Thanks all for your reactions. I have built my QCX for 30m. The fact that its output power is higher on 40m is unintended and not the result of some sort of deliberate compromise. My new LPF should be OK if you look to the PDF file: -3dB @11.3 MHz, and already virtually flat at 10.170 and lower. So the transmit behavior - gradually increasing output with decreasing frequency - is likely not the result of the LPF. I hope Steve, MW0KST, will conduct his experiment with L4 any time soon. Very curious to see what will happen.
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Regards/73,
Enno, PF5X
Enno Korma, PF5X