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How to properly ground a radio modem to protect from lightning

Following are some general guidelines for grounding a Digi PKG radio modem when the modem, antenna, conduit, or any component of the communication system is likely to be hit with a lightning strike: 

  • Establish a low impedance path to earth ground from the PKG's aluminum enclosure. Using metal screws on the end brackets should help you do this.
    The PKG's serial data reference ground and power supply ground are internally connected to the RF connector ground, and the RF connector is grounded to the aluminum case by the washer and nut on the exterior of the case.
  • Ground the voltage supply to a path to earth.
  • RS485 considerations: In an ideal RS485 system each device shares a common ground. This ground is connected to an earth ground at each device if the earth ground node at each device is the same potential.
    If the ground potentials at the devices are significantly different, the RS485 transceivers at each end could be damaged. In such cases it is recommended to find a better earth ground or install optical isolators (or another pair of radios--this might actually be cheaper).
  • Use an RF lightning arrestor such as the DSXL-MA from www.polyphaser.com inline with the antenna coaxial cable to protect the modem against energy surges on the antenna and coax. The arrestor should be installed on the end of the coax nearest the modem (not the antenna). Ground this lightning arrester to a path to earth.
  • Ground the antenna frame to a path to earth.
  • Ground all of the above together to a common node (such as the antenna mast) if each is a short distance away. This will reduce the possibility of ground loops through different earth grounds. However if distance from grounding point to common node is far (20 feet or greater) it may be better to use separate earth grounds. Test voltage at each earth ground node to ensure ground potentials are equal.
  • Remember that lightning strikes supply an impressive amount of current--18,000 Amps on average--Ensure that the connection to ground is secure at every location and proper gauge wire is used.

Each installation is different. Some great application notes on grounding can be found on the web site of Polyphaser. Please contact Technical Services if you have questions on specific applications.
Last updated: Aug 23, 2017

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