[TowerTalk] Antenna Tuner

[Paul Christensen]

> "But the antenna tuner does not change the feedpoint impedance of the
> antenna which in my view would be actually "tuning" the antenna."

All goods points and it's helpful to set definitions before engaging into a 
complex discussion like this.  W2DU used the term "'tuning" to establish 
system reactance cancellation (X = 0), the end result being resonance.  If 
the tuner is adjusted for complete antenna system  reactance cancellation, 
the net effect is as if the antenna is resonant when by itself, it isn't. 
Because of the wave mechanics involved, it's an appropriate term.  Maxwell 
used the term "matching" to establish both reactance cancellation and a 
target resistance value -- for most of our purposes, it's 50-ohms (50 +j0). 
So, tuner adjustment results in effectively adjusting the antenna to 
resonance.  By adjusting the tuner's controls, we can make a resonant 
antenna when it wasn't before.  However, tuner adjustment does not result in 
matching the Z between antenna and line.  With modest care taken to avoid 
impedance extremes -- and tuner loss as a result of an extreme Z at the 
tuner terminals (almost always because of a very low Z), then it doesn't 
matter.

Most of us would probably say that a commercially-made 5/8-wave or 1/2-wave 
mag-mounted 2m antenna is a "resonant antenna."  It's resonant because 
there's a small matching section in the base that results in an input Z of 
50+j0, or close to it.  The 2m steel whip is nowhere near resonant at the 
operating frequency for either mag type.  But we made it that way by placing 
a tuner in the base although there's no appreciable transmission line 
between the whip and base network.   With an HF dipole and transmission line 
with a tuner some distance from the antenna, we're doing the same thing --  
just more carefully to manage loss.  Like the base network that tuned the 
whip into resonance, the tuner used with an HF dipole is doing the same by 
"tuning" it into a resonant antenna.

Paul, W9AC