QCX acceptable load mismatch?

Johan Bodin


is there any knowledge/experience about how much abuse the QCX PA can withstand without damage? If a low loss SWR meter is used, the impedance seen by the PA will deviate wildly from the 50+j0 ohm target while searching for a perfect match with an antenna tuner. I am currently using a -6dB resistive SWR bridge (three 50 ohm resistors + a detector) while tuning so the worst case load seen by the QCX goes from 33 to 100 ohms when the load at the bridge output goes from shorted to open. Can I assume that this is safe?

My Mini is still alive and well but the lack of PA protection makes me a little worried. I just want to prevent failure in the field when messing around with makeshift antennas :)


Paul Ross

Well, the best bet is to make an initial "dry" tune -- in RECEIVE mode, adjust your tuner for MAXIMUM noise. This will be fairly close. THEN, key down and work from there. The resistive bridge type SWR meter does provide a reasonable level of protection...  I use a QRP Kits "Z" match tuner in the field with a built in resistive bridge. Just follow the foregoing, then key down and tune for minimum brightness of the LED. You should be able to get a good tune in less than a couple of seconds.

Curt wb8yyy


Paul beat me to it, find or build a tuner that includes resistive loading while adjusting it. Z match tuner might find a source in Eu maybe, or at least ideas. I use a ZM2 for portable operating. 

Maybe just me but I think the bs170 are more robust than classic NPN, but who wants to fix a mini. Keep your transmission short, also I peak my manual tuner for noise on receive to get close. 

Okay I see your message again, 33 to 100 ohms does not concern me, but key down lightly during adjustment.

Curt wb8yyy


Beacon mode switch on accidents must have killed a few PAs . If you have been using WSPR and switched off, The transceiver will automatically re-enter the WSPR transmit routine when switched on again. I know of about three, including mine which have blown the PA, because they fired up without an antenna load. Sometimes it is more than just the PA that is destroyed. Q6 won't like it and neither will Q5 perhaps.

I don't think momentary tuning mismatches will harm the PA, but the idea of using a resistive bridge circuit in the tuner is a good one. The PA heats up mighty quick if there is any sort of mismatch. That said, I use my QCX and QCX+ for wspr and I've had a few issues with the Classic QCX which, as you know has no heat sinking. I run a forty meter horizontal loop, and snow and wind can change swr over time. One time the wind blew down part of the antenna, creating a mis match. The other time the antenna wire was coated in snow. Mine runs 24/7. The uncooled QCX 40 PA certainly won't stand a bad swr during repeated, 2 minute, 100% duty cycle use. I can vouch for that. Fortunately, the BS170 is very cheap. Having replaced them a couple of times, I was thinking of fitting sockets if it happens again. Then I can just plug in some new ones. The disadvantage of that of course, is poorer heat transfer to the PCB.