On Sun, 2008-07-06 at 14:57 -0700, Alfred Lorona wrote:
> Jerry wrote:
> >it may be argued that the tuner at the antenna actually
> > resonates the antenna and so improves its radiation efficiency at some
> > cost from losses in the tuner.
> 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 feedline
current, if the antenna Q is higher.
> 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.
> The antenna wire remains at 120 ohms. If the
> feedline is 50 ohm coax, the tuner transforms 120 ohms to 50 ohms. The
> feedline is them terminated in 50 ohms which makes it a flat line with no
> loss due to a finite SWR. There is a gain in radiation but it is due to the
> elimination of power lost in the feedline that was due to a finite SWR and
> not because the antenna wire is brought to resonance.
I don't agree. See above.
> The feedline looks into 50 ohms at the input of the tuner. The antenna looks
> into 120 ohms at the output of the tuner. The two are said to be impedance
> matched. Both parties are happy and maximum power transfer occurs between
> them. Incidentally, an impedance match between the end of the feedline and
> the antenna occurs whether the tuner is at the junction of the two or at the
> rig end.
Because the tuner supplies the reactance needed to make the antenna
resistive and therefore resonant.
> 73, AL
73, Jerry, K0CQ
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