On Mon, 2008-07-07 at 10:25 +0100, Steve Hunt wrote:
>
> Dr. Gerald N. Johnson wrote:
> >>
> >> As Jerry points out, there are some amongst us who believe that a tuner at
> >> the antenna feed point tunes the antenna wire to resonance. It does not.
> >>
> >
> > Why not? The tuner can supply the needed series capacitive reactance to
> > bring the feed impedance resistive. Isn't a resistive feed impedance (no
> > matter what the magnitude) the definition of resonance? And when that
> > antenna is resonant, isn't the current in the wire maximized? Isn't
> > there a possibility of greater circulating current than feed line
> > current, if the antenna Q is higher.
> >
> Jerry,
>
> Mischievously ....... I just tried an interesting experiment. I took a
> 100 Ohm resistor in series with a 100pF capacitor and placed them across
> the "antenna" terminals of my tuner. I watched them carefully as I
> adjusted the tuner for a 50 Ohm match. At no time did I see any change
> in the physical appearance of the resistor or capacitor, so I don't see
> how the tuner was making them "resonant". I was disappointed - I'd hoped
> from your description I might see them "morph" into a 50 Ohm resistor
> before my very eyes :)
I never said the resonance condition included changing the radiation
resistance to 50 ohms. Only that that the phase angle of the load
becomes zero, e.g. current and voltage are in phase at the feed point or
alternatively that the energy stored in the inductance equals the energy
stored in the capacitance and that they swap every half cycle. At some
place in your tuner, it supplied inductive reactance to resonate the
series capacitor, leading to a resistive or resonant load of 100 ohms.
Resonance does not require 50 ohms, or 5000 ohms, just lack of reactance
or supply current in phase with the supply voltage.
The resonant AND matched antenna will show 50 ohms (or 60 if old EU
standard, or 75 ohms if CATV standard or 300 ohms if old TV receiving or
VOA transmitting station standard) impedance. It just depends on your
application.
>
> 73,
> Steve G3TXQ
>
73, Jerry, K0CQ
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