[TenTec] OT SWR VS Power Loss

[Alfred Lorona]

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