[TenTec] OT SWR VS Power Loss

[Dr. Gerald N. Johnson]

On Sun, 2008-07-06 at 21:30 -1000, Ken Brown 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.
> No. The feed point of the antenna still has the same impedance.

Even with the series reactance that takes out the residual reactance of
the antenna.

> >  Isn't a resistive feed impedance (no
> > matter what the magnitude) the definition of resonance?
> If it is resistive due to equal and opposite reactances at that that 
> frequency. If it is just resistive everywhere, I wouldn't call it 
> resonance.

By using a reactance to achieve resonance, its usually not resistive
everywhere.

> > And when that
> > antenna is resonant, isn't the current in the wire maximized? Isn't
> > there a possibility of greater circulating current than feedline
> > current, if the antenna Q is higher.
> >   
> I don't know, but adding a tuner (that is not a part of the antenna) 
> does not change the impedance of the antenna or the resonance of the 
> antenna, even if it does change the impedance and resonance of the whole 
> system.

Not even a variable coil in the mobile screwdriver? Sure acts like a
resonant antenna, works well at the perceived resonant frequency, works
badly away from that frequency.
> 
> The antenna still has the same length of wire, with the same wire 
> inductance and capacitance to ground and to other parts of the antenna 
> as without the tuner connected, so it still has the same feed point 
> impedance. The tuner has not changed the antenna. The tuner may increase 
> power delivered to the antenna, by increasing the efficiency of the 
> power transfer from the feedline to the antenna. Once the power gets to 
> the antenna, the antenna does exactly the same with that power (radiates 
> some, reflects some at a particular phase angle, turns some into heat) 
> as it did without the tuner there. The antenna has not been tuned. It 
> still has the same feed point impedance.

I think the antenna gets extended into the tuner's output parts that
provide the reactance (when the tuner is at the antenna) to achieve
resonance. And when that resonant Q is more than 1, there can be greater
current circulating in the antenna than would be if it wasn't resonated
by the tuner and so there can be a greater signal strength at a
distance.

> >> Consider a wire longer than a halfwave length with, say, a feedpoint 
> >> impedance of 120 ohms instead of the 70 ohms exhibited by a resonant dipole. 
> >> The tuner does not change the impedance of 120 ohms to 70 ohms which it 
> >> would have to do in order bring the wire to resonance. What the tuner does 
> >> do is transform the 120 ohm antenna impedance to the characteristic 
> >> impedance of the feedline.
> >>     
> >
> > Then the second duty of the tuner is to transform the resistive feed
> > point impedance to that desired by the transmitter and feed line.
> >   
> Unless you are hoping for some harmonic rejection or other bandpass 
> filtering action from the tuner, transforming the impedance is the first 
> and only duty of the tuner.
> >   
> > Because the tuner supplies the reactance needed to make the antenna
> > resistive and therefore resonant.
> >   
> No! The antenna still has the same physical dimensions, and RF applied 
> to the feed point of the antenna still travels through the conductors in 
> the antenna, affected by capacitance of nearby objects, the inductance 
> of the antenna conductors, and reflected from the ends of the conductors 
> the same way it does without the tuner in the line. The antenna is not 
> resonated by the tuner, although the system as a whole may be.
> 
> N6KB

73, Jerry, K0CQ