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Hi all,

I learned from the operating manual that the SWR measurement is from the center of the band. I had forgotten this trying to squeeze a POTA activation while on a business trip this week. My SWR reading was 2.00, on 20m . Not great but I was too lazy to pull my antenna out of the tree and use the shorter segment which usually gets me 1.4. (EFRW 71’ with 41’ segment) I have a EFHW which will work better but haven’t put it together yet.

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To test propagation, I decided to hook up the iPad and do a little FT8. I think the SWR was much worse down at that frequency because halfway through my first transmit cycle.. snap! I knew what happened right away. ?I did have SWR protection on and set at 3 but it didn’t stop the pop. Radio still received but I shut it down. ?I ordered some BS170’s from Digikey for a whopping 24 cents a piece and they’ll be here tomorrow.

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I think I should replace all four but thought I’d ask from those who know if it is likely the others are damaged too. Hopefully nothing else is damaged and I’ll recalibrate as well. Love this radio and pack it in my bag when I travel.

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I say do them all, they're super cheap. Make sure you get them from Mouser or Digikey. There are a ton of fake bs170 out there.

Lower your SWR protection to "2". If it's a 12-Volt build, run it on 9-Volts for digimodes. Doing those adds a larger margin of error.

Ouch!? A QRP antenna tuner is a good investment.? But some alternative suggestions are 1) reduce power supply from 12V to 9V (on a 12V QMX) or 2) use a cheap inline SWR bridge (e.g., ).? The bridge will significantly reduce your output power, but I've been amazed how many FT8 spots I get with the bridge inserted!? Better safe than sorry.

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73, Mike KK7ER

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When I damages my QMX+ finals, only 2 of the BS170s were bad. See my post on diagnosing which ones are bad and how to replace them safely without damaging your PC board.

Sorry, try this link./g/QRPLabs/message/144660

On Wed, May 21, 2025 at 07:15 PM, KF0OUH wrote:

?I did have SWR protection on and set at 3 but it didn’t stop the pop.

This surprises me.? I have my swr protection set at 3, and it has triggered every time it should.? I have several times forgotten to attach the dummy load before tx, and it saves me every time.? And when I forget and transmit on 12m or 17m on my 20/15/10 antenna, or outside the narrow passband on my 80m dipole, each time the swr protect triggers and saves me from damage.

Maybe you were at 2.9:1 during a FT8 transmit and that was enough to cause the damage...

Thanks all for the feedback!

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I received the BS170’s from Digikey today and made quick work of replacing the blown one. I thought I would replace just the one and test the results.?

Luckily everything else seems fine and all diagnostics measured normal. ?Made a few ?FT8 QSO’s on the QTH antenna and all is good. ?Now to tune rhe field antenna?