Isolated Antenna Mount (SMA/RP-SMA)

I designed this thing to isolate dc ground from chassis ground for antennas panel mounted on metal enclosures. It is basically a pair of 1mm thick 14.5mm flange washers, with 6.5mm ID, and a 9.2mm OD 0.5mm thick flange. It mounts standard panel mount SMA type antenna connectors through a 3/8" diameter hole.

The idea behind this thing is that metal enclosures are usually bonded to earth ground (many laws require this). An antenna panel connector mounted directly to the metal panel will make a connection from earth to the antenna ground. In the case of most hobbyist projects needing to get wireless signals outside a metal enclosure, the wireless device involved will have the antenna ground connected to DC ground (ie RPi, esp devices, etc). The result is a connection between a DC ground to earth ground, which in some cases can result in ground currents flowing through the rf section of your device (very bad for signal, and can potentially damage sensitive ic's). This is especially important if ground/earth bonding is not otherwise planned/executed, where the antenna shield may be the only path for current to flow.

Isolating the antenna body from the metal enclosure ensures no current can flow, regardless of your choice of ground reference. The decision to use earth as DC ground reference is common in hobbyist projects. If so, using a heavy guage wire to bond earth terminals to ground terminals at the "star point" of the bonding bus is best. This will ensure any ground current will not make its way through sensitive devices. In this case, the enclosure still serves as a massive grounding plane, but without a change for harmful current through rf circuits.

To use... print 2x washers; any filament works. Slide one washer onto the antenna connector, flange side facing away from antenna. Drill a 3/8" hole in the metal panel. Insert the antenna with washer into 3/8" hole from the inside of the enclosure, then slide the second washer onto the antenna with flange facing the panel. Both flanges should be inside the hole at this point. Keeping the flanges positioned inside the hole, tighten the antenna nut from the outside to secure the assembly to the enclosure. Before connecting the antenna cable, use a multimeter to verify infinite resistance between the antenna body and enclosure. If there is contact, the flanges did not stay in the hole... if so loosen the nut to recenter the flanges, and retighten more carefully.

Category: Electronics