Installation Hints for Large Antennas

There’s a general lack of understanding of how to provide lightning protection for radios, antennas and towers. Many installers either ignore the issue, do the minimum possible, or sometimes even cause significant problems with their installation. Below is a brief discussion of common issues.

Several websites exist with far more information about grounding procedures and hardware. The below comments are general in nature provided by Schneider Electric.

  • Mount the antenna on the mast or tower and, if a directional type, aim it with a compass towards the master site. Make a good connection with the clamp to the metal of the support structure. Ensure the clamp is tight without damaging it.

  • Run the heavy gage LMR-400 coaxial cable up to the antenna, tying it in place with black (NOT white!!) tie wraps every couple of feet. Some people put a couple of turns of electrical tape over the tie wraps too for added protection from the sun.

  • Connect the cable’s N male connector to the antenna’s N female connector. Ensure they mate properly then tighten them up by hand. (don’t use a wrench, it’s not necessary and may damage the connector!)

  • Ensure you run the cable DOWN away from the antenna so water can’t follow the cable down into the antenna itself. Also make sure that you don’t put tension on the connection.

  • As shown in the Sealing RF Connectors document, seal the connection with rubber splicing/fusing/mastic tape (vulcanizing tape) and form it by squeezing with your hand, then add a layer of electrical tape over top. The point is to keep water out.

NOTE: If desired you can connect the cable to the antenna and do the sealing on the ground, before going up the tower or to the top of the mast. But ensure you do not put too much pressure (eg by dangling the cable down from the antenna) on the connector.

  • Connect the long LMR-400 cable’s N male connector at the bottom to the lightning/surge arrestor’s N female connector. Again tighten firmly by hand only.

  • Include a drip loop so water following the cable falls off the cable before reaching the arrestor.

  • Seal the connection from the cable to the arrestor with overlapping layers of rubber splicing (vulcanizing) tape and cover that with an overlapping layer of electrical tape.

  • Connect the short jumper RF cable from the lightning arrestor (N male of the cable to N female of the arrestor) to the radio. Run this cable carefully in the equipment cabinet to keep it out of harm’s way.

NOTE: A radio may use any of several connector types eg N, TNC or SMA. Ensure you have the correct jumper cable for your radio type.

  • The lightning arrestor and the bottom of the mast or tower must be grounded. This typically requires a 10 foot ground rod or a good ground plate buried several feet deep. (A very tall tower may require more rods eg three evenly spaced around the tower) The cable must be a heavy gage (green) ground cable. The cable must be routed as short and straight as possible from the arrestor or mast/tower to the ground rod. Any bends not needed must be avoided. Definitely do NOT coil up any extra ground cable as this (and bends) causes impedance to the high energy discharges from lightning.

  • Typically the antenna itself does not need to be grounded by the installer. The antenna is mounted with a metal clamp on a metal pole, which must be grounded.

  • If the antenna is a yagi, (directional type) it’s a good idea to mount the antenna several feet below the top of the mast (spacing at least equal to antenna’s length) to allow the mast to take the main hit on a lightning strike.

  • If the antenna is an omnidirectional type it’s often mounted at the very top, though some are mounted on arms 2-3 feet out on the side of a tower. Many of these types are “DC grounded” which means the radiating element goes thru a coil to ground, allowing static buildup to be bled off as it tries to develop. This is often sufficient, but in some very heavy lightning areas installers may run a lightning rod around the omni and a few feet above. It must be kept well away from the antenna (a couple of feet) so it’s a bit awkward but hardware does exist.

  • If the tower is tall a cable grounding clamp should be installed every (approx) 75 feet which grounds the coax outer shield to the tower itself. This bares the outer shield (braid) so care must be taken to waterproof the cable/clamp afterwards.

  • A good quality surge arrestor (eg polyphaser) must be used at the building entrance. It may be mounted in a metal feed-through plate, may be a simple in-line type, but must be connected to a heavy gauge ground cable or strap.

  • The ground cable from the surge arrestor must be heavy green ground cable, kept as short and straight as possible with NO coils and no sharp bends. These add inductance, which acts as a resistance to the high frequency energy of a lightning strike.

  • Ground cables must not run through metal conduit. This adds impedance and can pick up noise.

  • Be aware that surge arrestors will fail after a period of time, either from absorbing one big surge or multiple smaller surges. This may appear as either an open circuit or more commonly as a source of high VSWR. (reflected power) Keep a spare handy and try swapping out the unit at site if high VSWR is seen.

  • A surge arrestor is not truly a lightning arrestor. A direct hit to the antenna may send enough energy through the system that the arrestor is overwhelmed, causing serious damage to the radio and attached equipment. A surge arrestor is primarily aimed at bleeding off the buildup of static charge to prevent a lightning strike in the first place.

  • The ground cable must go to an industrial quality ground rod or plate, installed according to local regulations or better. This must be an 8 or 10 foot rod.

  • All grounds should go to a common point eg a metal plate at the building cable entry point. Daisy-chained ground connections must be avoided.

  • The mast (if short eg 10-20 ft) must be grounded at the bottom with a short straight cable to a ground rod. Preferably a separate one but at a site where a short mast and the cabinet are close together often one rod is used.

  • If a small antenna structure is used a single long ground rod may be used at its base, but for a normal tower a ground ring should be installed with 3 or more ground rods, a ring of heavy copper wire connecting them, and several cables running out from the tower.

See also:
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DigiKey Antennas & Associated Products