On Monday, September 3, 2018, 11:51:11 AM PDT, Warren Allgyer <allgyer@...> wrote:
When I measured 80 meter harmonics the particularly troublesome ones were 5th at 18 MHz and 7th at 25 MHz. In both cases the attenuation of the stock uBitx at those frequencies was limited by the layout of the board and by the coupling of relays sharing both filter input and output in the same frame. In such cases the characteristics of the filters, both existing and any modifications are immaterial. The attenuation of the stock filter supplied is more than adequate. But the harmonics bypass the filter and go via I/O coupling straight to the output. Please measure the 5th and 7th harmonics on CW. I think you will find you have not done much to improve the stock situation.
Just a fyi ...see the picture of the 80 meter output from my unit ...after doing the L7,L8 mod and the output transformer mod .
A whole bunch of the cruft flying around in this radio is due to L7 and L8 having DC current flowing through them ...This DC biases the ferrite (shifting the BH curve who knows where) lowering the inductance and allowing RF everywhere ..
Put a proper inductor on the IRF510's that is NOT affected by DC current flowing through it and see what you get