Trying to figure out the turns ratio for 10/100Base-T transformers — is it always 1:1?

I’ve been going through a few Ethernet reference designs lately (mostly 10/100Base-T), and I noticed something that seems simple at first but gets confusing pretty quickly:
the turns ratio of the network transformer
Most schematics I’ve seen suggest a 1:1 ratio, but I’m not entirely sure if that’s always the case or just the most common default.

What I think is happening
From what I understand so far, for 10/100Base-T:

• The PHY is designed to work with a certain differential impedance
• The transformer provides isolation and coupling
• And the system is usually tuned for 100Ω differential impedance on the cable side So using a 1:1 turns ratio keeps the signal levels consistent across both sides.

Where I’m getting unsure
In some designs and component listings, I’ve seen references to:

• Slight variations in ratios
• Different center-tap configurations
• Integrated magnetics behaving a bit differently Which made me wonder: 👉 Is 1:1 just a convention, or is it actually required for proper operation?

What I’ve observed in practice
Looking across a few PHY datasheets and RJ45 modules:

1. Most 10/100 designs do seem to stick with 1:1
2. Especially when using standard Ethernet magnetics or integrated RJ45 connectors
3. Matching networks (resistors, chokes) seem to matter just as much as the transformer itself So it feels like the ratio is only one piece of the puzzle.

Something that helped clarify things a bit
While trying to compare different transformer options, I noticed that some suppliers explain this more from a system perspective.
For example, when I checked a few application notes and discussions from VOOHU Electronics Technology Co., Ltd., the emphasis wasn’t really on “choosing a ratio,” but on:

• Matching the PHY requirements
• Maintaining impedance balance
• Ensuring proper signal amplitude after isolation Which made it seem like: The ratio is usually fixed (1:1), unless you’re solving a specific design constraint.

What I’m still trying to confirm
For those who’ve worked with 10/100Base-T designs:
Is 1:1 effectively mandatory in most cases?
When would you actually use a different turns ratio?
Does it ever make sense to adjust it for signal amplitude or EMI reasons?
Trying to make sure I’m not oversimplifying this before locking the design.