Trio Radio Installation - Grounding Antennas, Towers and Cabinets.

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.

1) 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.

2) 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.

3) 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.

4) 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.

5) 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.

6) 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.

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

8) 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.

9) 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.

10) 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.

11) 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.

12) 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.

13) 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.

The above comments are general in nature. Several very good websites exist with far more information about grounding procedures and hardware. A quick Google search should find plenty of good reading.

Publicerad för:Schneider Electric Sverige