[Elecraft] Tuner efficiency question

[Bill Coleman]

On May 13, 2005, at 9:32 AM, Stephen W. Kercel wrote:

> Comment on baluns: If you drive a ferrite core to saturation, it  
> will overheat.

True.

> Once the core overheats, the inductance changes and you lose your  
> match (quite severely, in my experience).

In order to change the inductance permanently, it must reach the  
Curie temperature. The inductance will change much at a much lower  
temperature than this, but the effects aren't lasting.

> It doesn't really take much to do it; a few minutes of normal CW  
> operating with 100 watts into a 5:1 SWR on 20 meters will do the  
> trick for me.  You are much more likely to drive a balun core into  
> saturation on the high SWR output of a tuner than on the low SWR  
> input of the tuner.

Note that a normal current-type balun feeding a non-pathological  
antenna (one that has made a reasonable attempt to be balanced) will  
show little or no flux on the core. The object of the current-type  
balun is to discourage current from flowing by creating a high- 
impedance path -- if there's high impedance there's little current  
and therefore less chance of heating.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901