Building a SatNOGS groundstation: Grounding and lightning protection

Dealing with grounding and lightning protection is obligatory when setting up a roof antenna. Bad grounding might cause not only damage to your antenna and equipment but even your complete home electronics or ignite fire.

Lightning is dangerous for two reasons. Direct lightning conduction can cause impulse voltages of higher than 20 Megavolts, whileas induced voltages can also reach up to 10 Kilovolts. Very interesting I found also this quote [1]:

A lightning has  an impulse voltage of > 20 Megavolts and an impulse current above 100.00 Ampere. Multiplying both numbers, the resulting yields a power that would instantaneosly vaporize any conductor. It does not, because the condctor does not actually conduct the energy of the lightning. The a cable with a diameter of 10mm alumium or 8mm copper only lays out a path, through which the surrounding air gets ionized, through which the actual charge surge current flows. This ionized stream will has a diameter of 2-5 cm and will not ignite anything, as the grounding conductor has a resistance <3 ohms.

TL;RL

To fullfill lightning protection according IEC 62305 (DIN EN 62305) your setup must at least have:

  • Lightning protection: A lightning rod or conductor connected to the antenna mast, >16mm2 directly to an earth grounding element.
  • Overvoltage surge protection: on lines entering a building, protecting against the PEN (protective earth neutral) inside of your house with >4mm2.

General antenna grounding

Please note that this is not a complete guide for house and antenna grounding. For a proper house grounding or if you feel unsure please contact a qualified electrician! All suggestions are at own risk!

A roof antenna is part of the lightning zone, or EMC safety zone 0, which is directly exposed and can not be shielded. Inside the house you have different safety zones (1-3) and it important that you do not connect these zones directly. This means, your roof antenna must be grounded not through your ground which is provided from the inside of the house, but with an external grounding. Depending if there is an existing external lightning rod, you may connect to it, if there is none, you must put your own lightning rod.

Earth ground element

Generally grounding must be performed by a thick wire (diameter recommended is 8mm copper / 10 mm alumium, however IEC only states 16mm2) in an distance from the wall into an earth grounding element with at least 3 meters depth. You can get those elements starting at little money.

Ham radio amateurs will connect their antenna setup directly with the lightning rods. For protection of the antenna cable however exist several mechanisms to redirect the currents inside the cable. But most operators will still disconnect their expensive radio equipment to avoid damage.

Proposal for Grounding a SatNogs

  1. Connect the metallic parts of the antenna mast directly with the lightning protection (>16mm2)
  2. Connect the antenna and/or electronics to the lightning protection over an gas discharge tube (>16mm2)
  3. Use power line surge protector (5-10€)
  4. Use ethernet surge protector (20€) connector.

SatNogs requires more than an antenna cable. You have to supply power for the electronics, motors, receivers. Probably you want to use a ethernet connection to transmit the data. Let’s dont be foolish: If a lightning strucks your groundstation, it will definetly be bricked. But without grounding and further protection, it may travel into your house wiring and destroy .. well .. all your appliances.

Overvoltage protection devices

There exists a variety of power surge products to safe your computer or other equipment. However these are supposed to protect the connected device, not the supplying net. These devices will not offer sufficient protection if the lightning travels through the house, i.e. when your antenna mast is not grounded externally!

Update 13.03.2015: I have contacted APC Home and asked if their “Essential series” product line can fullfil this. In response they provided the Dekra test certificates.

For the overvoltage protection you must provide a copper wire of at least >4mm2. This implies that the power plug ground is not sufficient as these usually only are connect through a 1.5mm2 wire to the house earth potential rail. You might be tempted to use a heating or water pipe since these must be grounded, but it is strongly recommended not to do so. (There could be plastic elements in between etc.). Instead you should have a 4mm2 copper cable directly to your earth potential rail (PAS – Potentialausgleichsschiene) which is connected to your house ground foundation earth.

Ethernet connection

For ethernet you can find commercial surge protectors (Ubiquiti) for around 20€.

 Don’t: Isolated safety transformer
Isolated safety transformator
Isolated safety transformator

Previously I proposed a setup using safety transformers. This icon shows isolated safety transformer with short-circuit protection.

However, despite the calming words isolation and safety, these have a breakthrough voltage of about 4kV which is definetly not sufficient for direct lightning protection.

(EN)

[2] http://electrical-engineering-portal.com/lightning-and-surge-protection-for-cable-networks-and-antennas-for-tv-part-1

 (DE)

[1] http://www.ib-haertling.de/amateurfunk/Blitzschutz_und_Potentialausgleich.pdf

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