As I posted earlier the duobander inv-L was finished before the RA-DX contest. I planned to do full time SOAB LP but I had a socking moment to realize that the 80m bandwidth is quite narrow. It was very interesting to see how the duobander operates differently compared to the single band 160 and 80m versions. All the measured data were correct but somehow the “together” version did not want to be as it should. I dismantled the LC matching boxes to re-measure the L and C parts to see whether the values have some discrepancies. For first I made all first measurements with the MFJ-259B capacitance and inductance modes. To be more precise I bought a new high precision L/C meter to avoid the further tool errors. These are the calculated values for the LC circuits:
…and for 80m:
The LC meter has 1-3% accuracy class so I can claim that the MFJ could not (or I made mistake) measure well this low inductance value. The original L on the MFJ was ~0.45 uH which I considered exact enough to go for the on site tuning. Funny is that this coil which has 0.21 uH inductance on the picture is already squeezed into half to increase its L value. The original was as low as 0.1 uH.
On the other hand 160m had very similar SWR curve and band width as the single bander brother. The problem came with the 80m which had such a narrow one below: