Having changed the 4 m antenna to a single band one there’s no need to use my trusty coax diplexer anymore (one half of it was described here). It served very well eliminating all the interstation interference between 6 & 4 m. But being a notch filter it did not attenuate any HF signals, of course. Depending on many factors I still had a bit of interference at times when some HF bands were used around the station. Not much and certainly nothing I couldn’t live with but aren’t we radio amateurs always trying to perfect things? 😉 After the good experience with our 2 m helical BPFs I ordered another band pass filter for 4 m, this time from Serbia’s Antenna Amplifiers.
Measurements showed a very good performance in attenuating HF signals but 6 m as the neighbouring band could certainly use some more attenuation. Therefor I combined the filter with the other half of the diplexer (i.e. a 6 m coax notch filter) to get the best out of both. Now even the smallest kind of interference is gone, mission accomplished. 😎