Jim AB7VF suggests that much of the harmonic cruft in the µBITx is from DC current flowing through the L7 and L8 ferrites that effectively lowers the inductance as current increases and allows RF to go everywhere.
Jim replaced the electrolytics as can be seen from the picture ..
He suggests that the transformer is effectively a centre tap. He wanted tighter coupling between the two windings …. The electromagnetic field set up by the DC passing through the coil will bias the ferrite “magnetic domains” causing a shift in the B-H field resulting in loss of inductance and the generation of spurs .
When feeding the center tap – current flows up toward the “dot” or start of the top winding “left hand rule” will give you the polarity of the magnetic field around the top coil … current also flows through the bottom coil away from the dot or start of the bottom winding creating a magnetic field opposing the one created by the top coil. The net result is no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.
The following photo shows the 80 meter output of Jim’s unit after doing the L7,L8 mod and the output transformer mod.
Jim suggests putting a proper inductor on the IRF510’s that is NOT affected by DC current flowing through it and you will get legal output on 80m CW.
uBITx.net would be interested in whether this approach works for others in cleaning up the harmonics, because it will be a lot cheaper and easier than sorting the LPFs.