The maximum coupling onto the cable occurs when the polarization of the interfering signal matches the cable run. In most cases, intermittent interference sources, such as handheld two-way radios or tablet computers, are naturally vertically polarized because of the way we hold these items during normal usage. As a result, installations with long vertical runs of cable are more susceptible to EMI/RFI than horizontal runs of similar length.
Figure 1 illustrates how to install cables to improve immunity against vertically polarized interference, and Figure 2 illustrates cable installations that increase susceptibility to vertically polarized interference.
Figure 1. RFI source cross-polarized with long cable run (minimum interference)
Figure 2. RFI source polarization coincident with long cable run (maximum interference)