Physics people. FTP CABLE inherintly has significant disadvantages for RF digital performance. Shields are ONLY useful if you have noise ingress or egress that deteriorate performance over a UTP counterpart. UTP cable is well superior at electrical stability as the ground reference, and it’s variation, is far, far lower.
A RF cables reactive variables are controlled by the proximity to the electromagnetic ground reference. Capacitance is a power law formula that says the CHANGE in the location of a shield the WORSE the capacitance will be at that location in an RF cable. The closer the shield to the core pairs, the larger the change in capacitance. Thus, the WORSE the impedance stability. And, a shield will impact internal pair to pair cross talk up to 6 dB or more and this limits the Shannon law bandwidth. The noise floor is closer to the signal.
Shields are good if the ingress noise is significant enough to upset the balanced CMRR built onto Ethernet cables. Proper balance in pairs mitigates the need for shields. If there is no noise source, more stable performance is seen in UTP Ethernet cable.
Where ingress noise reduces the Shannon law bandwidth more than the 6 dB impact shields have on internal cross talk, and the increased return loss due to worse impedance control is acceptable, then use shield, but not till then.
10g Ethernet uses FTP design to allow tighter cable to cable spacing, but still mitigates ANEXT between cable that is cable to cable magnetic field controlled. A UTP will work is fine if it is sized to increase the spacing to account for no shield. Special designs need to be used to precisely place the shield to mitigate RL reflection degradation from the use of shields in 10g cable.
10g can’t tolerate what is called alien cross talk from adjacent cables to a target reference cable as it is unpredictable. Internal NEXT, RL and other variables can be noise cancelled with electronics.
There is still a lot of misunderstanding on how cables work.