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I have some UHF, microwave antennas and an antenna rotator on a roof mounted mast. The "radio room" is in the second floor just below the antennas. Also the service entrance for building electricity comes up the outer wall and enters the building in the second floor near the cables for the roof mounted antennas. The location of the service entrance is quite different from most descriptions I find on the web, which makes me a bit confused about how to best connect things for best protection.

I do not want to propagate the coaxial cables for UHF and microwave all the way down to ground to protect them before propagating them up again because of cable loss. My thinking was that I should add a single point ground panel on the outside wall on the second floor, connect the mast and all antenna and rotator cables through it and ground the panel with a wide copper conductor all the way to ground. At ground level I connect it to a new grid of ground rods + the electricity safety ground rod. Since the service entrance for building electricity is very close to where the ground panel on the second floor will be, I was thinking to bind the ground wires together also at this point. This means there will be two ground conductors going on that outer wall quite close to each other all the way to ground - one thick new one, and the existing one from the electricity provider. They are connected to each other in both ends.

Are there any problems associated with having two down conductors for grounding close to each other - vertically like 7 meters? Should I ensure a maximum/minimum distance between the two? Is it correct to connect them together also in the top? I am mainly thinking in terms of lightning safety + and also not screw up the building electrical safety.

I hope someone with more experience can answer these questions.

rubund
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Your two ground conductors must be connected at both ends.

They must be connected at the bottom because that's where the ground is and they aren't ground conductors if they aren't connected there.

And they must be connected at the top because if they are not, you don't have a single point ground. Without a single point ground the potentials between the two ground conductors will be different, and the lightning protection will not be effective.

I would route them as closely together as possible. The idea is to minimize the area of the loop they create. A larger loop area means higher inductance, meaning more induced voltage due to changing magnetic fields, and higher voltages with changing current.

Remember the goal of a lightning protection scheme is to divert as much as possible of the strike current away from the protected equipment, and to ensure everything within the protected equipment stays at about the same voltage. If you haven't already, see How can I protect equipment against a lightning strike?

Phil Frost - W8II
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    Thanks, Phil! My concern regarding having the ground wires close to each other is also that the existing ground conductor is tightly adjacent to the wires carrying electricity into the building (3 phases 230V 50Hz) all the way down to ground. Maybe that can cause noise entering via the ground wire (not directly lightning related though)? Any other risks associated with this? – rubund May 07 '20 at 17:47
  • @Ruben your home is probably entirely wired with NM cable which has a ground conductor immediately between the hot and netural wires. Worrying about noise between the two just in this one section is futile. For the purposes of lightning protection, you want all the conductors (ground or otherwise) to be run together to minimize that loop area. – Phil Frost - W8II May 08 '20 at 16:03
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    @Ruben if you did want to reduce noise, best would be to track down the sources (switching power supplies, etc) and either replace them or try to contain the noise with ferrites, thus preventing the noise from getting on the wiring in the first place. – Phil Frost - W8II May 08 '20 at 16:05
  • So making the spacing between the two wires small, minimizes the total inductance of a loop - meaning inductance for two wires having current in opposite directions. But I thought lightning would make the current go in the same direction in those two wires (when they are bonded in both ends). In that case, the total inductance towards ground is reduced the wider apart the wires are placed from each other (infinitely spaced same-type wires: L/2) Why do we want to minimize the inductance of the loop - and not the to-ground total inductance? – rubund May 08 '20 at 18:18
  • @Ruben What will you use to connect these two infinitely far apart wires together? – Phil Frost - W8II May 08 '20 at 18:29
  • Hehe, really good question.. :) I'm not seriously thinking of putting them infinitely far apart. My point is just that the to-ground total inductance decreases with increased spacing. I was thinking that if they then go a few meters apart from each other, and are connected to ground at two different points (and bonded under ground), the total inductance towards ground would be lower (even though the loop inductance is higher). And why is not that what we want? – rubund May 08 '20 at 18:38
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    @Ruben consider the consequences of a large loop area in combination with Faraday's law of induction in an environment surrounded by extremely high di/dt driven by a lightning strike. Also consider what happens when the lowest impedance path between ground rod A and ground rod B is not through the soil, but through one of your conductors, up to the 2nd floor, and back down to the other ground rod. https://www.w8ji.com/lightning_strikes.htm – Phil Frost - W8II May 08 '20 at 21:55
  • If rod A and rod B are bonded with a thick wire under ground and this wire is much shorter than the loop, the loop will not be the one with the lowest inductance.

    The way I understand it now: For indirect strikes, minimising loop inductance is more important - and indirect strikes are the most common ones... For direct antenna strikes, the inductance towards ground may be a bit smaller if the inductors are spread apart, but it does not weigh up for the fact that indirect strikes get more dangerous. Therefore I should keep the wires together.

     – rubund May 09 '20 at 21:06