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.
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.