[TenTec] SLIGHTLY OT
Dr. Gerald N. Johnson
geraldj at weather.net
Fri Apr 8 14:01:35 PDT 2011
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
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