A common thing I see is people running these part-15 (in the US) devices with upgraded antenna systems, which isn’t permitted. Either run it under amateur rules or under part 15.

Have you tried a 1/4 wave vertical with 4 radials sloping towards the ground? (all wire)? or even a vertical dipole fed halfway up? J-poles show significant coupling to their feedline and mounting arrangements and, as you discovered, are taller than they need to be for their gain.

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personally, I think the limit should’ve been higher, at least 5 kHz, when the AM Phone guys are occupying 10 to 15 kHz with their hi-fi broadcast set ups.

And a hard bandwidth limit really does limit total data rate, the symbol rate limit didn’t - exactly the opposite effect that the loud anti-baudrate-limit people wanted