Take 1 cm off your cable to reduce PIM

A new study has discovered that cable lengths in mobile base station structures are much more important than previously thought, revealing that adjusting a jumper cable length by as little as 1 cm can significantly influence passive intermodulation (PIM).

PIM is a series of resulting signal harmonics and distortions that emerge between two or more interacting signals when transmitted through passive components such as cables and connectors, causing unwanted interference and degradation of wireless communications. 4G, 5G and future generation networks require stricter signal-to-noise ratios (SNRs) and the issue of PIM must be addressed if cellular capacity is to be maintained.

Now, a study by Hughes Electronics and the Biomedical and Communications (BiMEC) research group at London South Bank University suggests that standard metre-denominated lengths of cables are rarely phase congruent with commonly used (cellular telecom) frequencies and cause higher levels of PIM than are necessary.

“During our prior research into connector-related PIM we discovered that small adjustments to cable length made a significant difference to PIM results,” explained Greg Rymar, head of research and product development at Hughes Electronics. “In particular, we found that industry-standard jumper and feeder cables denominated in multiples of 0.5 or 1 m lengths did not produce the most desirable results when it came to PIM.

“Our investigations showed the reason for the phenomenon was that these standard cable lengths are not in harmony with frequency wavelengths, ie, the cable length does not match the compound length of the carrier frequency sine wave, causing ‘phase offsets’ and triggering phase angle reflections, which can be seen as PIM.

“In addition, each reflection causes a loss of amplitude (VSWR) to the received signal and intermodulation (IM) becomes more acute as a result of the difference in power between transmitted and received signals. The more impoverished received signal strength is, the more vulnerable it is to PIM distortion levels.”

Calculating overall phase displacement and adjusting cable length to harmonise phase in the operating frequency range is therefore a must for improving PIM performance, according to Hughes Electronics. In light of this, the company has added a length calculation service to its Network Approved rapid reaction non-standard jumper cable manufacturing service.

The addition of the service provides clients with the performance advantage of frequency- and phase-matched cables. The frequency-matched jumper cables are prepared in Hughes’ central London workshop, using high-temperature inductive soldering, and incorporate an IP68 gastight moulded finish.

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