What are the Issues?
An unexpectedly high number of Cat 5E Return Loss failures are being reported from the field.

Field measurements can show a high degree of variability.

Lack of link margin concerns.

Links constructed of compliant components can fail the draft category 5E specification.

The TIA has revised Return Loss component requirements.

Difficult problem resolution

What is Return Loss?
Return Loss is a measure of the ratio of signal power transmitted into a system to the power reflected (i.e. 'returned'). In simple terms, it can be thought of as an echo that is reflected back by impedance changes in the link. Any variation in impedance from the source results in some returned signal. Real-life cabling systems do not have perfect impedance structure and matching, and therefore have a measurable return loss. Twisted pairs are not completely uniform in impedance. Factors that can contribute to slight variations in cable impedance include:
1.Changes in twist
2.Distance between conductors
3.Cabling handling
4.Cable structure
5.Length of link
6.Patch cord variation
7.Varying copper diameter
8.Dielectric composition
9.Thickness variations.

In addition, not all connecting hardware components in a link may have equal impedance. At every connection point there is the potential for a change in impedance. Each change in the impedance of the link causes part of the signal to be reflected back to the source. Return loss is a measure of all the reflected energy caused by variations in impedance of a link relative to a source impedance of 100 ohms. Each impedance change contributes to signal loss (attenuation) and directly causes return loss.

Return Loss is an important parameter for simultaneous bi-directional transmission systems.

How is Return Loss measured?
Return Loss measurements involve measuring signals simultaneously going into and out of the same pair of a link simultaneously on the same pair, requiring special measurement techniques and equipment. The standard approach is to use an RF network analyzer, a directional coupler, and a balun to which the pair under test is connected. In the lab, this is also called an s11 measurement. A directional coupler separates a sample of transmitted and reflected signals passing through it to two separate ports, which are connected back to the analyzer. The transmitted and reflected signals are measured, and their ratio may be interpreted as return loss. Because directional couplers are not perfect (25 to 30 dB of directivity is common), and baluns do not have good return loss characteristics themselves (20 to 25 dB typically), a fair amount of correction is required to mathematically 'remove' the non-ideal effects of the coupler and balun. For these and other reasons, return loss is difficult to measure accurately with level II field testers. While sufficient for NEXT and attenuation measurements, these testers are not capable of making the complex mathematical corrections necessary for truly accurate return loss results.

What can I do if a link fails Return Loss?
1. Review installation practices. Inspect terminations. Are wire twists tight to all connection points on the affected pair(s)?

2. Check the patch or test cord. Retest with a different physical orientation and see if that changes the result. Check the patch cord rating (Cat 5, 5E, or Cat 6). Try swapping the patch cord (if channel) or swapping ends with the test cord (if basic or permanent link). If the problem moves with the cord, the cord may be suspect. However, in many cases slight variations in cords may only be revealing a marginal link.

3. Verify all components (cable, patch cords, connecting hardware) are intended to meet Cat 5E or Cat 6 requirements.

4. Diagnostic approaches. If all links are failing this points to sub-category performance of a link element. Frequency domain plots that hug the limit line and do not exhibit deep nulls suggest the connector is the major contributor to return loss. Curves that exhibit deep nulls and high peaks, with significant amplitude variations, suggest the cable. However these are guidelines, not absolute.

5. If failures occur at high frequencies, this tends to suggest the connector is at fault.

6. If failures occur at low frequencies, this tends to suggest the cable is at fault.

7. Look at the attenuation frequency domain plot. If you see ripple (> +/- 1 dB) in the attenuation at higher frequencies rather than a smooth line this suggests there may be a structure problem in the cable.

8. Use a TDR. This will indicate where the largest impedance changes occur, but can often be misleading in diagnosing Cat 5E return loss problems. They are often not sensitive enough to isolate Category 6 impedance irregularities either. TDR's tell you nothing about return loss limits, and cannot clearly differentiate whether a single point source of impedance change is more of an issue than a series of smaller distributed impedance changes.