The presence of the sweet spot was traced to overhead CATV cables acting as passive radiators.
Hence it was decided to create a tuned passive radiator to further improve performance.
One half of my 40 m inverted 'V' dipole antenna was chosen as the passive radiator. The series-tuned circuit was connected to the core of the coax at the shack end and earthed as shown.
|Tuned Passive Radiator - Schematic|
After tuning the passive radiator to resonance, performance of the portable crystal radio was found to be quite good across the shack roof.