Re: Antenna tuner

James Daldry W4JED

Hi, James

Take a low voltage transformer, connect a couple wires to the low voltage winding, and while holding the wire ends, connect a common 1.5 volt battery between the wires. When you disconnect the wires you will drop them, because the collapse of the magnetic field in the coil will generate a voltage spike several hundred volts in magnitude. Now, try it again with a 100 ohm resistor across the winding. No spike. Now - makes no difference if the switch is a relay controlled by a microcontroller or a manual knob switching the inductance - it's a switch, a voltage, and an inductance.

Add a 3 db 50 ohm pad to the input of the tuner when tuning. The tuner will still receive 2+ watts to play with, and the resistors will greatly reduce the voltage spikes.



On 7/13/22 17:50, james jones wrote:

Blew my QDX finals using an ATU-100 set up for QRP.  Bad mismatch on 40 but ATU found tuning solution.  Power in to tuner was shown as 6.8 watts.  Power into to 50 ohm load had been 4.5 watts.  Apparently the QDX saw the mismatch before the solution was attained.  

Pulled the bs170s and radio receives great.  New transistors tomorrow; hope for the best!

Jim w0nkn 

On Jul 13, 2022, at 3:54 PM, bojan.naglic via <bojan.naglic@...> wrote:

I have assembled a DIY kit ATU-100 ext 7x7 designed by N7DCC.  Originally it is for 100W and needs at least 5W input to work well.  Then I changed it for QRP use.  It is an easy job.  It works fine up to 40W with a minimum needed input power of 1W.
Hope it helps.

73 Bojan S53DZ

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