Thanks for correcting the lan2 typo.
I am fairly sure it is bad ethernet cables causing the issue. I have tried about 18 cables now -all I can find and mostly supplied with routers. 10 are within 0.0-0.1db difference when tested i.e. probably no difference, 4 are with 0.1-0,2 difference and 2 with 0.3 and just 2 with ~1 or ~2 db, I conclude 2 or 6 of the cables should be binned. I have a second HG612 and I will try that another day but what every that shows some cables work as I would expect i.e. no impact but a minority are found to have an impact in this test. Below is some Texas Instruments text relation to ethernet noise emissions.
4 Sources of EMI in Ethernet Applications
Ideally, if differential signals are perfectly balanced, no common mode energy exists in the system. In
single ended signal systems, ideally all forward energy is contained within the signal wire or trace and
return energy is contained within a ground wire or plane in close proximity to the signal.
The source of unwanted emissions in network applications is common mode energy, radiating either from
differential signal wires, or directly from the system chassis.
This common mode energy can originate from any of three sources:
1. Imbalance in the differential signal path
2. Noise coupled to or from the system chassis or power supply system
3. Noise coupled to or from the network interface cable
4.1 Differential Signal Path Imbalance
Signal path imbalance can occur in two ways: across the differential signal pair or between the signal
source and destination.
Imbalance that occurs across a twisted pair can result from the cable medium itself being unbalanced, or
from signal termination imbalance. Imbalance across a signal pair results in one signal having a larger
magnitude than the opposite signal, which manifests itself as common mode noise.
5. Differential Signal Path Imbalance
End to end or longitudinal path imbalance can occur if the source impedance, transmission line
impedance, and destination impedance are not exactly matched in a system. This form of mismatch
causes energy reflections across the cable from end to end, which also results in common mode noise.
Recommendations for preventing differential signal path imbalance include:
• Use high quality symmetrically and tightly wound cable. ISO CAT5E or better quality cable is
recommended for 10/100 applications.
• Use equal length differential MDI signal traces with a strip line impedance of 50 ohms.
• Closely match the values and physical placement of signal termination components.