
> His 110ft of open wire will transform 11-j930 to around 7+j290
On my Smith chart slide rule, that 11-j930 a long ways from the low R 
side of the chart spins over a quarter wave to about 4 + j2.75 though 
the resistive component is practically on the outside edge of the chart.
> > So your balun
> > sees high impedance that may be causing core saturation (for sure in a
> > voltage balun) and you see high Z at the tuner.
> A true 1:1 Guanella balun will not saturate because of a High Z
> differential load. The core flux is driven by *Common-Mode* current, not
> by differential-mode voltage.
>
> Perhaps surprisingly the 10ft of LMR600 then has a major effect -
> transforming 7+j290 to 206+j1573
>
> If you look at the L-network needed to match that impedance to 50 Ohms
> it requires much more inductance than the 238B is capable of. I wonder
> if your tuner is at max L as well as being at max C?
>
> 73,
> Steve G3TXQ
73, Jerry, K0CQ