I'm working on a product design which includes an Ethernet jack (with integrated magnetics) that will need to be remote from the controller. The controller and jack are contained in different parts of the product that rotate relative to each other. Because of this I was thinking to use FFC cable. The total cable distance would be 10-15cm. I'm fine with only 10Mbps so I thought it should work OK.
I was planning to alternate signal with ground in the cable (as described here: I2C on FFC or IDE cable - interference?) but I'm a little concerned about the impedance of the FFC. I would normally use a calculator like this https://www.eeweb.com/toolbox/microstrip-impedance to determine trace widths, but they assume a ground plane and I wanted to use a standard FFC cable with no shielding. What would the optimal pitch and conductor width be to achieve 50Ω impedance, assuming the signals were G S G S G, etc.?
The controller would likely be something like the LAN9514, with integrated MAC and PHY. If the above approach is a major problem, would splitting up the MAC and PHY (placing the PHY by the jack) be a better solution?