Inductance of the QCX 20m low pass filter inductors
At the end of 2017 I’ve ordered two QCX QRP TRX kits from QRP Labs. The QCX is a 5 watt CW single band transceiver with WSPR beacon and built-in alignment/test equipment. QCX kits are available for the 80m, 60m, 40m, 30m, 20m and 17m bands. I’ve ordered the 20m and 40m kits.
After winding the 20m low pass filter toroids, I’ve measured the inductance values, because there have been reported low output power problems in the QRP Labs group at Groups.io, which seem to be related to the L1, L2 and L3 low pass filter inductors.
The L1, L2, L3 inductors as well as the L4 drain inductor in my 20m kit (serial #2585) needed 1 turn less to nearly meet the specs mentioned in the manual. I’ve measured the inductance with an AADE type L/C meter before soldering the inductors to the PCB. My final values are
L1 = 0.71 µH (should be 0.77 µH)
L2 = 0.89 µH (should be 0.90 µH)
L3 = 0.74 µH (should be 0.77 µH)
L4 = 0.41 µH (should be 0.40 µH)
The final output power of my QCX with these inductance values is
2.5 W @ 11 V
3.0 W @ 12 V
3.5 W @ 13 V
4.0 W @ 14 V
4.6 W @ 15 V
5.2 W @ 16 V
This was measured with an LP-100A into a 50 Ohm dummy load. As the thin enamelled copper wire is the only fixation of the inductors and this seemed a bit loose to me, I’ve fixed them with hot glue to the PCB.
Toroids of the 20m QCX low pass filter