Integration of QCX+ and 50W PA

With my QRP radios I’ve been struggling to make myself heard at times even on CW from my home QTH. The problem is the location is far from ideal and my attempts to chase SOTA activators in an effort to improve my CW skills are being frustrated somewhat both by the geography and my limited power. I can often hear people but they don’t seem to be able to hear me.

So the answer was to look for a few more Watts. :slight_smile:

I already had a QCX+ built for the 20m band so I decided to buy one of the QRP Labs 50W PA kits which cost around $30 although with shipping it came to around £40 delivered to the UK.

The 50W PA can be fitted into its own enclosure but I had read it would also fit into the QCX+ enclosure and to cut a long story short this is what I’ve done. Some pictures of the finished assembly follow.

This shows the original QCX+ on the left and on the right the 50W PA fitted to the underside of the QCX+ top cover.

In the photo above you can see I have fitted the rear panel from the 50W PA enclosure on the rear of the QCX+. You can’t buy the panel separately but Hans Summers generously added it to my order for free when I explained what I wanted to try.

This is it put together. The finned structure on the top is the heat sink which comes with the 50W PA kit.

And a close-up of the rear panel showing the connections. There are different approaches which can be taken but I chose to keep it simple and keep the QCX+ and the 50W PA separate without any internal connections. Hence the short coax joining the RF output of the QCX+ to the RF input of the 50W PA.

The power supplies are different voltages. The QCX+ is running on a 12 volt lithium battery but the 50W PA needs a 20 volt supply. It will run at a lower voltage but power drops off significantly, especially at 20m, which was the band I’d chosen. The 50W PA works best at 40m and power drops off either side of that.

To create a 20 volt supply I took the 13.2 volt 4.2 Ah LiFePO4 battery I already had and connected it in series with a 6.6 volt LiFePO4 battery. As I couldn’t find a 4.2 Ah battery I’ve used two 2.1 Ah batteries and connected them in parallel. I used various connectors to achieve this as I wanted to do it without butchering the battery terminals so they remained standard batteries.

It is a bit of a mess and I need to look at the leads again to shorten a few of them but it produces a nominal 20 volts and from my measurements the PA is producing around 35W which is in line with what QRP Labs say it should produce on 20m. The amplifier only produces 50W at 40m.

And it works. I had a few normal ham radio QSOs this afternoon and one SOTA contact, @EI9KY/P on EI/IE-064.

The batteries, including the 12 volt lithium for the QCX+ weigh a total of 1.25Kg which isn’t too bad and shouldn’t prevent me trying this out from the top of a hill sometime. On send it draws 3.3 Amps, so with 4.2 Ah of capacity it should last a few hours I think.


Nice. But where are the fuses?

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Good point. :slight_smile: I will need to address that in Version 2. :slight_smile: You can get in-line mini blade fuses which might be ideal.

I run my QCX amp from a 5S lipo. I’m seeing 40w on 20m.

After a 2 hour solid activation a couple of weeks ago my 3aH 5S battery still had some life left…

I thought the amp was quite a difficult build but I actually rate the finished article as one of the best things I’ve built. I’ve been waiting for such a product for so long!

In fact I like the amp so much I built another!

Oh FAO Andy MM0FMF, I run a very short power lead and an inline fuse with my amp :grinning:. I made a special power lead with Power Pole connectors, it’s not a connector I use often.

73, Colin


That’s very good. I need to look at my dipole, it’s a SotaBeams Tri-bander and could probably benefit from some fine tweaking to bring it into the CW end of the band. It also isn’t strung in an ideal way in my garden.

I found the trifolar winding awkward although the instructions say it doesn’t have to look pretty to work. It was only afterwards I realised I should have plaited it. :slight_smile:

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I did this too - but for activations rather than chasing. Several years ago I was offered an HF Packer V2 Amp kit that had never been built. I got it completed and then didn’t use it very much until this year. Now it’s a standard part of my SOTA kit for HF activations.
It gives a nominal 50W from a 12v battery which it doubles internally. The top board is the doubler and the lower one the actual PA, the switchable bandpass filters are mounted vertically at one end. I run it from a 5Ah 3S LiPo and it easily does two activations on a charge.


A little update on progress. I did some more chasing today and managed 3 QSOs with activators F/DM3FAM/P on FL/VO-125, @EI6FR/P on EI/IE-021 and @G4YSS/P on G/NP-008. This also taught me you need to be patient to be a chaser - there were some long pile-ups to wait to end before I could get in. :slight_smile:

I also ran my NanoVNA over the Tri-Bander and as I half suspected it appears to be optimised for the SSB end of the 20m band with lowest SWR at 14.3MHz. Adding about 5cm of wire to the end of each crocodile clip brought it down to almost exactly 14.06MHz which is much better for CW activity. But I need to check this all again with the antenna mounted in an open space and away from obstructions - which I can’t achieve in my garden.

I’ve also realised a useful source for a 20 volt supply to run the 50W PA are the lithium batteries sold for portable tools like drills. Many of these are 20 volt and you can buy separate sockets the batteries clip into as a convenient way of connecting the battery to your system. The downside is they seem to lack capacity compared to the LiFePO4 - but they are cheaper and easier to find.

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Nice job and thanks for the contact. FYI, I had a KX1 with 2W and a vertical antenna.