Topband: Adding 80 m to 160 m quarterwave vertical

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I have up and operational a vertical for 160 m. It’s about 30 m of 250 mm triangular mast (Deeco) topped by about 5 m of 50 mm aluminium mast with a small top hat to resonate around 1840 kHz. Fed directly above the base insulator, it presents a reasonable match across the band, with about 32 ohms at resonance.
I also want to operate 80 m DX. So far, I’ve been experimenting with a wire, offset by 1 metre, about 20.2 m long, parallel to the mast. I’ve tried grounding the 160 m mast and feeding the wire separately, floating the mast and feeding separately, various L-matches, and now feeding in parallel with the main mast with no matching. The latter gives me a reasonable match on the CW end of the band, and it doesn’t affect the 160 m operation.
The complicating factor is the approximately halfwave antenna on 80 m in close proximity to the wire.
Previously, I had a mast about 22 m high, fed through a series capacitor for 80 m, with an offset wire going up to a top hat at the top of the mast for 160 m. That seemed to work ok. Now I have the reverse situation.
I made a couple of QSO on the 80 m DX window last week, with the offset wire about 19 m long, and it seemed to work ok. But my interest is not SSB DX. Adding lumped constants to try and match that wire on the CW end of the band didn’t work as expected, i.e., some base loading to bring down the resonant frequency, presumably due to the effect of the tower, grounded or floating. I lengthened the wire to resonate on the lower end of 80 m and made a few CW QSO, but either it’s performance or the propagation was disappointing.
After all that, I’m getting a bit lost. What arrangements have been successfully used by others? Again, I have a near-enough to quarter wave base-insulated 160 m vertical, and want to operate also on 80 m CW, with a secondary priority of SSB DX Window.
73, Luke VK3HJ.

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