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Grounding a Tower Mounted on Rock

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0SJE, Jun 30, 2021.

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  1. W0SJE

    W0SJE Ham Member QRZ Page

    Hello all-

    I've started installing my Rohn 25 tower on my property and looking for some advice on grounding. I live in the Colorado mountains and the tower will be up on some granite with no way to do conventional grounding with rods. I doubt I can even drive something to 3 feet without hitting rock.
    I'm concerned with grounding as we get significant lightning and I want to do the best job possible, even if a compromise.
    Does anyone have similar experience, or suggestions?

  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    I am thinking that a concrete encased ground rod in what little dirt you have. Did the Google Earth sattelite & you have very little. What is the makeup of the rock, mineral wise?

  3. W0SJE

    W0SJE Ham Member QRZ Page

    Hi Ed-
    It's solid granite. I've drilled into it for the mounting plate with "Hilte"-type bolts.
    I've seen some approaches with several shallow rods connected in a circle with copper or other screen arrangements.
    I wonder what commercial installations do?
  4. W9IQ

    W9IQ Ham Member QRZ Page

    The MIL-HDBK-419A has rather extensive coverage of this topic. You can download it for free in a searchable PDF version. It is over 800 pages so you want to take advantage of searching for suitable terms such as "rock".

    - Glenn W9IQ
    N0TZU likes this.
  5. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Last edited: Jun 30, 2021
    N0TZU likes this.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    R56 section 4.11 addresses this and other difficult situations. Basically for a stone ground situation the rods or plates are laid horizontally and radially, buried as deep as the soil will allow.

    Some years ago a friend of mine had a similar problem for a cabin AC service rod in the Front Range foothills. The electrical inspector had him install a horizontal rod, but because of conditions it was partially in soil and partially in a shallow trench jack hammered out of the granite and back filled with grout.

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