Working on a homebrew 40M QRP superhet transceiver based loosely on the BITX40 design with a KK7B Classic 40 receiver front end. Recently modified the PA stage from 2N2222 to IRF510 for better efficiency, but now experiencing significant receiver sensitivity loss - MDS went from -125dBm to about -110dBm.

Using 6MHz IF with discrete mixer and crystal filter. T/R switching is done with PIN diodes. Anyone experienced similar issues after PA modifications? Wondering if increased RF leakage from the MOSFET stage is desensing the receiver during standby.

PA isolation is showing only 30dB on my homebrew directional coupler - should be closer to 60dB. Any suggestions for improving T/R isolation in QRP transceivers?

Check your PIN diode bias networks first - IRF510s can have significant gate leakage that affects bias voltages. Also verify ground connections between PA and mixer stages - poor grounding is a common culprit for desense issues.

Had the exact same problem with my scratch-built 20M rig. Added a series ferrite bead on the PA drain lead and improved the low-pass filter response - gained back 10dB of isolation. Sometimes a simple toroid choke does wonders.

Make sure you test all your PIN diodes individually before troubleshooting further - I've seen switching diodes fail in subtle ways that only show up under RF drive conditions. A simple forward/reverse resistance check isn't always enough.