Re: 80 Meters Harmonics Fix Proposal
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Ok I did replace the electrolytics as can be seen from the picture ..It is effectively a center tap I wanted tighter coupleing between the two windings ....The reason why you feed from the center tap is because the elecrtomagnetic 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 ..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 ...Result no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.
Try it and see. can't cost a lot .Maybe it'll work
On Monday, September 3, 2018, 6:27:45 PM PDT, Howard Fidel <sonic1@...> wrote:
You can't argue with success! However, I don't quite understand a few things. Why remove the electrolytics, the bulk capacitance can't hurt? Why not center tap the transformer primary and feed the DC there? It should have the same result and is simpler.
On 9/3/2018 9:04 PM, jim via Groups.Io wrote:
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