There’s another special connector type that can confuse designers at first: the reverse polarity (RP) connector. The standard connector configuration is to have a male (pin) center contact in the plug, and a corresponding female (receptacle) in the jack. But in the US, Federal Communications Commission (FCC) regulations mandate reverse gender “polarity” in some unique cases. The situation dates back several decades when wireless Wi-Fi routers for consumer use were introduced. They were designed for limited range using a small antenna having a connector at its base which screwed directly into the Wi-Fi unit’s antenna connection, and thus with no ability to relocate it. However, the FCC was concerned that end-users would attempt to boost the device’s range with add-on amplifiers and/ or external antennas, causing Wi-Fi band interference. Their “solution” was to attempt to prevent easy connection of such add-ons by mandating the use of RP connectors on these wireless devices (which often used SMA connectors) to make them incompatible with standard add-ons (Figure 13). Within a short time, however, cable assemblies terminated with RP connector pairs became widely available and were standard add-ons for devices such as external, relocatable Wi- Fi antennas (Figure 14).
Figure 13: RP SMA plug and jack connectors have the opposite center conductor gender compared to conventional SMA connectors; (left to right) standard SMA male connector, SMA standard female connector, RP-SMA female connector, RP-SMA male connector. (Image source: Wikipedia)
Figure 14: This external Wi-Fi antenna can be moved around to find an optimal location and is connector compatible with the antenna interface on the Wi-Fi router due to its RP-SMA connector. (Image source: Amazon)
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