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SOTA tuner kit prob

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I like making kit stuff but am really no good at it. Just got a little SOTA tuner kit from qrp kits.com. Works great except if I try to 'tune' the SWR led is lit bright all the time I xmit; it never dims when I cross resonance. I hook an antenna analyzer to it and it actually tunes my efhw fine. Any ideas?

Doesn't bother me much; I always set tuner with the analyzer before connecting to radio as I also am inclined to burn up radios.

Dennis W5WIL

Just a quick thought Dennis - are the diode and led in circuit the right way round?


Thanks for the suggestion. I doubled checked all I could and it appears correct. For now I just disconnected the SWR indicator board from the circuit and connected the output coil tap directly to the input BNC. Works great for my needs tuning EFHWs I'm using in the park. When I get some extra money I may buy just the SWR indicator kit for $20 and try that again..

Dennis, W5WIL

When I had an FT-817 I built a ZM2 ATU. It worked very well and would handle long-wire and coax feeds. It was also light as a feather.


I built that kit a few years ago.  It's simple, and works well.

The tuning aid is a wheatstone bridge with 50 ohm resistors on three of the legs, and the (tuned) antenna on the fourth leg.  The LED is across the bridge.  When the antenna is presenting a 50 ohm load, the bridge is balanced, with 50 ohms on each leg, and no current should flow through the LED, so the LED should go out.  Realistically, it's close enough if the LED just dims and doesn't completely extinguish.

About the only thing I can think of that would go wrong is if one or more of your 50 ohm resistors isn't really 50 ohms, or if something is not soldered in correctly.  Another possibility is that your antenna is never coming close to presenting a 50 ohm load to the bridge. 

If you can short out the antenna leg of the bridge, the input should see 50 ohms in parallel with 100 ohms, or 33 ohms presented to the input.  That's easy to check with a multimeter.  If you could substitute a 50 ohm resistor for the antenna leg of the bridge, you should be able to check with a multimeter that the input resistance of the bridge is 50 ohms (each leg should be 100 ohms, both legs in parallel make 50 ohms).  If those don't check out, check soldering, and maybe disconnect the resistors to check each one's individual resistance.


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