- This topic has 1 reply, 1 voice, and was last updated 4 years, 10 months ago by
.
-
Posts
-
I’m intending to install a 2m/70cm antenna on the side of my house. The coax run will be about 12’ down the side of my house, into a weather-head & pvc conduit across 30’ of my finished garage and into my office/shack. The purpose will be to serve a mobile radio.
I’m imagining a design with a bus bar outside (think CPI or Alpha Delta) and a lighting arrestor for my frequencies and to drive a ground rod proximally there. This side of the house is not close to the utility service entrance, however my office is. Is best practice to bond by connecting the above bus bar with a separate piece of copper (in separate PVC conduit), to a bus bar in my office and then bond to the utility ground? Or is this simply overkill for a desk mobile install?
Hello Jeff,
The guiding principle of lightning protection is if two wires are at the same potential, even if it is 50kV, there cannot be current flow between them. Without current flow there is no heat or damage.
In your case the two “wires” are the antenna and the utility wires including each of their associated grounds. Given your proposed arrangement if the utility wires or your antenna is struck there will be a significant potential difference between these “wires” and significant current will flow, much of it across the operating bench and through your radio.
We will consider the utility service and the feed’s point of entry ground rods first. A typical ground rod to earth resistance is around 10 ohms. With a 10kA strike the voltage at the associated ground rod is (I*R) 100kV while the other ground rod is near zero volts. Assuming the coax is connected to one ground rod and to the radio, and the radio is connected to the utility service either through the shack ground or simply a 3 prong power plug a high voltage will appear across the radio.
A simple solution is to locate the ground buss bar and coax entry near and connected to the utility ground. This isn’t always practical.
Depending on the actual layout of your shack a potential solution is to retain the ground rod, surge protector and buss bar as planned at the feed’s point of entry. The feed and a 4 or 6 AWG ground wire would continue from the buss bar to a station ground It is important this 2nd buss bar is connected to the service ground. From there the feed and ground would continue to the operating position. All of the equipment would be connected to this ground.
All of the wires must be connected together during a strike. Each of the 110VAC lines via MOVs in a surge protector, the radio, computer’s case, and PSU via ground straps, the coax shield and even the coax center conductor assuming a coax surge protector is used, are all referenced to a common ground. The key is all of the wires are at the same potential during an event. It doesn’t matter this voltage could be over 20kV.
A single device on the operating table not on the common ground, for example a table lamp or a wall wart not plugged into the surge protector, could provide a current path and a brilliant arc. USB cables from radios to computers that are not on a common ground are a frequent cause of damage. You need to stand back and consider all of the current paths.
There is a lot more to lightning protection than what I can capture here. The ARRL has a good book on the topic.
Hamilton,
What a detailed and informative response! I enjoyed reading it and learning something new. Thank you very much.
Merry Christmas and Happy Holidays.
/Jeff
For further study of the science of grounding, Motorola created a standard for radio communications sites. It is called R56 and can be found on several websites. Here is one from the Bureau of Land Management: https://www.blm.gov/sites/blm.gov/files/Lands_ROW_Motorola_R56_2005_manual.pdf
See Chapter 4.7
The Motorola Type B site target is a ground resistance of 5 ohms. Since I have 3 repeaters at my home site on 7×24, I have multiple ground rods, one on each tower leg and guy wire, connected in a ground ring which also contains my electrical service ground. My Single Point Station Ground bar is then connected to that ground ring. Perhaps overkill for a light-duty station, but it has saved me, on a hill top, thru many storms since installation in 1991.73,
Bill, NE1BI agree with Bill, NE1B. I also have several repeaters / digitpeaters at my location and have a 160′ tower. Five guy points on the tower every 30′. Tower base has 4 6′ ground rods. Each guy point also have 4 ground rods.
Keeps static charge, therefore lightning static buildup to a minimum. ALWAYS check your ground with an OhmMeter.
Have fun!!!
73,
Jim – KA1SU
NH ARRL Technical Coordinator
603-491-2132
- You must be logged in to reply to this topic.