As I tested building 20m version one more time and I still found a little bit self-oscillation. As I said this is the nature of this architecture to use only one NE602 for both RX and TX. AA7EE also mentioned about this when he was building his DSB radios:
I had thought I found a perfect fix by muting the front end but it seems not working very stable for 20m as the frequency is higher and the self-oscillation gets easier, I suppose.
What I am going to do, is that I will hold the shipment and announcement for 20m version before the actual shipment starts. I will need about one week to find a proper fix. If it does not happen, I will ask if you would like to switch to 40m or accept refund.
The 40m version is stable and I think you will enjoy this tiny radio to make dozens of QSO’s.