I trued a series modulator to see how easy or not could it be. Here is the circuit so far, but the LPF will change to a 160m LPF when I finish with the rest of the TX. I removed the final stage and kept just three bjts to experiment at lower power first. Without any audio and without any LPF, I measure 1.7W (square wave) when the output is loaded with a 50R resistor.
I found this modulator in another circuit on the net and since it used the bd139, I thought it would be powerful enough to modulate a bd139 RF final.
Now it does not have another 1k from the base of the bd139 to the ground, to bias the bjt at 6v but sine it worked for him, I tried to test it as it is.
It did not work ok for me though. I can get some modulation only on tones, but on music only a highly distorted drum is heard, no music. I believe the 1M makes the bjt starve, so I will try a lower value tomorrow. But let me know if you have other ideas or things to comment.
Thank you