Lightning Protection and Surge Suppressors
When you deploy an RF link you typically mount antennas at high elevation points, possibly on the rooftop of a tall building, telecommunications tower, or mountain top. During thunderstorms there is a strong possibility for these locations to be hit by lightning strikes. This application note describes the effects of lightning, and provides installation guidelines to reduce the likelihood of damage to the equipment.
All the Afar wireless ethernet bridges are housed in waterproof enclosures so that you can install them outdoors, in close proximity to the antenna. This is our recommended deployment approach because it reduces the coaxial cable losses between the radio and antenna. Even so, some users prefer installing the radio indoors, down below, and run a long coaxial cable between the radio and antenna. These two deployment strategies require very different lightning protection approaches.
Independent of the deployment strategy, the first step in preventing damage to the equipment is to divert lightning from hitting your antenna directly. Preventing a direct hit requires providing an alternate target on the
tower or building for the lightning to strike. This requires the installation of a lightning rod or equivalent at a higher elevation than the antenna, and provide an effective path to earth ground. There are several companies that provide equipment and guidance for an effective lightning protection installation. One such company is Erico which provides an illustrated six-point lightning protection plan.
Radio Mounted at the topWhen lightning hits the structure, there is a sudden “Ground Potential Rise (GPR)” at the top of the structure in relation to the earth ground. With the radio mounted near the antenna, and with its enclosure electrically connected to the tower or structure, its ground reference follows this GPR. Any differential voltage between the RF port center conductor and the ground will be small and easily handled by the quarter wave stub internal to the radio.
For this type of installation – where the coaxial cable is less than 10 ft (3 m) – you DO NOT need an external RF Lightning Arrestor . But make sure you provide a good connection between the radio enclosure and the earth ground (for the PulsAR radios this is explained in the Operators Manual, section 3.2.2).
With the radio at the top of the tower you will likely have a long cat5 cable down the tower and into the power inserter (and your equipment) installed indoors. This cable includes four twisted pair wires, two for the Ethernet Tx and Rx data, the other two used to carry the power to the radio. When this cat5 cable is long – greater than 20 ft (7 m) – we recommend using our Ethernet Surge Suppressor device installed at the point where the cable enters the building. Make sure you connect the ground wire of this surge suppressor directly to an earth ground with a straight wire (no loops).
Radio Mounted at the bottom
If you deploy the radio at the bottom of the tower with a long coaxial cable between the radio and antenna, then we recommend that you install an RF Lightning Arrestor at the radio N-type connector.
The reason is that a coaxial cable shield can carry large currents at speeds in excess of 90% of the speed of light while the velocity factor of the center conductor can be as low as 66% of the speed of light. An electromagnetic pulse induced at the top of the tower can result in a large differential voltage between the shield and the center conductor at the input of the radio with enough energy to "burn-through" the quarter wave stub inside the radio. The external surge suppressor is designed to withstand much higher energy levels.
All the Afar wireless ethernet bridges include a quarter wave stub protection on each of the two antenna ports. The quarter wave stub acts as a short to ground for any signals with frequencies other than the operating frequency (900 MHz or 2.4 GHz) and their multiples. This shunts to ground all the undesired electromagnetic energy induced on the antenna central conductor. On the other hand, at 900 MHz or 2.4 GHz the quarter wave stub acts as an open circuit and therefore lets the desired signal pass to the radio front end.
Energy from a lightning strike spreads from DC to 1 GHz with most of the energy staying below 1 MHz.
References:RF Lightning Arrestor
Ethernet Surge Suppressor
AFAR Communications, Inc | 81 David Love Place | Santa Barbara, CA | USA 93117
T: +1 805 681-1993 | F: +1 805 681-1994
T: +1 805 681-1993 | F: +1 805 681-1994