WSPR Transmitter - update

It has been an interesting few days with lots happening keeping me away from the workshop. In that time I did some reading and discovered a flaw in my logic in relation to the output power of this project. So with a slight circuit and layout revisions I have made some progress.

Firstly, the intended output power was 500mW pep. Since this is a DSB transmitter I incorrectly assumed I would have 250mW in each sideband. This is wrong. I would in fact have 125mW in each sideband. The reason is that when the voltage of both signals is in phase the output voltage doubles, giving a fourfold increase in power.

So I revisited the circuit and concluded that 1Wpep was as far as I could stretch the class A broadband final stage before matching networks would be needed. Since the long term goal was to use this board for other bands I didn't want to move away from the concept of a broadband transmitter. It appears that the 2W of heat the transistor is dissipating is manageable and the device is still operating in it's safe operating region. So back to 250mW of usable output.

I did look at using a broadband push-pull arrangement, and even parallel transistors as in the JBOT (just a bunch of transistors) concept. But the first problem was to source ferrite material at reasonable cost. I have ordered some material that may be suitable but it will take weeks to arrive and I want to finalise the boards. So despite having great success in the past using ferrite tubes removed from EMI filters on VGA cables, something I will write up for you one day, I will just push on. The only problem with junk-box ferrite is each transformer is bespoke because the properties of the ferrite were unknown. As much as I love experimenting I strive for reproducible results at all times.

One step forward was to revisit the layout. I now have the layout dimensions as  110mm x 60mm so it will slide into a commercial extruded aluminium case.While my intended housing is an old VHF commercial radio case it is good to have a nice case like this as an option, albeit more expensive than what I will be using.

That's it from me for today.
Richard