Jerry wrote:
'snip'
>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.
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. 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.
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.
73, AL
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