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QCX Mini Part 2

It’s annoying when it says that… ignore it as you are posted interesting information.

It has been interesting and helpful to follow this thread. I got my 20 meter kit built up late last week.
With an Elecraft dummy load the power out was only 1.5 watts or so. I mistakenly took two turns off L2 which lowered the power. Trimming L3 got me up to about 3 watts. Compressing the turns to half the toroid on L2 produced 5.3 watts. The supply voltage was set to 12 V. Next I tried my Bioenno LiFePo battery. The measured power was 7.3 watts! No more QRP, hi.
A Siglent SSA 3021X was available so I thought I’d try measure the harmonic filtering. But I don’t have any suitable attenuators. Instead I laid the dummy load board on a pick up coil plugged into the SA.
On 14.060 the signal read -23.9 dBm. On 28.120 the reading was -39 dBm. On 42.180, -50.5 dBm.
Having never done this before I don’t know if there is something wrong with my measurement method.
Are these values meaningful?
Is there some calculation involved to find the actual attenuation value?
Does the higher power output coincide with higher harmonic suppression?

I may try opening the turns on L2 to decrease the power output to 5 watts or less.

I have an Elecraft XG1 which I used to check the receiver. The difference between 1 uV and 50 uV on the S meter set for 10 segments was gratifying, no indication at 1 uV and full segments at 50 uV.
Because I have no antenna HOA restrictions the radio has yet to hear real signals.

Hopefully, I can try my QCX mini out with an antenna soon.
David N6AN

1 Like

Hi David @N6AN

I don’t think your measurement is valid, the pickup coil will have a different coupling at different frequencies, so it’s only an indication …

Is your dummyload the DL-1 ?
You can easily modify it into a 40 dB attenuator (or other value), more or less like this one I once made (but this is not the one I used in my measurements).

The load is a 20W 50 Ohm resistor in a TO-220 housing
The input is the lower BNC.
Then to the right, in series, is 1500 Ohm + 2 x 4700 Ohm in parallel, so a total series resistance of 3850 Ohm.
Finally in parallel another 51.1 Ohm resistor.
The SMD resistors were from my junkbox. Other resistors will limit the frequency range, but should be OK for HF work.

It’s not perfectly 50 Ohm from both sides, but good enough, and surely better than that pickup coil.
Attenuation values can easily be calibrated with your Spectrum Analyzer, using the Tracking Generator (I use the same model SA here).

For a better flatness, I added a piece of PC board on the top, same size as the base board, connected to ground at the left side with a stiff wire, which holds the “screen” just above the BNC connectors.

I get -40.07 dB at 30 MHz, -39.46 dB at 145 MHz, so still very good, only at 435 MHz attenuation gets worse with -36.40 dB.

EDIT : Another option is to build a Power Sampler like this one:

Google for “Power Sampler”, and you’ll find several other similar designs.

Luc - ON7DQ


Thank you, Luc.

That makes sense. I need to get a broadband flat response pickup coil, hi!
My parts stock is very limited. Perhaps I can scrounge what’s needed for a low wattage attenuator.
Kudos on your homemade unit.

I widened the turns on L2 to drop the power to around 5 watts with a 13.2 volt supply.

David N6AN

DL9SCO has published a couple of very nice videos on youtube:

  1. QCX Mini - Assembly: https://youtu.be/xprCjFVvSqo
  2. QCX Mini - Smoke test and alignment: https://youtu.be/0GElK3qaxog
  3. QCX Mini - Installation in the optional QCX Mini enclosure: https://youtu.be/xOn-3BW9Zjc

Have fun!
73, Roman


Yesterday, I made a small Battery Box, matching my QCX Mini
(for ON & PA hams, I found those small “Radio & TV” theme boxes in the WIBRA shop).
It has almost the same size and thickness as the Mini, and weighs 180 gram … “batteries included” as they say.

Inside are 3 Li-Ion cells (2200 mAh) for 12V output, the box has on ON/OFF switch, to reduce the number of times inserting or extracting the DC plug at the QCX Mini.
And @G0UPL, it even has its own FUSE , hi.

The insides of the box are covered with a thick plastic film (ex laptop motherboard protection), and all is fixed with generous amounts of hot glue.

I charge it by connecting the plug to a 12.6V stabilized & current protected power supply, so no balanced charger … (note: for the picture, it was not fully charged yet)
I know it’s against all logic, but my experience is that cells of the same batch charge reasonably equal. And if I ruin the cells, I have many more where those came from, they are easily replaced. Or if I think I need it, I can connect a balancing connector.


Due to it being perishing cold on the summits today, I decided to have a poke round in the QCX Mini. It had developed an intermittent problem with the volume, depending on how you touched the knob. After removing the QCX Mini from the case and turning it on, I found the problem had spread to the VFO. I noticed a dry joint at the header pins of the VFO controller. After remelting and adding a bit more solder, the problem was completely sorted. I had another play with the toroids and got TX up to 5W at 12V, and over 8W at 15V. I’ll call it good at that. As I was putting everything back together, I noted how the knobs can work loose, slide down the shaft and contact with the enclosure. This means that the click function on the VFO no longer works. I was thinking about what I could shove down the hole to create more clearance, and suddenly realised that the holes are about the same size as a paper punch:

I took an old ski pass and punched out about 4 discs per knob and shoved them in with a hex key. Voila, the knobs always have the right clearance, even if the grub screw works loose:

I got some new speakers recently and they came with small plugs in the banana sockets that presumably keep the dust out:

With a little trimming they fitted perfectly in the CAT & PTT sockets; and looked rather smart too:

Just minor tweaks really I suppose. My QCX Mini is now completely finished and operational. It will be coming with me on most of my motorbike activations when I put the bike back on the road at the end of March.

       73 de OE6FEG

Great work Matt.

I unexpectedly acquired a dead QCX-mini 40 this week. I have spent a few hours with a solder sucker removing all the excess solder from the through hole components. I re-wound the toroids and bridged across the lifted tracks on T1 (reason for scrapping and entering my possession).

I fired up the rig this evening and I’m seeing 4 watts output at 12v without even adjusting the toroids. Much to my surprise the RX alignment went absolutely fine.

I swapped out the 5v regulator for a branded one.

Good progress for today, next I have to fit the case and do the GPS frequency calibration.

73, Colin


Neat winding on the toroids. :slight_smile: I struggled to keep count of the turns on T1 and ended up taking a photograph, printing it out then counting them on the image. :slight_smile:


Yes, that’s a good technique!
I wound my toroids in the dark yesterday evening whilst XYL was watching telly. I took a photo of a part wound T1 to count the turns.


T1 looks like it was “machine wound”. Absolutely beautiful job.


A piece of tapered dowel (or paintbrush) in a vice makes life a hell of a lot easier:

       73 Matt

For the 3D printer folks, there is this toroid winder aid

Btw. did anyone perform the SSB (usdx-mod) for the QCX Mini?

Might give it a try to have a monoband lightweight SSB/CW unit for my longer hikes.

73 Joe


I think I will order one for fun.

Yesterday during sota activation I had two “freeze” with my new 20m QCX Mini. need to reset. Have you any feedback about this ?? I activated severall time with my 40m QCX pro and I never noticed this problem.
Best 73,

Did you notice if a “G” popped up on the screen when it froze? I’ve seen that happen before when RF gets into the paddle cable. Since the GPS and paddle use the same IO ports on the processor, if the Mini thinks that a GPS is connected it will lock out the paddle.

This was with an original QCX, but the design is pretty much the same.

I didn’t notice any “G” … but as I have forgotten my glasses in my car 300m down :grin: … the paddle continues to work, Rx too … but no way to modify the screen with buttons

I did the first field test with my QCX Mini 17m yesterday, on ON/ON-013. @G0UPL

Power was from the 3-cell battery box (see an earlier post), antenna a half-wave “Fuchs” antenna (length 8.9m), sloping from abt. 6m above ground to 0.5m (tied to the arm of my chair … ensuring “armchair” copy, hi). A 4m counterpoise completed the setup.

[ @F5MDY No “freeze” noticed, but maybe you used the “turn the speed to ZERO” trick for tuning up with a carrier. After unkeying, the rig does not transmit any more, but RX stays muted. The EXIT button gets you back in RX mode. It does not happen if you set the key to Straight via the menu, or if you use full-QSK (Semi QSK OFF). ]

Strange enough, despite many CQ’s on 18.092, and being spotted on SOTAwatch, I got only one reply from UR5IRM, who gave me a 559.

Time for plan B.
Adding 2.2m of wire to the antenna, and retuning my Fuchs tuner, I started CQ’ing on 14.062, with reduced power of 4W, and reduced RX sensitivity of about 1 S-point.
I almost fell off my chair when the first reply was from no one else than … ZL1TM !
He was a good 539 with me, and he gave me a 509.
That was the first ZL in my SOTA log. WOW.

Further QSO’s with OH5RP, EA5K, EA7GV, OH6JUM, EA7TG, SV2OXS and YL2TQ proved that “abusing” a QCX on the “one lower” band is certainly feasible.

To please the non-CW chasers, I switched to the KX3 to calll a bit more on 40m SSB, and ended the activation with 21 QSO’s in the log.

I will make a longer report of my SOTA excursion later.



Ok, probably not the same issue. With me, I would start keying, the “G” would pop up, and no response from the paddles. It only happened during one outing.