Start here: An antenna may seem noisy because its pattern or polarization hears a local source strongly, its feed line carries common-mode noise, or it simply captures more real band noise than the previous antenna.
Why this changes a real station
Noise level alone does not prove an antenna is worse; the useful comparison is signal-to-noise ratio on the same signals at the same time.
What it looks like in practice
A larger or more efficient antenna can raise both signals and noise, while a directional null can improve copy even with less raw signal.
What to do next
Compare SNR, rotate or relocate where possible, test a feed-point choke, isolate local circuits, and hunt confirmed noise sources before changing antennas.
Evidence boundary
Keep frequency, reference plane, antenna geometry, losses, measurement method, and safety conditions attached to the conclusion. A match, a DC check, or one field symptom does not prove efficiency or identify the complete RF path.
