Again :-( Drift question (Ultimate3S)

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DK5LH
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Again :-( Drift question (Ultimate3S)
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Hi,

since some days I'm again on the air in WSPR now using a Ultimate3S and the Si5351A (formerly a Pi)

I'm getting mostly spots with 0dB drift from my transmissions on 40m (rarely -1 or 1) but on 20m, 15m or 10m mostly -4 or -3dB.

But during my transmission I´m receiving myself using a different antenna and a FT817. Here I always have a drift not above -2 or 1dB.

So my question is why do I receive mostly spots with a drift of -4 or -3dB on 20, 15 or 10m from other OM's but my own spots are ok?

Some additionally infos:

My Synthesizer is warmed up for many many hours and covered with a piece of Styrodur/XPS foam. I don't know if it was a good idea to place 100 Ohm resistor on the crystal surface (50mA)...
I did that in the beginning when I changed from 1 BS170 to 3 using 5V and getting also drift reports of -4dB. After fitting the resistor I got mostly -2dB on 20-10m.

Now I changed to 10V for the BS170. Each of them is now covered with a small tube of thin copper.
Output power is now adjusted to 750mW (40m), 250mW on 20m, 200mW on 15m and around 100mW on 10m.

Would be great to get an idea what to do...

Vy73 de Hartmut / Dk5LH