Dc Grounded antenna question

[sp5it]

Vertical. No choke. No balun. Good match. Bad noise

So coax is part of the antenna. On RX and TX.
It is possible, that antenna is not picking the noise, but coax is.
Another thing is proximity of the antenna to metallic objects, the way the coax goes and so on.
Some reading: https://www.w8ji.com/common-mode_noise.htm

The broadband common mode choke, 1:1 balun and 1:1 unun - PA9X

Choosing a common mode choke. A look at different variants, air core, ferrite beads and ferrite toroid core for ham radio HF band.

www.pa9x.com

The common mode choke lowered the noise floor with 12-15 dB, equal to 2-2.5 S-points.

Mike